Liquid Effluents in Agriculture - Portfolio by Mario Prieto

PORTFOLIO

CPE 408 – LIQUID EFFLUENTS

Liquid Effluents in Agriculture May 2013

Green Country Industries Nathan Carew Huw Ellis Sergio Hernández Bonilla Robbie Lewis Mario Prieto Fernández

Contents Page

1. Preliminary Wastewater Treatment

2. Primary Wastewater Treatment

3. Secondary Wastewater Treatment

4. Tertiary Wastewater Treatment

5. Sludge Treatment

6. Fertilizer Production

7. Effluent Treatment

8. Legislation

101

9. Case Study Review

112

+ INTERACTIVE DVD

1. PRELIMINARY WASTEWATER TREATMENT

[Source: http://www.voltas.com/voltas_water/images/pdf/grit_removal_mechanism.pdf]

1. Introduction Sewage is fairly uniform in nature but anything may be present from large bulky objects, grit, organic materials and clothing materials. The composition of these materials may vary from one time to another depending on the presence of foul sewage or storm water. Therefore, the purpose of preliminary treatment is to screen easily separated bulky solids from the water and separate out the grit in order to protect pipework, pumps and treatment equipment as the water flows onto downstream treatment plants. As the composition of sewage is so varied; multiple cleaning units are usually required, firstly removing the large bulky objects and then down to smaller objects and grit.

A simplified typical preliminary treatment process outlining this is shown in the figure below:

Sewer Inlet

To Primary Treatment

Solids Screening

Grit Removal

Screenings Dewatering and Disposal

Grit Cleaning and Recycling/Disposal

Figure 1 A simplified preliminary stage effluent treatment process

This report will look at the different types of preliminary water treatment units including different screens and channels used in the sewage treatment sector and will outline their main features, application and design considerations.

2. Screening A screen, generally speaking, is a device with uniform openings through which raw sewage is passed. Screens vary from broadly spaced (6-50 mm) inclined bars to finer perforated metal sheets and can be placed in series in order to successively remove bulky objects such as pieces of wood to smaller objects such as paper and plastic. Screens need to be regularly cleared of screenings to prevent blockages. Bar screens are usually mechanically cleaned with manual cleaning made possible as a backup although manual cleaning may be done entirely at smaller treatment works. Bar screens are usually inclined to make it easier to manually clean. Perforated screens are usually mechanically or manually brushed.

2.1

Screening Units

(a) Curved Bar Screen (Mechanically Raked)

Mechanically Rotating Rake Assembly

Smaller solids pass through

Raw Sewage Flow Curved Bar Screen

Figure 2.1 A Schematic of the mechanically raked curved bar screen

Features  

Large solids are screened by a series of 15-20mm spaced curved parallel bars which extend through the entire channel width. Smaller solids pass through the screen. The screens are mechanically cleaned by rotating or partially rotating rake(s). The screenings are then removed from the rake(s) into discharge by a bar known as a ‘doctor bar’ 5

  

A simple construction with few moving parts. Suitable for small to medium scale treatment plants. Should be used in series with a manual bar screen in the case of mechanical or electrical failure.

(b) Vertical Bar Screen (Mechanically raked)

Figure 2.2 A schematic of a mechanically raked vertical bar screen highlighting the sequential process within the unit. [Source: http://www.aqualitec.com/aqualitec_screentec.html]

Features  



Suitable for deep channels. Large solids are screened by a series of vertical (or slightly inclined) bars which extend across the entire width of the channel. Smaller solids pass through the screen. The screenings are mechanically raked from the bottom up by a rake set up on a conveyer. The screenings are then moved from the rake into discharge by the ‘doctor’ bar. Should be used in series with a manual bar screen in the case of mechanical or electrical failure.

Figure 2.3 A photograph of a commercial band screen unit in operation [Source: http://www.ovivowater.com/content/files/data/prds_9_faf0217814014b808ade5ffb e546258b.pdf]

Features 

  

Large solids are screened by a perforated (4-6mm openings) band of durable metal where they are trapped and forced off the screen by a jet of water and into discharge. Smaller solids are able to pass through the screen. Screenings will contain a considerable amount of water content which provides a significant disadvantage of handling cost. A relatively high maintenance unit. Should be used in series with a manual bar screen in the case of mechanical or electrical failure.

(d) Drum Screen / Cup Screen

Water Jets Effluent Inlet

Effluent Outlet

Rotating Drum with Mesh Perimeter

Figure 2.4 A schematic showing a principle drum screen unit.

Features  

Consists of a mechanically rotated cylinder or serrated cone geometry extending across the entire channel width with mesh surrounding the perimeter. Effluent enters through the outside of the drum screen and flows through the mesh into the drum where suspended solids are trapped. Smaller solids may pass through the mesh. 8



 

The caught solids are then washed off with water jets into discharge which is usually a pit below the drum where solids can be pumped or mechanically transported away. The screenings will contain a lot of water which will increase handling costs. Should be used in series with a manual bar screen in the case of mechanical or electrical failure.

(e) Run-down screen

Figure 2.5 A labelled schematic of a run-down screen system. [Source: http://www.screensystems.com/RunDownScreens.aspx]

Features 

This unit consists of several tapered wires or bars grouped horizontally and parallel to one another. Effluent flows down though the tapered wires and small suspended solids and fibrous material become trapped in between the wire wedges and slowly pass down to the lower ends of the bars until they can be collected at the bottom.

  

Used for removing finer solids and fibrous material. This unit would normally follow bulkier solid screens. A flow depth of around 1m is required for this design. Grease and fat blockages may occur among the tapered wires.

(f) Fine Brushed screen

Features   

2.2

A curved finely (2-6mm) perforated metal plate in the form of a trough. The screen is regularly cleaned by rotating brushes. Any thin or fibrous solids such as paper and plastics are unlikely to pass through where they might have passed through a more conventional bar screen. Power is required for the mechanical rotating brushes.

Screening Design Considerations

This section will introduce some of the main design parameters to consider when designing an effluent screening system.

(a)

Approach Velocity, Va

The velocity of the effluent stream as it approaches the screen.  

(b)

Should not be too high (>1.0m/s) which would cause dislodgement of caught solid material on the screens. Should not be too low (<0.5m/s) which would result in solid settlement before reaching the screen

Hydraulic Head Loss, HL

The hydraulic head loss of the effluent stream across the screening unit is an essential parameter for treatment plant design as the correct approach velocity can be derived 10

from it. It is important to consider how this head loss will increase with time as solids become trapped on the screens and the rate of screen cleaning should be large enough such that the head loss does not become too big however this can be unpredictable due to the changing nature of the sewage. The head loss for a clean bar screen can be given by [1]:

Where: W = Total closed width of the screen B = Total open width of the screen hv = approach velocity head = Va2/2g Î˛ = Bar shape coefficient (around 2.5 for cheaper rectangular bars and 1.7 for more expensive streamlined bars) = angle of the screen to the horizontal [Source: http://wedc.lboro.ac.uk/resources/units/WWT_Unit_3_Preliminary_and_Primary_Treatment.pdf]

Channel Width, W The width of channel required to facilitate a certain screening unit can be estimated by an equation given by [1].

Where W = Channel Width (m) C = Side Frame Allowance (m) Q = Maximum flow rate (m3/s) H= Maximum water depth (m) V= Flow velocity (m/s) t = (b+s/s) b = bar thickness (mm) s = space between the adjacent bars (mm) 11

2.3

Disposal of Solid Screenings

Solid screenings are typically disposed of by incineration or sent to landfill. However, the discharge stream usually needs to be separated from the water used from the water jets to reduce handling volume and sufficiently then dried to a low enough moisture content if screenings are to be incinerated. As mentioned before, as the nature of sewage varies from day to day, it can be difficult to predict how much will be processed but typically ranges from 1025kg per 1000 people per day [1].

3. Grit Disposal The removal and subsequent washing of small granular inorganic particulates such as sand, silt or ash from wastewater is done in order to increase the valuable volume load on the primary sedimentation tanks and to prevent any blockages in pipework due to grit build-up. The amount of grit present in wastewater effluent depends widely on the location of the plant. Grit content tends to be higher in rural areas than urban as one might expect. On a wider scale, developing countries tend to have a wider practice of using sand as cleaning aids for example which heavily influences the grit load on treatment plants.

3.1

Grit Removal Units

(a) Grit Channels Parabolic shape based cross section geometry

Length of Channel â&#x2030;&#x2C6;20 to 25*Channel depth

Figure 3.1 Labelled cross section geometry of a typical grit channel used in industry. 12

Features  



Usually more than one grit channel is required per application such that at least one is operating while the other(s) is being cleaned. The flow travels along the length of the channel at a constant velocity and grit settles and builds at the bottom rectangular section of the channel where it is either mechanically scraped or pumped away or manually cleaned. The main disadvantages are organic material is likely to be trapped amongst the grit and the unit requires a large surface area of land.

(b) Detritors

Figure 3.2 A photograph of an industrial detritor unit with rotating steel arm scrapers. [Source: http://www.voltas.com/voltas_water/images/pdf/grit_removal_mechanism.pdf]

Feature 

A detritor is a large, shallow concrete tank with sloped corners. The effluent flows into the unit via adjustable vertical gates for a controllable constant velocity throughout the unit. The lower velocity allows grit to settle to the bottom of the

   

tank and water to flow through to the opposite side which may flow over an outlet weir. The settled grit is mechanically scraped onto a collection sump by slowly rotating steel arms at the bottom of the tank. The grit is then mechanically raised or scraped from the sump into discharge where water is drained and the grit is cleaned in a separate cleaning unit. Dimensions of the tank depend on the manufacturer but the surface area can be anywhere from 3m2 and above. Up to 95% removal efficiency is achieved.

Figure 2.3 An aerated grit channel unit model schematic [Source: http://www.westechinc.com/en-usa/products/aerated-grit-chamber#media] Features 

This unit consists of a deep channel with an approximately rectangular crosssection. 14

 

Air is introduced into one side of the channel via a pipe producing a spiralling motion of effluent flow along the channel. As a result of this flow pattern, grit is deposited at the bottom of the channel where flow velocities are typically around 0.3m/s as lighter water and organic material can rise to the surface. The grit is then mechanically scraped and lifted into discharge for dewatering and cleaning.

(e) PISTA ® Grit Traps (Vortex)

Figure 2.3 A vortex grit removal system schematic model [Source: http://www.smithandloveless.com/Products.aspx?CategoryUid=31&ProductUid=205]

Features 

 

This unit works in the same way as an industrial vortex unit in that wastewater effluent enters into a circular tank tangentially, producing a turbulent swirling vortex which effectively separates the grit from the wastewater which exits radially out of the top of the tank. The grit settles into a central sump at the bottom of the tank where it is air lifted into discharge. Lighter organic matter which may settle with the grit is separated at the bottom by a stream of air flowing through. 15



In larger treatment plants, smaller and multiple units may be used over large single units to treat high volumes rates of effluent as this achieves higher efficiencies. Since this unit doesn’t rely on settling effects unlike units such as the aerated grit channel, higher efficiencies are achievable especially at lower particle sizes.

3.2 Grit Removal Design Considerations Design Velocity, Vdesign This section will introduce some of the main design parameters to consider when designing a grit removal unit. For grit removal systems relying on particle settlement for separation, the flow velocity must be low enough for the grit particles to settle but fast enough for the organic matter to remain in suspension. Stokes Law theory is used to calculate the maximum settling velocity (Vmax) for a spherical particle in a liquid. This theory can be assumed to sufficiently describe the behaviour of grit settlement due to its relatively low concentration in the wastewater stream. The settlement behaviour of organic material on the other hand is usually hindered by intermolecular forces forming a unique solidliquid interface. It is these differences in settling behaviour that makes it possible for separation of grit in settlement tank units. Stokes Law:

Where: Vmax = Maximum velocity of settling partical (m/s) ρ = Density of settling partical (kg/m3) r = Radius of settling partical (m) ρ’ = Density of liquid (kg/m3) g = Gravitational constant (m/s) = Dynamic viscosity of liquid (N.s/m2) [1]

Therefore, the design velocity for a constant velocity grit channel can be calculated for its specific geometry dimensions making sure to allow for the size distribution of particles and turbulence effects to achieve a sensible balance between removal efficiency and cost.

3.3 Grit Disposal Once separated, grit is usually dewatered, cleaned and either sent to landfill or recycled; usually in the manufacturing industries. Extra care must be taken in the grit treatment process as pathogenic or parasitic organisms will be present as in all sewage derived solids.

4. References [1] WEDC Preliminary and Primary Treatment. Available: http://wedc.lboro.ac.uk/resources/units/WWT_Unit_3_Preliminary_and_Primary_Treatment.pdf. Last accessed 04/05/2013. [2] Aqualitec. Available: http://www.aqualitec.com/aqualitec_screentec.html. Last accessed 04/05/2013. [3] Smith and Loveless Inc. Available: http://www.smithandloveless.com/Products.aspx?CategoryUid=31&ProductUid=205. Last accessed 04/05/2013. [4] Water Online Africa. Available: http://wateronline.co.za/wastewater/conventional/preliminary/screening/curved/curved-barscreen.html. Last accessed 04/05/2013. [5] Aqualitec. Available: http://www.aqualitec.com/aqualitec_screentec.html Last accessed 04/05/2013. [6] Westech-in. Available: http://www.westech-inc.com/en-usa/products/aerated-gritchamber. Last accessed 04/05/2013. [7] Voltas Water. Available:http://www.voltas.com/voltas_water/images/pdf/grit_removal_mechanism.pdf Last accessed 04/05/2013. [8] Screen Systems.com. Available:http://www.screensystems.com/RunDownScreens.aspx Last accessed 04/05/2013.

2. PRIMARY WASTEWATER TREATMENT

[Source: http://www.watura.net/uploads/94/picture.jpg]

Contents 1.0 Introduction 2.0 Designing a Primary Sedimentation Tank 3.0 Particle Settling Theory 4.0 Particle Settling Velocity, Surface Loading Rate and Retention Time 5.0 Tank Zones, Types, Loading and Efficiency 6.0 Primary Sludge Treatment and Disposal/Usage

1. Introduction Primary waste water treatment proceeds where preliminary treatment left off. Generally speaking, the purpose of a primary treatment plant is to reduce the load on the succeeding biological secondary treatment plant by reducing the biochemical oxygen demand in the flow which is done by separating out some of those suspended (typically by 40-60%) and settleable(typically by 90-95%) oxygen demanding solids. [3] This process is achieved primarily in continuous sedimentation tanks (also known as clarifiers) where wastewater is stored for a period of time and allowed to be in a relatively stagnant state. The calm conditions allow light solids to sink to the bottom of the storage tank as ‘sludge’ and floating material such as grease and oil to come up to the surface as ‘scum’. These phases are then physically separated and removed allowing for a reduced load on the biological processes of the secondary treatment that follows.

Scum (Grease, oil etc.) Preliminary Treated Wastewater

Sedimentation Tank

To Secondary Biological Treatment

(Primary) Sludge

Figure.1A simplified diagram of a typical primary separation treatment process.

2. Designing a Primary Sedimentation Tank There are a number of factors to consider when designing a primary treatment tank. A well designed tank will look to relieve the load on upstream secondary treatment units by around 25-40% by reducing suspended oxygen demanding solids in the flow by around 50-70%. [1] Additionally, the tank must be flexible enough to operate under different flow properties which are subject to change throughout the year. These property changes may include particle and liquid density, particle size distribution and stream temperature. It is this unpredictability of the incoming flow properties which provide the biggest challenge to tank design. Most often, designs in industry tend to make use of engineering rules of thumb and empirical data based on existing designs.CFD modelling can be usedalongside these methods to simulate flow velocity and temperature profiles and predict possible problems of tank operation such as short circuiting.

The initialfundamental designsteps for a primary sedimentation tank usuallyinclude: 1) Understanding the underlying types of particle settling theory and deciding which type of settling relates to the flow composition being dealt with. 2) From step 1 and from additional experimental tests, finding the average settling velocity of the solid particles within the flow. 3) From the settling velocity, and hence the loading rate, as well as other factors, deciding on the design tank retention time. 4) Once these fundamental steps have been decided; the shape, sizing and flow characteristic of the tank to balance tank efficiency with cost can be made and also make any fine tuning that may be needed.

Each of these design considerations will be covered in more detail in the remaining sub sections.

3. Particle Settling Theory As primary tanks fundamentally speaking; allow the settling of suspended particles to the bottom of the tank for separation; it is important to appreciate and understand the different types of particle settling behaviour that may occur within a sedimentation tank.

a) Discrete – This type of behaviour can be modelled by stokes law. Particles are in such low concentration that the settling behaviour of a single particle is not affected by its neighbouring particles. Grit and sand are typical examples of particle which follow this behaviour. This type of settling may occur in primary sedimentation tanks if there is some grit remaining from preliminary treatment. b) Flocculant – Smaller particles clump together with larger particles and therefore settle more quickly. This results in a distribution of settling times as particles clump together in different ways as they settle. This type of settling occurs in primary and secondary sedimentation tanks. c) Hindered – Intermolecular forces of particles play a part in binding particles together and form a unique solid-liquid interface which hinders particle settlement. This type of settling may occur for activated sludge in secondary sedimentation tanks. d) Compressive – Occurs when a packing arrangement occurs at the bottom of the tank forming a compressed blanket of sediment where liquid is pushed upwards in order to maintain the structure of the sediment. This behaviour usually only occurs with thick sludge in sludge thickening plants.

The type of settling behaviour a solid might portray will have a strong impact on the average settling velocity of the solids in the flow and hence the loading rate and retention time design parameters of the tank. The types of settling behaviour that tends to occur in a primary sedimentation tank are flocculent and hindered flows which normally cannot be predicted to a satisfactory accuracy using models. In this case, it is normal to carry out sedimentation tests in order to create particle depth/time curves and derive an appropriate average settlement time for the separation efficiency needed. Additionally, since settlement behaviour is not always ideal due to turbulent flows in the tank etc, it is good design practice to add a safety factor (usually 1.7-2.5 [1]) to the result.

4. Particle Settling Velocity, Surface Loading Rate and Tank Retention Time

Q W Vh

Vp L Figure X.Ideal rectangular tank geometry with labelled dimensions, constant horizontal velocity, Vh, constant particle settling velocity, Vpand constant flow volumetric flow rate, Q If we consider the figure given above, then we calculate the time for the particle to settle as: ts = H/Vp For an ideal design, this particle settling time should then be equated to the time it takes for the flow to travel from the tank inlet to the tank outlet: t = H/Vp = L/Vh But since Q = VA, then Vh = Q/WH = Q/A And Vh = (Vp.L)/H Therefore: Vp=

Q/A = Surface Loading Rate

Surface Loading Rate is therefore a fundamental design parameter which is equal to the particle settling velocity as is shown in the derivation. Another point made apparent is that the depth of the tank is not a fundamental design parameter for sedimentation tank designtherefore this parameter is chosen for a balance between tank efficiency and cost of land but is usually set to at least 1.5m to avoid sludge scouring and to allow for sludge accumulation. The tank retention time is typically at least 2 hours by which most of the sedimentation required would have been achieved at peak flow conditions however this value may be increased to account for dry weather conditions or decreased to reduce the likelihood of anaerobic conditions occurring which would result in foul odours.

5. Tank Zones, Types, Sizing and Efficiency

5.1 Tank Function Zones Sedimentation tanks can be split into four fundamental operating zones which should be considered carefully during the sizing process as this will have an effect on the tank efficiency. a) Inlet Zone â&#x20AC;&#x201C; The zone in which the kinetic energy of the incoming flow is dissipated so that settling within the fluid can occur and flow is evenly distributed along the tank cross section. A well designed inlet would keep turbulent flow to a minimum yet allow enough mixing for a good average settling velocity to be achieved. This zone should be kept to a minimum in order to maximise the effective settling zone of the tank. b) Settlement Zone â&#x20AC;&#x201C; The zone where the flow conditions are relatively calm as to facilitate solid settlement. This zone specifies the effective tank capacity. It should be maximised in order to increase the overall efficiency of the unit. c) Outlet Collection Zone (Weir) â&#x20AC;&#x201C; The zone where the settled wastewater is collected by what is called an effluent launder and pumped onto upstream biological treatment units. Weirs are installed along the outlet, usually V-notch, which ensures an even and relatively calm run off, which will minimise this zone volume and increase the tank efficiency. An important sizing parameter of the tank is therefore the weir height. It is important to keep the weir plates level and clear of blockages to prevent short circuiting from occurring. 23

d) Sludge Collection Zone â&#x20AC;&#x201C; Where sludge is periodically collected and withdrawn at the bottom of the tank; usually by pumping. A designated percentage of up to 25% of the tank volume should be allocated for sludge accumulation. The Sludge blanket should be regularly measured and controlled (usually every 2-4 hours [3]) as to minimise anaerobic digestion occurring which would release gas bubbles to the surface and cause foul odours. Additionally, if the sludge zone is minimised, then the settling zone and hence tank efficiency is kept high

Settling Zone Inlet Zone

Outlet Zone

Sludge Zone

Figure 5.1.A horizontally flowing rectangular tank with four function zones.

5.2.Types of Sedimentation Tank Sedimentation tanks used in primary treatment differ in their shape and the nature of flow within the unit. There are basically three types of sedimentation tank which have respective shapes and flow characteristics. The two most common tanks are types 1 and 2. 1) Rectangular horizontal flow tanks 2) Circular radial flow tanks. 3) Rectangular or radial upward flow tanks

5.21 Rectangular Horizontal Flow Tank

Energy Dissipater

Effluent Outlet

Constant Forward Velocity

Effluent Inlet

Settling Velocity

Outlet Weir

Mechanical Scraper

Sludge

Figure 5.2aA labelled schematic of a rectangular tank design Features 

Uniform forward flow between the tank inlet and outlet zones.



Settled sludge is scraped along the sloped bottom into a collection hopper usually at the inlet zone end. Scum settled on the surface is separated by mechanical skimming into troughs and is usually treated along with the sludge



Rectangular horizontal flow tank types have an advantage of a more compact design and therefore less susceptible to flow disturbances.



A disadvantage is limited outlet weir size.



The maximum forward velocity is typically somewhere between 10-15mm/s for a length to width ratio of 3:1 resulting in a total retention time of around 2 hours. The surface loading is around 30m3/m2 [1]



The depth of the tank is calculated based on the tank retention time and surface loading rate but is usually no less than 1.5m to avoid sludge scouring

5.22 Circular Radial Flow Tank

Figure 5.2b.A Schematic of a circular tank design [Source: [1]] Features ď Ź

Waste water enters into the top of a central well within the tank designed to distribute flow evenly in all directions and minimise turbulent motions within the settlement zone of the tank.

ď Ź

Settled material at the sloped bottom of the tank is relatively quickly mechanically scraped into a central hopper where it is collected and removed.

ď Ź

The upper perimeter of the tank serves as the effluent outlet and is in the form a V notch weir to ensure even run, calm run off; ensuring a better tank efficiency. Scum separation in the form of scum trapsor scum scrappers usually occurs immediately after of while flowing over the height of the weir or from the tank surface. The scum is then usually sent to be treated along with the separated sludge 26



The tank diameter can be anywhere up to 50m. Larger tank diameters result in higher efficiencies. In terms of tank depth; it is usually no more than 1.5m at the wall with a slope to the central sump of gradient 7.5-10. [1]



The main disadvantage of this design is a higher susceptibility to flow disturbance resulting in a lower efficiency.



The main advantage of this design is the long outlet weir length which forms the upper perimeter of the tank.



Usually designed for a maximum surface loading of no more than 45 m3/m2/day and a tank retention time of 2 hours or more. [1]

6. Primary Sludge Treatment and Disposal/Reuse The discharge of settled solids separated from the primary sedimentation process forms a watery sludge containing 96-99% water by weight and is known as “Primary Sludge”. Sewage sludge contains nitrogen, phosphorus and potassium giving sludge good fertilizing benefits for growing plants with typical concentrations in the UK shown below [5]: Nitrogen (N) - 50 mg/L Phosphorus (P) - 10 mg/L Potassium (K) - 30 mg/L

However, sludge also tends to accumulate a number of other substances such as pathogens, heavy metals and PAH (Polycyclic aromatic hydrocarbons) which would be harmful if introduced into the food chain as well as have an effect on crop growth and yield. The European directive 86 / 278 / EEC and Sludge (Use in Agriculture) Regulations 1989 was introduced to protect humans and the soil from these substances by making it a requirement to pre-treat sewage sludge before it is used on farmland. Sufficient sludge treatment is therefore considered an integral part of wastewater treatment. The use of bio-solids for agricultural use is heavily environmentally favoured and around 80% is already being applied in the UK due to its sustainable nature however there are some technical and social barriers still to overcome such as storage and public perception.

The Sludge Treatment Process The treated sewage sludge is also known as ‘bio-solids’ and there are currently two different forms of treatment. 1) Conventional Treatment – removes up to 99% of pathogens present. 2) Enhanced/Advanced Treatment – removes virtually all the pathogens present. The later treatment is therefore suitable for application to food crops There are separate rules and regulations as to how bio-solids may be used depending on whether conventional or enhanced treatment has been used.

The process of a typical sludge treatment plant in Europe can be outlined as follows[5]: 1. Preliminary treatment (Initial screening, pulverisation) 2. Primary thickening (dewatering) 3. Liquid sludge stabilisation (Pasteurisation at 75 degrees C for at least 30 minutes to kill pathogens, Biological anaerobic digestion at 35 degrees C to remove foul odours, addition of lime to raise the pH level to greater than 12.0) 4. Secondary thickening 5. Conditioning 6. Drying 7. Final treatment 8. Transport of enhanced “bio-solids” to final destination.

7. References [1]WEDC Preliminary and Primary Treatment. Available: http://wedc.lboro.ac.uk/resources/units/WWT_Unit_3_Preliminary_and_Primary_Treatment.pdf. Last accessed 04/05/2013.

[2] J.P. Guyer. (2011). Introduction to Primary Wastewater Treatment. Available: http://www.cedengineering.com/upload/Primary%20Wastewater%20Treatment.pdf. Last accessed 6th May 2013. [3] Ragsdale and Associates Training Specialists, LLC. Primary Treatment. Available: http://ragsdaleassociates.com/WastewaterSystemOperatorsManual/Chapter%203%20%20Primary%20Treatment.pdf. Last accessed 6th May 2013.

[4] Foundation For Water Research. (2010). Sewage Treatment. Available: http://www.euwfd.com/html/sewage_treatment.html. Last accessed 6th May 2013.

[5] D. Fytili, A. Zabaniotou. (2008). Utilization of sewage sludge in EU application of old and new methods—A review. Renewable and Sustainable Energy Reviews.12 (1), p116–140.

3. SECONDARY WASTEWATER TREATMENT AEROBIC AND ANAEROBIC BIOLOGICAL TREATMENT Biological treatment is the degradation of the organic matter by biological processes. Both procedures require a source of energy but the electron receptor is different: In the aerobic process it is oxygen while in the anaerobic process, the receptors are sulphates and nitrogen (Autonoma University, 2012).The degradation of the organic matter results in the generation of biomass. Some differences can be mentioned between aerobic and anaerobic processes (ibid):  Aerobic processes are quicker than anaerobic ones which leads to the need for smaller reactor volumes.  The amount of biomass produced is slightly bigger in aerobic processes.  Aerobic processes are commonly used for the water treatment, while anaerobic processes are orientated to the sludge treatment.

AEROBIC BIOLOGICAL PROCESSES In aerobic processes, the substrate is decomposed into CO 2 & H2O, along with an increase in biomass as microorganisms grow exponentially. According to Degremont 1973, two important steps can be distinguished in this process:  

The development of the bacteria within the tanks which can be collected in films or by the use of flocculants and then they are fed to the waste. Separation of the sludge by biological filters and rotatory contactors

All the biological treatments are based on trickling filters, aerated lagoons, contact and stabilization and they differ in terms of the operation conditions. 

Biological filters Fluid goes through a porous material where the solids in suspension remain attached. The filter is aerated to allow microorganism growth. Sometimes additional ventilation is used to improve the separation. CO 2 and by-products are released from the biological film into the air and effluent. Sometimes an anaerobic layer is formed below the aerobic one (Ibid) Biological filters as well as the rotatory contactors are used with low flow rates although they work with high concentrations (higher than 2000 mg/l) Typical efficiencies are between 60 and 65 % when the depth is around 2 meters and over 66 % when the filters have up to 4 meters. Since the values are not too high, the 30

effluent is recirculated to pass through the filter again. The recirculation allows the autowashing of the filter and it permits the dilution of the BOD wastes. The typical way to carry out the recirculation is drawing a stream from the final tank which is placed after the biological filter towards the entrance of the primary tank placed before the biological filter. ď&#x201A;ˇ

Activated sludge This is a process in which the growth of sludge takes place when the tank is aerated. It is continuously stirred to maintain the sludge in suspension and therefore an oxidation reaction can occur. The population of the sludge is moribund since it does not go through a reproduction process, they consume nutrients to provide energy. Such nutrients are basically degradable carbonaceous material (BOD), with a proper relation of nitrogen (0.03-0.06 kg N/kg BOD) and phosphorous (0.007-0.001 kg P/kg BOD) to ensure a good operation. (Winkler, 1981) After the biodegradable process, the outlet stream is led to a secondary settling tank with the purpose of separating out a clarified effluent from the sludge. The sludge, containing activated solids, is drawn from the bottom of the settler and most of it is recirculated back to the aeration tank to maintain its concentration in the reactor, however a small part is withdrawn and disposed of. Winkler (1981) highlights the importance of the activated sludge flocculation for a successful agglomeration of the particles in the waste and for an economical separation from the liquid phase. There are different possibilities in terms of the collocation of the inlet streams in the aeration tank, but the following scheme (Figure 1) shows the most typical process according to Degremon,1973 which is the design that ensures a better mixing among the effluent to be treated, the oxygen supply and the activated sludge that is continuously injected back for the whole tank. The continuous operation is usually more used than semi-batch (Winkler,1981)

From primary treatment

Reactor

Secondary settler

Recirculation Figure 1, Schematic of a typical Secondary Biological Treatment process

According to Autonoma University 2012 ; X u = Xw = XR where, Q0 is the inlet effluent from the primary treatment [volume/time], Q R is the recirculated sludge [volume/time] Q e is the clarified effluent, V is the reactor volume [volume], X, Xe, Xu, Xw and Xr are referred to the suspended solids concentration [mass/volume], S0 and Se are the BOD5 concentrations before and after the reactor respectively [mass/volume]. This process, however requires great amounts of oxygen in the biodegradation of the effluent, so a suggestive improvement, in terms of the consumption of oxygen, could be the injection of some air upstream instead of downstream (Degremont 1973).

ď&#x201A;ˇ

When plants are larger, the difficulty of achieving good mixing, along with the increase in economic cost, leads to both the aeration process and the settling process within the same unit, which ultimately achieves higher purification efficiencies. These combined tanks usually have channel shape with clarification areas containing settling solids, surrounding the central aeration zone. The most typical kind of combined tanks arrangement according to Degremont (1973) are: Oxycontact tanks: Where there is a direct passage between the settling and the aeration area and the settled sludge in the sedimentation area returns to the aeration area by gravity since it is at a lower level. The slopes of the settlers areas are typically between



45 to 50 ° and unit dimensions are usually between 15 and 70 m in length and 4 m in depth. Oxyrapid tanks: The settled sludge is directed back to the aeration area by rising ducts. Within oxyrapid tanks, inclination slopes are within the range of 50 to 55 °C, but a slightly deeper depth is present than in comparison to oxycontact tanks. The secondary settling tanks carry out the separation between the water and the sludge by the precipitation of the solids. It is important to ensure the complete precipitation of the solids, but meanwhile avoid the anaerobiosis of the sludge. The BOD5 purification efficiencies achieved in the clarified effluent leaving the secondary settler depends not only on the sludge loading in the aeration reactor, but also in the secondary settler performing. Degremont,1973 fixes the efficiencies ranges:

  

95 %, when there is more than 0.4 kg of BOD per kilo of sludge. 93-90%, when there is less than 0.4 kg of BOD per kilo of sludge. 90%, when there is less than 0.1 kg of BOD per kilo of sludge. An important design parameter within the design of the secondary settling tank is the upward flow rate which in any case should be above 2.5m/hr. For the tank inclination 45° is suitable (ibid). During operation of the settler, to mechanically enhance separation of the active solids from treated effluent, a rotational scraper with several blades is used to collect the settled sludge. Activated sludge design  Sludge concentration, is usually referred to suspended solids concentration. In the reactor it can be expressed as MLSS (mixed liquor suspended solids). If inorganic mater is not taken into account, it would be expressed then as MLVSS (mixed-liquor volatile suspended solids) although it does not distinguish how much is biologically degradable. According to Winkley (1981) the values of MLSS are unlikely to exceed 2 to 3 kg (dry weight) per m3.  Reactor volume: Establishing a mass balance across the reactor:

Inlet Q0 S 0 + QR S e

Outlet -

(Q0 + QR) Se

Reaction =

-rS V

Accumulation +

(Equation 1) The reaction rate can be compared with the substrate elimination rate according to the Michaelis - Menten equation:

(Equation 2) where K and Ks are kinetic parameters. According to Autonoma University (2012), K is falls within the range of 2 to 3 days-1 while Ks is between 100 and 120 mg/l. Both equations can be related and expressed in terms of the specific velocity for the substrate elimination (U) which is equivalent to, -rs / X

(Equation 3) ď&#x201A;§

Biomass production According to Henze et al (2008) and Autonoma University (2012), the biomass production [mass / time] can be expressed by the mass of the system and the sludge age:

(Equation 4) Another method of calculating biomass production is to undertake a mass balance across the whole active sludge system, the biomass generation would be obtained as follows (ibid):

(Equation 5) Autonoma University,2012 establishes another expression in relation to the biomass production:

(Equation 6) Where Y is the cellular growth coefficient [kg of biomass/kg of substrate] which and kd is the biomass decline coefficient [kg of biomass/kg of biomass. t] ď&#x201A;§

Sludge retention time (SRT) [days] (also known as sludge age if the microbial cells in the volatile suspended solids are considered as constant). An accurate way to calculate it takes into account the feed, settlers, pipes and the loss rate both in the clarified effluent and the waste sludge, however this is difficult to accurately quantify. Alternatively according to Henze et al (2008) and Winkler (1981) a simplified version would be: 34

SRT =

= (Equation 7)

Where V is the volume of the liquid in the reactor (m3), X and Xw are the mixedliquor volatile solids in the reactor and volatile suspended solids in the waste stream respectively (mg/m3) and Qw is the flow rate of the waste stream (m3/day). According to Autonoma University (2012), the sludge age (SRT) can be related to with the specific velocity for the substrate elimination:

(Equation 8) The sludge age is one of the most important parameters to bear in mind when an activated sludge system is being designed. It largely depends on the degree of purification of the effluent required at the outlet stream. According to Henze et al (2008), the main point of the process is the COD or BOD5 removal, the sludge ages normally fall within the range of 1 to 3 days with reduction efficiencies between 75 and 95 %. Such efficiencies are mainly based on the sludge transfer from the reactor to the secondary settler and the settler efficiency. If phosphorous has to be taken away, a sludge age of 3 to 5 days should be enough at temperatures between 14 and 20 °C. However lower operational temperatures would allow for a higher sludge age (ibid). If nitrification is required, its reactions will lead to an increase of the sludge age to a period of 10 - 15 days (ibid) 

Hydraulic retention time (HRT) [hours], Is the average time that the liquid stays in the aerated reactor. It can be defined as the relation between the volume of the reactor and the inlet flow rate in the reactor which is the combination of influent flow rate in the plant and the recycled stream from the secondary tank (ibid)

(Equation 9) According to Autonoma University (2012), the value is in the range of 4-5 hours. 

Oxygen demand

Oxygen is needed to be injected in the reactor in order to oxidize the biodegradable organic matter into water, CO2 and some quantities of ammonia. It is important to ensure stirring throughout the tank to avoid "dead zones" which would not undergo oxidation and also to provide homogeneity in the mixed effluent. According to Winkley (1981), there are two important mixing regimes when activated sludge is going to be treated: 



Complete mixing: The main points are, the reactor incorporates buffer to face up to increases in loadings (especially useful if the waste is toxic) and it is easier to maintain constant the operational conditions. Plug flow: It consists of an aeration device within a long (between 30 to 300m) and narrow (6 to 10 m) vessel. The most noticeable difference is the decrease of oxygen demand along the reactor since the nutrients are depleted so a more complicated design is required.

Sometimes an intermediate option between the two just mentioned is the introduction of the feed at different points along the plug flow tank, this design allows for a well-mixed system. Thus you can take advantage of the whole volume of the vessel and even the oxygen demand throughout the tank. There are different types of aerators systems which can be included into two different groups: -

Bubble Aeration Systems: The oxygen is present in bubbles formed in the liquid by three different ways: the air is made to pass through nozzles, or through media or it is dissolved in the liquid under pressure. The sizes of the bubbles formed are between 1 2 mm in diameter if dissolution in the liquid is carried out up to 5 mm in the diameter if the air is forced through nozzles. Mechanical Aeration systems: There are 3 main possibilities: surface aerators, liquid jets or combined systems. Winkley (1981) states that surface aerators are the option usually preferred for sewage which rotation system can have a vertical-axis in a horizontal plane (turbine aerators) or an horizontal axis in a vertical plane (paddle)

The most common one according to Degremémont (1973) is a slow, vertical-axis which rotates at around 5m/s. The water can pass through the aerator and it is forced to the sides. The power required for the agitation depends on the immersion depth, the rotational speed and the tank shape in which circular tanks are the suitable in terms of power requirements while the oxygen input is determined by the injection power, tank dimension and the type of reactor used. Autonoma University (2012) stated that the oxygen demand can be expressed as follows: O2 Requirement = Q0 (S0 - Se) + 1.42 KD X V (Equation 10) The first part of the equation is referred to as the substrate elimination and the second, as the biomass decline because of microorganism respiration. 36

The value 1.42 comes from the mole relation in the reaction of the microorganism with the oxygen: C5H7NO2 + 5 O2 â&#x2020;&#x2019; 5 CO2 + H2O + NH3

Molecular weights: C5H7NO2 =113 g/mol ; O2 = 160 (g/mol) Which means:

ď&#x201A;§

= 1.42 kg O2 per kg of microorganism

Processes at different loading rates: The loading rates are significantly different from some activated sludge plants to others. The following Table 1, is a summary of the approximate values of the characteristic parameters mentioned before depending on the loading (Winkler, 1981).

kg-BOD/m3/d

0.5-1,5

1.5-3.5

DispersedExtended growth Aeration Aeration (low rate) (ultra-high rate) Very high 0.25

SRT(days)

4-10

0.5

HRT (hours)

~10

Conventional Treatment

High-rate Treatment

Up to 24

~2 ~0.25 Up to 48 (much higher with industrial wastes) Table 1, Conventional values of the aforementioned Biological treatment factors, BOD concentration, Sludge Retention Time (SRT), Hydraulic Retention Time (HRT)

The control of the biological process is a tricky issue so it has to be carefully designed in order to save money and maintain the clarifying efficiencies even though the pollution concentrations may increase. If the contamination decreases, the energy demand should be reduced to make the process more profitable. It is essential to ensure a constant growth rate of the microorganism. The oxygen injection and the sludge loading by spreading have to be jointly analyzed (Degremont ,1973).

Example of a problem posed in an activated sludge plant . Autónoma University (2012) A treatment plant processes 20,000 m3/day of wastewater. The chemical oxygen demand (COD) of the wastewater is 510 (mg/l), while the biological oxygen demand in five days (BOD5) is 300 mg/l. The biomass concentration in the reactor (X) is 3500 mg/l. The efficiency of the settler is considered as 100%. The suspended sludge consistence(X u = Xw = XR) is 2% (0.02 kg/l = 20,000 mg/l). The air flow rate is 200,000 m3/day at 25°C and 1atm. The efficacy in terms of the air utilization is 13%. Calculate: 1. 2. 3. 4.

COD at the outlet stream of the treatment plant Volume of the reactor Purge and recirculation flow rates. Hydraulic retention time (HRT)

Given: Kinetic parameters: Y = 0.5 ; Kd = 0.05 days-1; K = 1.8 days -1 ; Ks = 120 mg/l

1 &2) Firstly it is needed to be calculated the real biological oxygen demand. According to Autonoma University (2012), BOD = S 0 = 3/2 ) . Hence: So = BOD = 450 mg/l ; → Matter not biodegradable is 510 - 450 = 60 mg/l The COD at the outlet stream would be the sum of the organic matter not biodegraded (S e) and the matter not biodegradable: CODe = Se + 60 To calculate Se two equations are needed to be established: 



O2 Requirement = Q0 (S0 - Se) + 1.42 KD X V

→

(1)

Air flow rate is known, but O2 flow rate is required itself in (kg/day) 21 % of O 2 in air and at atmospheric conditions: 25°C ad 1 atm; 1 kmol = 22.4 m3 . Efficiency = 13 % Hence; Q0 =200,000 m3/day 1kmol/22.4m3 32 kg/kmol 0.21 7800 (kg/day) and; O2 = 7.8 109 (mg/day) 3500 mg/l

+ 1.42 (2)

Thus, solving the equations 1 and 2: S e = 92.5 mg/l , V = 2616 m3 and CODe = Se + 60 = 92.5 + 60 = 152.5 mg/l 3) In the same way, it is necessary to solve several equations at the same time to calculate the purge and recirculation flow rates: 

Establishing a mass balance across the settler: (Q o + QR) X = Qe Xe + (QR + QW ) XR Since the efficiency in the settler is 100%, X e = 0 mg/l. →(Qo + QR) X = (QR + QW) XR → (20,000 m3/day) + QR )



=(QR + QW)

20 kg/m3 (3)

The biomass production can be expressed as : ∆B = Q eXe + Qw Xw

Since the efficiency in the settler is 100 %, X e = 0 mg/l →∆B = + Qw Xw (4) The biomass production can also be related as follows: ∆B (kg/day) = Y Q 0 (S0 - Se ) - Kd X V = = 0.5 20,000 (m3/day) (0.45 -0.0925 kg/m3) - 0.05 day-1

3.5 kg/m3

→ Qw = ∆B/ Xw = 3574.5 (kg/day)/ 20kg/m3 = 155.9 m3/day. Hence, QR can be obtained from equation 3: QR = 4053.5 m3/day 4) HRT =

= 0.109 day = 2.6 hours

2616 m3 = 3117.2

ANAEROBIC DIGESTION In this process digestion takes place in absence of oxygen, the biodegradable matter is transformed leaving products such as NH4 +, PO43- and S2-. Its performance is plain and it can be designed for big scales. The excess sludge is very small (excess sludge reduced in 90%) and there is a production of biogas by the microorganisms (Henze et al, 2008). This kind of digestion makes use of little quantities of chemicals and the clarifying efficiencies are higher. Anaerobic digestion is developed in four important steps (Henze et al, 2008) -

Hydrolysis of biopolymers (proteins and polysaccharides). Which is carried out by hydrolytic bacteria. Acidogenesis, oxidation of amino acids, sugars and alcohol. It is led by fermentative bacteria. Liquefaction. In other words, Acetogenesis. Great amounts of volatile acids are produced. Alcohols, CO2 and H2 are obtained. Gasification, also known as methanogenesis. The volatile acids and alcohols are converted into methane gas by anaerobic methanogenic bacteria. For an optimums performance, pH must be between 6.8 and 7.2.

Two kinds of micro-organism can be distinguished in the anaerobic process according to Edyvean (2013): - Mesophilic: Their performance activity is between 20 and 45 °C. Successfully effective for the methanogenic bacteria activity. - Thermophilic: Temperatures from 50 to 60 °C. Truly optimal for hydrolysis and acidogenesis although the overall process is also enhanced. - The loading rate for both processes is in the range of 480 to 640 kg/m 3.

There are many reactions taking place in the acetogenesis and methanogenesis process, but the following ones are the most representative (Henze et al, 2013): - Acetogenesis: CH3CH2CH2 COO - + 2H2O → 2CH3COO- + H+ + 2H2 CH3CH2COO- + 3H2O → CH3COO- +HCO3- + H+ + 3H2 -

Methanogenic:

CH3- COO- + H2O → CH4 + HCO3CO2 + 4 H2 → CH4 + 2H2O

The gas produced in the gasification is mainly CH 4 (65-70 %) and CO2 (25-30%) with small quantities of CO, N2 , hydrocarbons (Degremont,1973). The heat content of the gas after the digestion process has between 18000 and 26000 KJ/ m 3 (EPA)

Parameters which affect the quantity of gas produced and therefore the efficiencies of the anaerobic process are: - Temperature: High and constant temperature is requried. - Capacity and retention time in the reactor: It should be large enough to ensure the degradation of the matter. - Sludge consistency: The thicker the sludge is, the most profitable and the faster the digestion becomes. - Stirring: Steady agitation throughout the tank eases the contact between the microorganism and the waste improving the degradation, also maintaining the suspension of the active solids. - Inlet feed and removal of the sludge: Variations in terms of the substance nature (heavy cations, NH4 excess, sulphides, organic compounds) lead to interferences between the microorganism and the matter. According to Degremont (1973), there are different reactor systems which can be classified in terms of the rate of the digestion. ď&#x201A;ˇ

ď&#x201A;ˇ

Low-rate tanks: One of the oldest digesters developed was Imhoff tank in which the digestion and the settling occur in the same tank. The drawbacks are that it needs big treatment zones and the operational temperature is low since convection currents occur. The sludge is poorly settled since the gases try to go upwards, giving solids buoyancy. Scum layer is sometimes deposited at the top being difficult to be removed. Anaerobic ponds are included in this group with loads ranging from 0.025 to 0.5 kg COD per m3 (ibid). These digesters, however find problems related to the odour and there is also energy losses due to the CH 4 release to the atmosphere, which is also a severe greenhouse gas. Medium - rate digestion: The digestion and the settling stages usually occur in different tanks, the digester needs to be heated up. In the digester it can be distinguished by different layers: a superficial scum, an intermediate layer with less quantity of solids in suspension and a bottom one where the sludge is becoming thicker. The design of the digester with a slope prevents from the accumulation of solids. The gas produced is usually used in the heating boiler to provide energy. High - rate digestion: It is commonly carried out in different stages. The second stage digester does not have to be closed. If it is, the gas released is led to the boilers to provide the energy. In order to ensure a good performance with high organic loadings, Henze et al (2008) established some important points which should be achieved: Ensure a high content of viable sludge at the operational conditions Good contact between the bacteria and the biomass (i.e. stirring throughout the tank) The gases produced should be easily released and thick biofilms have to be avoided. The operational conditions (temperature and pH) for optimum performance of the microorganism is essential. The temperature maintenance is essential. 41

There is a great range of microorganisms taking part, so the conditions to be reached in the different reactor parts, has to be carefully considered and designed.

A single stage performing system has traditionally been utilised, but multi-stage anaerobic digestion has emerged and nowadays it is the most common choice when anaerobic plants are constructed. In the first stage, the hydrolysis of the organic matter and the formation of the volatile fatty acid take place in acidic conditions and at the highest temperature conditions while in the second stage acetic acid is formed and later methane is produced at a lower temperature (EPA). The selection of a multi-stage digestion system leads to some advantages with respect to single-stage digesters. According to Edyvean (2013) the methane yield achieved is higher since the breakdown of the feedstock is better, the retention time is lower (from 2 to 5 days according to EPA) and there is a higher flexibility in terms of the feedstock nature. In these digesters, higher loading rates are tolerated, and the volatile solid content is reduced which means better odour (EPA). However, more complex equipment for the piping, control and operation is needed to be designed (ibid). The operation temperature is an important issue as it has been mentioned before, so heating procedures play an important role. Degremont (1973) states that nowadays hot-water sludge heaters are mostly used. Coil exchangers with a slow recirculation (1 to 2 m/s) or fast circulation tubular exchangers (2 to 4 m/s) are heaters typically employed. External heat loses have to be taken into account; an additional hollow- brick wall outside the concrete wall can be utilised to minimize the heat loses (ibid). Part of the gas produced in the digestion is employed for the own heating requirements of the plant and also to provide energy in drying units. If the gas furthermore passes through a scrubber it could be labelled as natural gas and added to the gas grid (EPA). It is important to provide good mixing in order to ensure an easy stabilization after the treatment, which is essential to be then used in other applications. Mixing can be applied either by the sludge recirculation or by gas recirculation (Degremont,1973) -

By sludge recirculation: The sludge is drawn from the bottom of the tank and it is reinjected in the top part preventing from the formation of an undesirable scum layer (EPA). Besides, in the recirculation, the sludge passes through a heat exchanger to maintain the operational temperature (Degremont 1973) By recirculating the gas: The own gas produced in the digestion is compressed and inserted back by diffusers or draft tubes inside of the tank to provide mixing (EPA)

REFERENCES OF AEROBIC AND ANAEROBIC BIOLOGICAL TREATMENT SLUDGE TREATMENT

Aut贸noma university, Madrid engineering. Unpublished.

(Spain),2012

Waste

effluents

treatment.

AND

Environmental

Degr茅mont, Gilbert, 1973. Water treatment handbook. Fourth edition. Published Britain by Austin, Stephan.

in Great

Edyvean,2013. University of Sheffield. EPA. United States Environment Protection Agency. Bio solids Technology Fact Sheet. Multistage Anaerobic Digestion. [Online]. Available at: http://water.epa.gov/scitech/wastetech/upload/2006_10_16_mtb_multi-stage.pdf Accessed: 10/03/2013 Henze et al, 2008. Biological wastewater treatment: principles, modelling and design. Published in London

Mountain Empire, community college, 2000. Water/Wastewater distance learning website. Lesson 13. Advanced Wastewater treatment. Stabilization. Acceded the 30 th March. [Online]. Available at: http://water.me.vccs.edu/courses/ENV149/stabilization2.htm Valladolid university (Spain), 2010. Chemical engineering and environmental technology department. Chapter 8 - Sludge stabilization. Torre, R.M and Encina, P.G. Acceded the 30 th March. [Online]. Available at: http://iqtma.uva.es/instrat/Presentaciones_pdf/Chapter%208%20Sludge.pdf Van Haandel, Adrianus and Van der Lubbe , Jeroen 2007. Handbook Biological Wastewater treatment. Design and Optimization of activated sludge systems. Published in Netherlands. Winkler, M. Biological treatment of Waste-Water,1981. Department of Chemical Engineering. University of Surrey. Published by Horwood,E in England.

4. TERTIARY TREATMENT BIOLOGICAL NUTRIENT REMOVAL Introduction The presence of nitrogen based compounds can pose many problems with the discharge and reuse of wastewater as different kinds of nitrogenous compounds can be both beneficial to plant growth but also toxic, although the addition of all nitrogenous compounds to a body of surface water will almost certainly always have negative implications on aquatic life. Specifically if wastewater containing traces of ammonia and nitrate is discharged to a river or surface body of water, there are several possible negative impacts that can occur: 1. The ammonia (NH3) will be immediately oxidised by bacteria already present within the surface water, to produce both nitrate and nitrite compounds (NO 3-and NO2- ). Which in turn will lead to a notable reduction in dissolved oxygen within the body of water, therefore conditions are no longer sustainable for aquatic life so fish will eventually die out via asphyxiation. 2. Ammonia and ammonium ions will be in continuous equilibrium (NH3 + H+ â&#x2020;&#x201D; NH4+ ), which in turn will increase the pH and temperature of the water, creating toxic condition for fish. 3. The presence of nitrate simultaneously stimulates the synthesis and growth of algae, which thrive wherever there is a surplus concentration of nitrate, the continuous exponential growth results in eutrophication, induction of severe hypoxia which prevents sufficient oxygen diffusing into cell tissues, resulting again in asphyxiation. 4. If high concentrations of nitrate and nitrite are present within drinking water this can cause methemoglobinemia, which is elevated numbers of methemoglobin a reduced form of haemoglobin, which in turn has a greater affinity for oxygen being in a reduced form, thus it prevents the release of oxygen to the tissues as the red blood cell is in an over-reduced form, again causing tissue hypoxia and ultimately asphyxiation. Babies and younger children are highly susceptible to this disorder. However if nitrate and nitrite compounds are ingested by human via drinking water, it can result in the formation of carcinogenic nitrosamines, resulting in gastric cancer. Therefore it is of the utmost importance for the environment, ecosystems and mankind to properly treat waste effluent properly, removing sufficient nitrogenous compounds, before discharging treated effluents further to rivers, lakes or the sea. Metcalf & Eddy(2002) states that the maximum contaminant level (MCL) within drinking water is currently set at 45 mg/L for nitrate, 10 mg/L for nitrogen and 25-45 mg/L for ammonia (Wiesmann, 1994).

Nitrogen Removal: Overall Process Wastewater requiring treatment will (almost) always come from either domestic or industrial sources, nitrogenous compounds will be ever present within human urine and stool, commonly as urea ((NH2)2CO), as humans utilize the ornithine cycle to eject waste ammonia from their bodies in the form of urea. However in industry urea is an important compound used in agricultural fertiliser to optimise plant growth and ultimately crop yield, therefore wastewater coming from the manufacture of agricultural fertilisers will obviously contain urea. Urea can be hydrolysed to ammonia and carbon dioxide, thus ammonia can enter a nitrification process converting it to a nitrate form followed by a denitrification process, thus removing it in the form of nitrogen gas. The overall process of removing nitrogenous compounds from wastewater, involves two separate stages first nitrification, which ensures that compoundsare in nitrate form, i.e. oxidises ammonia to nitrate and nitrites, before denitrification. This is commonly carried out on an industrial scale through the use of nitrifying bacteria in aerobic conditions. Directly following nitrification, denitrification then takes the nitrates and using denitrifying microorganisms converts it to nitrogen gas, via a series of reduction reactions and intermediate compounds. This is achieved through the use of denitrifying bacteria in anaerobic conditions (Sedlak, 1991). The overall process of nitrogen removal can be summarised via Figure 1.

Figure 1, Overall schematic for the Biological Removal of Nitrogenous Compounds (Sedlak, 1991) 45

NITRIFICATION Nitrification is a biological process that involves the simultaneous oxidation of ammonium ions to nitrite and nitrate compounds. The process commonly occurs within aerobic conditions as the microorganisms carrying out the chemical reaction are obligate aerobic lithotrophic, which essentially means they require oxygen and utilise inorganic compounds such as ammonium ions (NH4+ ) for their nitrogen requirements, to grow and synthesise. However the fraction of ammonium ions utilised by the nitrifying bacteria for biomass incorporation can be considered as negligible, in fact it only accounts for about 2% maximum, therefore the microorganisms should only be considered as biological catalysts that catalyse the nitrifying reactions, oxidising ammonium ions to nitrates. The primary microorganism responsible for this transition are Nitrosomonas and thrive in a slightly alkaline environment (pH 6 - 9), therefore are ideally suited as ammonium ions exhibit basic properties. Nitrosomonas utilise oxygen molecules as the final electron acceptor from the final complex (IV) in electron transport chain, the final process within cellular respiration, hence are able to readily synthesis energy in the form of ATP for growth (Sedlak, 1991). The common reaction for converting ammonium to nitrate is: 2NH4+ + 3O2 →

2NO2-+ 2H2O + 4H+

Followed by 2NO2- + O2

→

NO3-

Therefore the total oxidation reaction scheme is NH4+ + 2O2

→

NO3-+ H2O + 2H+

However although this is an overall reaction scheme, bacteria are continuously respiring and CO2 is being produced, thus further modifications to this reaction scheme can be made assuming that incorporate synthesis of microbial mass or biomass: CO2 + H2O

↔

H2CO3

↔

HCO3- + H+

↔

CO32- + 2H+

Along with carbonate molecules, a fraction of the ammonium ions is also incorporated into the cellular tissue of the microorganism in the form of biomass (C 5H7O2N), which can be summarised by the following reaction scheme: 4CO2 + HCO3- + NH4+ + H2O

→

C5H7O2N + 5O2

The biomass can be assumed to be the building blocks for the synthesis of new cells.

Nitrification Process Description To treat nutrient-containing waste effluents, the nitrification process can be split into two processes, first the waste effluent will enter an aeration tank (aerobic zone) which will contain the microorganisms capable of oxidizing ammonia/ammonium ions into nitrate or nitrite compounds, Figure 2. Once a sufficient retention time of effluent has been achieved to ensure a high conversion efficiency of ammonium compounds, the sludge containing the treated waste effluent then passes into a clarifier where the suspended solids and microorganisms can be separated out via gravity. Once the activated sludge has settled at the bottom of the clarifier, it can then be returned to the initial aeration tank, although a portion of the sludge can be wasted to mitigate the rate of cell synthesis and prevent excessive bacteria growth. The aeration tank is configured as a plug flow to prevent short circuiting and ensure that a consistent tank retention time is achieved (Hammer, 2003).

Figure2, Schematic diagram for the Nitrification process

Environmental Factors in Nitrification Because microorganisms are being used in the nitrification, all cells are sensitive to variable range of conditions, such as temperature, pH and toxic metals, the microorganisms responsible for nitrification are Nitrosomonas, meaning that during the nitrification process conditions should be manipulated to ensure an optimum environment for the conversion of ammonium ions and a highly efficient nitrification process overall. When considering the optimum process pH the aforementioned pH range between 6-9 was previously stated, however to truly ensure optimal nitrification rates a pH between the range of 7.5-8, would suffice, although fairly reasonable nitrification rates are attained between the pH range of 7-7.2. So slightly alkaline conditions should be maintained for the best nitrification rates, however if the untreated effluent is of low alkalinity, alkaline solutions can be added on site at the wastewater treatment plant. Examples of alkaline solutions that can be added to improve the alkalinity of the 47

waste effluent are Lime, Sodium Bicarbonate or Magnesium Hydroxide. The direct result of reduced alkalinity is the wastewater to â&#x20AC;&#x2DC;attackâ&#x20AC;&#x2122; the concrete surface of the tanks, as well as corrode ferrous metals used in the final clarification stage of nitrification (Hammer, 2003). The nitrification process is highly sensitive to a vast range of both organic and inorganic compounds, however it is not possible to identify the extent to which nitrification is affected by the presence of these compounds. However the inhibiting effects of these compounds is certainly evident during the process, some compounds which are toxic to the microorganisms involved in nitrification include, proteins, amines, alcohols, ether and phenolic or aromatic compounds. Metallic compounds which have a detrimental effect on the nitrification process, enduring total inhibition of the process are chromium, nickel and copper at concentrations of 25mg/L, 25mg/L and 10mg/L, respectively (Sedlak, 1991). As the nitrification is an aerobic stage the dissolved oxygen concentration should always be of greater magnitude than 1mg/L, however a minimum oxygen concentration of 2mg/L should be maintained to reduce the occurrence of sustained denitrification in the settling tank, which ultimately results in the maintained flotation of settling solids from the release of nitrogen gas, ultimately reducing the overall performance of the final clarification stage. Common loadings of ammonia into the aeration tank are between 160-320 g/m3.day, with exhibited effluent temperatures of between 10-20oC, however this is also dependant on the location and climate of the treatment plant. Within the aeration tank, typical effluent aeration time can vary between 4-6 hours (Sedlak, 1991)(Hammer, 2003)(Wiesmann, 1994).

Options for Nitrification Before commencing nitrification, first the carbonaceous biological oxygen demand must be removed, this can be done either prior to the nitrification stage or simultaneously, by using a single sludge nitrogen removal system, which is able to incorporate both the removal of BOD and the subsequent nitrification stage, it can also incorporate a further denitrification stage. With this type of sludge system these stages of carbon oxidation and nitrification are completed within aerobic zones, as nitrifying bacteria are obligate aerobes, before the sludge can then move onto denitrification processing. The design of the facility to accomplish the initial carbon oxidation and nitrification stages can be as simple as an aeration basin, which allows for air to be bubbled into the solution. The system could be designed and configured as a plug flow or a homogeneously mixed basin. When considering the oxygen loading within the aerobic zone, there are different classes of aeration equipment that are suitable for use within an activated sludge aeration system.

The three most prominent technologies are: â&#x2014;? â&#x2014;? â&#x2014;?

Mechanical Surface Aerators Fine or Bubble Coarse Diffused Air Systems Submerged Turbine Aerators

Each technology has various associated pro and cons, which include variable oxygen transfer efficiencies, however depending on the basin capacity these transfer efficiencies are often overlooked as priority is given to operational and maintenance characteristics. Mechanical Surface Aerators, require little maintenance but complete aeration of the basin is often unachievable, therefore undesirable oxygen transfer efficiencies are achieved. The setup results in great losses in heat during cold weather and limited turndown capability, therefore seasonal variations in dissolved oxygen will arise. Fine or Coarse Bubble Diffused Air Systems, are highly suitable to nitrification systems as the technology offers great turndown capability, in turn tapered aeration is possible for plug flow configurations. However fine bubbles are preferred to coarse bubbles due to the high aeration requirement during nitrification, as fine bubbles allow for more efficient oxygen transfer. Although due to the finesse of the bubble produced from the equipment, surface fouling issues can arise which in turn can lower the overall oxygen transfer efficiency, therefore greater maintenance costs are associated with this oxygen delivery technology. Submerged Turbine Aerators, allows for efficient oxygen diffusion throughout the entire basin, due to good mixer/agitation characteristics, thus with this technology, aeration and anoxic zones are easily inter-convertible by simply turning off the oxygen supply. When selecting the appropriate aeration system, energy savings from aeration turndown during a period of lower demand, can easily offset the higher costs associated with high quality aeration technologies, thus the degree of control over the oxygen output is important both from an operational and economic standpoint (Sedlak, 1991).

DENITRIFICATION The denitrification process, further converts or reduces the nitrates produced from the nitrification stage to nitrogen gas. Denitrification falls directly, after a nitrification stage, and in combination with a nitrification stage is commonly the most cost effective method for removal of ammonia and nitrates from wastewater, in comparison to other methods such as ammonia stripping. Before denitrifying a waste effluent two methods of nitrate removal are highly prevalent depending on the characteristics and conditions of the wastewater. Firstly nitrates could be assimilated to produce ammonia, via assimilation, effectively reversing the nitrification process, 49

which the ammonia nitrogen (NH4-N) can then be utilised by autotrophic cells for growth and synthesis. However in a solution containing a high concentration of ammonium ions, cells can only uptake a small fraction (≤ 2%) of ammonium ions to synthesise microbial mass, therefore if this occurs in an overwhelming manner, the effluent will return to an essentially untreated state. Alternatively the second, but most common, denitrification method is via biological dissimilation, which involves utilising the nitrite and nitrate compounds, NO 3-and NO2- , produced via nitrification as oxidising agents within respiration, specifically as the final electron acceptor in electron transport chain. Termed ‘Anoxic Denitrification’, meaning that denitrification occurs in an anoxic environment (oxygen deficient), this encourages heterotrophic Nitrobacter cells to utilise nitrate compounds for their oxygen content, however for the cells to respire they also require a carbonaceous source, such as methanol, which is added to the process. This organic feedstock added provides sufficient biological oxygen demand (BOD), for the nitrate reduction, thus increasing the overall rate of denitrification. The addition of organic compound is done so in a proportionate manner with the nitrate concentration, in a one step process although many different types of reactor can be utilised. The most common reactor for this process is an activated sludge reactor, with two stages, the first being an anoxic stage and the second an aerobic stage. Thus ultimately nitrate is reduced to nitrogen gas: 5CH3OH + 6NO3-

→

3N2 + 5CO2 + 7H2O + 6OH-

However depending on the capacity of the treatment plant, the cost of methanol can eventually prove uneconomical in large quantities, thus organic alternatives may have to be used Hammer, 2003)(Sedlak, 1991). Conventional denitrification systems consist of a plug flow tank, with embedded agitation tools, that ensure a homogeneously mixed solution, whilst maintaining the microbial floc in continuous suspension. Nitrogen gas produced from the cells, is buoyant thus will float to the surface, however when produced in large quantities it can carry matter with it, thus the nitrogen must be removed before the final clarification stage. The stripping of nitrogen from the activated sludge system can be carried out using an aeration chamber or a degasifier, Figure 3. The usual detention time for the denitrification process usually falls between 2 - 4 hours, however various external factors such as temperature and effluent nitrate concentration can cause variations.

Figure3, Schematic diagram for the Denitrification process

COMBINED NITRIFICATION-DENITRIFICATION Process Options As previously defined the removal of nitrogenous compounds involves three processes: ● ● ●

Synthesis - Where nitrogen is incorporated into a cells biomass as a result of cell growth Nitrification - Oxidation of ammonia to nitrate by nitrifying microorganisms (nitrosomonas) Denitrification - Reduction of nitrate to nitrogen gas by denitrifying microorganisms

When considering biological nitrogen removal there are commonly two major approaches: ●

Separate Stage →

●

Nitrification

→

Denitrification →

Combined Nitrification & Denitrification →

Nitrification & Denitrification →

Combined Nitrification-Denitrification consists of two stages, an anoxic and aerobic stage, an exact combination of the aforementioned nitrification and denitrification processes, the configuration of the stages can be either way, however commonly the anoxic zone is placed first in the process followed by the aerobic zone, i.e. denitrification before nitrification. With this configuration the biological floc returned via the recycle stream from the clarifier, is mixed with untreated effluent so the incoming effluent will contain a moderate concentration of nitrates for denitrification. 51

Considering the configuration of denitrification first, then nitrification, the nitrates within the returned activated sludge and untreated effluent will then enter the anoxic zone, made up of a series of anoxic chambers, which provide agitation to the effluent to encourage suspended solid growth and homogenous mixing. In the anoxic zone the nitrates will be utilised as the primary oxygen source due to the oxygen deficient conditions, thus nitrogen gas will be released. Following the anoxic zone, the effluent will enter the aerobic zone, which will contain aeration technology to provide oxygen to the microbes within the suspension. The delivery mechanism for oxygen is usually Fine Bubble Diffused Air Systems, as this technology is suited to the overall plug flow configuration of the treatment facility and the fine bubble allows for a high efficiency of oxygen transfer throughout the effluent. The nitrifying bacteria within the aerobic zone will utilise the oxygen to reduce ammonia based compounds to nitrates. During both the anoxic and aerobic stage the wastewater biological oxygen demand (BOD) will be diminished, via the uptake of dissolved oxygen and aerobic respiration in the aerobic zone, and via uptake of organic matter for cell growth and synthesis during denitrification. Further denitrification occurs in the clarifying stage as the anoxic environment encourages microbes to utilise newly converted nitrates (from the aerobic zone) as a oxidising agent within respiration. As nitrates are recycled from the clarifier to the anoxic zone via the Return Activated Sludge, the rate of nitrogen removal is directly controlled by the rate of recirculated flow. However a finite balance must be found between the rate of recirculation and the rate of denitrification by the denitrifying bacteria, as if the recirculation rate is too great an insufficient detention time for the nitrates and denitrifying bacteria will be attained in the aerobic zone, thus reducing microbial activity and overall nitrogen removal. Conversely if the recirculation rate is too fast it will reduce the detention time at all stages, thus the overall nitrogen removal efficiency. The overall oxygen removal efficiency will also be dependent on a number of different aspects, such as, nitrogen concentration (both ammonia nitrogen and nitrate nitrogen), BOD, temperature and pH, thus all these aspects must be controlled simultaneously to ensure optimum nitrogen removal from the liquid effluent.

Four-stage Bardenpho Process A process specifically built for denitrification, however combines both nitrification with denitrification through a series of anoxic and aerobic zones, Figure 4. The Bardenpho system uses both wastewater organic matter and endogenous organic decay matter, to achieve denitrification. The endogenous decay matter is utilised for denitrification through a recycle stream between the first two zones within the Bardenpho system, this allows for a high concentration of organic matter to be achieved within the first stage, thus providing optimum conditions for denitrification and reducing operational costs as no external organic source is required such as methanol.

Fresh untreated effluent is mixed with activated sludge from the recycle stream (Return Activated Sludge), which contains a high concentration of nitrates. Eventually when the mixed solution enters the first anoxic stage, the organic matter present within the sludgeand nitratescan be utilised by the denitrifying bacteria, for respiration and metabolism, thus producing nitrogen gas which can be vented via the aeration basin (anoxic basin not open to the atmosphere), directly into the atmosphere as it poses little environmental concern. Ammonia present in the raw wastewater will remain untouched through the anoxic zone, thus will pass straight through into the first aerobic zone, where nitrification can commence. Again fine bubbles are used to deliver the oxygen during this stage. The nitrified mixture will then enter the second anoxic zone, where denitrification will occur at a slower rate, as there is less endogenous carbon present in the suspension. Then final aerobic zone aims to provide aeration to encourage nitrogen release and enhance the settleability of the sludge prior to sedimentation in the clarifier, although some further nitrification will occur, but not in the same magnitude as the first aerobic zone.

Figure 4,Schematic diagram of the Four-stage Bardenpho Process

PHOSPHORUS REMOVAL Phosphorus in Wastewater Depending on the effluent source, phosphates can originate in wastewater effluents through a variety of sources. Considering two types of effluent source, municipal wastewater and industrial wastewater, for a municipal wastewater phosphate generally originates from fecal matter and waste materials. Whereas for industrial sources the origin of phosphate compounds depend on the industry type, generally the highest content of phosphorus compounds will originate from the agricultural industry, in particular phosphate based fertilizer production. Phosphorus is commonly abundant in wastewater effluents in the form of orthophosphates (PO4), condensed phosphates and organically bound phosphates, although the latter two are eventually converted to the orthophosphate form via gradual hydrolysation and bacterial decomposition. Although secondary biological treatment removes phosphorus via biological uptake, the quantity of phosphorus is far in excess of the cell requirement for synthesis, therefore only a small fraction of phosphorus is removed in secondary treatment. Therefore conventional methods of biological treatment only remove from 20 to 40% of phosphorus from the wastewater effluent, thus further tertiary treatment is required, specifically focussing on the removal of nutrients such as nitrogenous and phosphorus compounds. The overall removal of phosphorus from waste effluent involves incorporating the phosphate into a suspended solid form, then removal of the suspended solid can commence, either by biological (microorganisms) or chemical (precipitation) methods. Phosphorus is incorporated into into microbial biomass during cell growth, cells will readily uptake orthophosphate, the most common form of phosphate species, for this purpose. However chemical precipitation with metal salts of Aluminium or Iron, can also be utilised to remove phosphorus from wastewater effluent. Strict effluent limits for phosphorus discharge into surface water bodies such as, lakes or rivers, range from as little as 0.1 mg/L up to 2 mg/L, with the most common limit set as 1 mg/L. Therefore phosphorus remediation techniques must be highly efficient in reclaiming dissolved orthophosphate, to comply with strict regulation.

Biological Removal of Phosphorus Biological phosphorus removal via microorganisms is based upon some key facts metabolic facts: â&#x2014;? Bacteria possess the capability for storing excess amounts of phosphorus as polyphosphates (condensed phosphates). â&#x2014;? Bacteria can uptake simple fermentation substrates such as, acetate or short chain fatty acids, in the anaerobic zones and assimilate them into carbonaceous storage product, but must expend energy via ATP (adenosine triphosphate), therefore there is a release of phosphorus.

●

In the aerobic zone energy is produced through the oxidation of the carbonaceous stored products, resulting in an increase in polyphosphate storage through ATP.

Despite the phosphorus content within cells only accounting for ~2%, all microorganisms are capable of uptaking levels of phosphorus which surpass the stoichiometric requirements for biosynthesis, some species can uptake phosphorus levels equivalent to 11-12% of the dry weight of the cell. However within wastewater treatment the microorganism specifically associated with enhanced phosphate removal is acinetobacter. In anaerobic conditions the aforementioned fermentation products are uptaken and stored intracellularly as PHB (Polyhydroxybutyrate), which is formed directly from acetoacetate and cellular function is an electron acceptor allowing for the reoxidation of the coenzyme NADH → NAD+, outside of the oxidative electron transport chain. However microorganisms must expend energy during the uptake and storage of these organic molecules. This energy is acquired through the bond cleavage within an ATP molecule which accounts for about 30.5 KJ/mol in energy, the product of this cleavage is an organic phosphate or orthophosphate and a molecule of adenosine diphosphate (ADP): ATP

→

∆H = -30.5 KJ/mol

ADP + Pi

The product of the bond cleavage Pi (orthophosphate), is released from the cells into the wastewater effluent, with organic fermentation products uptaken simultaneously utilising the energy from the bond cleavage to do so. In the subsequent aerobic stage that follows the anaerobic stage, orthophosphate is immediately uptaken from the surrounding effluent for resynthesis of ATP, during cellular respiration. During respiration the PHB stored during the anaerobic stage, is aerobically oxidized or metabolized, to allow for cell growth and synthesis. To optimize the performance of the aerobic stage the detention time of the anaerobic stage should be sufficient to ensure the presence of excess orthophosphate release, ultimately meaning a favorable ratio of organic matter to phosphate ratio, which will ensure a rapid uptake of phosphorous during the aerobic stage for cell growth and synthesis. To summarize the processes involved in Biological Phosphorus removal: Anaerobic Zone 1. Fermentation

- Soluble Biological Oxygen Demand (SBOD) converted to - Volatile Fatty Acids (VFA).

2. Bacteria uptakes VFA

- Cells uptake VFA but requires energy - ATP cleavage, Orthophosphate product released from cell - VFA converted to PHB

Aerobic Zone 1. Phosphorus Uptake

- PHB Oxidized - ATP resynthesised - Orthophosphate removed from solution to replenish that lost in ATP synthesis

2. New Cells synthesised

- Cellular metabolism & Respiration - Cell synthesis

Adapted from R. Sedlak, 1991

BIOLOGICAL PHOSPHORUS REMOVAL SYSTEMS Although many phosphorus removal systems exist, one of the most prominent is the Phostrip process which will be considered in detail as the primary biological phosphorus removal system.

Figure 5, Schematic diagram of the Phostrip process

Central to the Phostrip process is the Stripper Tank, a portion of the return activated sludge is diverted through the stripper tank, where it is subjected to a period of anaerobic detention. Within the stripper tank sedimentation takes place and the anaerobic conditions promote an associated release of orthophosphate, from the microorganisms within the sludge. The tank contains an agitation device to mechanically distribute the phosphate compound equally throughout the sludge. However up to 50% of the released orthophosphate is carried away via the supernatant, the remaining subnatant is rerouted back into the return activated sludge stream, which flows directly into the inlet stream containing the raw untreated effluent entering the process discharged directly into the aeration basin. The supernatant outlet via the stripper tank, concentrated in phosphate, is then chemically treated with lime, to precipitate out the dissolved phosphorous within the supernatant. This chemical treatment accounts for about two thirds of phosphorous removal from the Phostrip process, the remaining third is accounted for by the wasted sludge containing microorganisms that have reached their phosphate absorption capacity. After a period of sludge ‘conditioning’, via numerous aerobic/anaerobic cycles, through gene regulation the microorganisms are able to turn on genes that allow for the synthesis of enzymes that enables the microorganisms to collectively absorb all of the excess phosphate, during the aeration zone. The sludge exits the aerobic zone and into the clarifier, where sedimentation occurs. The sludge containing suspended solids i.e. cells, will sediment first leaving behind a supernatant of low phosphate content for discharge as the final ‘treated’ effluent. Wasted Activated Sludge rich in intracellular phosphate, is discharged through the Waste Sludge Stream, as it has exhausted its capacity for further phosphate removal. This proven phosphorus removal system, can be operated at a range of temperatures from 7 to 30oC. It has been proven to reduce effluents containing phosphorus concentrations of 20 mg/L, to under 1 mg/L total phosphorus (Levin, 1987)(Kaschka,1999).

CHEMICAL PHOSPHORUS REMOVAL As opposed to biological removal of phosphorus with the use of microorganisms, phosphorus can be effectively removed chemically through a series of coagulation reactions with metal salts. This chemical precipitation of phosphorus is conventionally carried out using aluminium or iron coagulants, specifically aluminium of iron based acids i.e. alum or ferric chloride, although aluminium based alkalis and lime are also effective. However the specific selection of the metal salt is dependent on the observed performance and cost, which is specific economical with metal based acids. The treatment plant facilities required for chemical phosphorus removal consist primarily of only chemical storage and feeding equipment, rather simplistic. The coagulants are dosed with the wastewater effluent to precipitate out phosphorus allowing for subsequent removal. With either alum or ferric salts, the reactions are primarily with orthophosphate and are as follows: 57

Fe3+ + 3Cl- + HPO42- + 2H+ Al3+ + 3Cl- + HPO42- + 2H+

→ →

FePO4 + 3HCl AlPO4 + 3HCl

Although chloride ions can be switched for sulphate ions, and a similar reaction will proceed. Thus in the main reaction insoluble iron phosphate is produced, this same reaction is evident with Aluminium based acids. Although the addition of metal salt consumes alkalinity of the the wastewater, therefore the addition of these salts can depress the overall pH of the effluent. To counteract this supplementary alkalinity must be added by the addition of an alkaline substance such as lime, must be added before effluent discharge to ensure that the treat wastewater meets the discharge pH limits set by regulation. Unfortunately during the main precipitation reaction, unwanted by-reactions occur simultaneously with this main precipitation reaction, thus some form of chemical regulation is required to optimise the main precipitation reaction. This can be attained through the addition of precipitating agent to aid solid flocculation, this agent is usually an anionic polymer. When considering dose rates of the metal salts, the dose rate will vary with the varying phosphate concentration of the raw effluent. In terms of process design, the outlined design should provide a high degree of flexibility with the types of chemicals used, dose points and dosage range that can be accommodated (Kaschka, 1999) (Sedlak, 1991) (Hammer, 2003). Example For example, for total effluent outlet concentrations of less than 0.5 mg-P/L, up to 6 moles of metal salt must be added per mole of phosphorus removed, and on a stoichiometric basis, 9.6 g-AlCl3 per g-PO42- removed and 5.2 g-FeCl3 per g-PO42-removed is required. Performing a simple calculation of the dosage required to precipitate out phosphorus in wastewater, assuming: ● Wastewater Phosphorus Concentration = 10 mg/L ● Wastewater Treatment Plant throughput = 20 m3/hr ● 2 moles of Aluminium required to remove 1 mole of Phosphorus ● Liquid Aluminium Sulphate as the metal salt, Al 2(SO4)3 (aq) ● 40% strength of Al2(SO4)3 (aq) ● Density of liquid Al2(SO4)3 (aq) = 1200 kg/m3 Begin by calculating the weight of Al2(SO4)3 per litre of solution: Mass (Al2(SO4)3) /Litre =

Next calculate the weight of Aluminium (Al), per litre of Al 2(SO4)3 assuming molecular weights: Mr (Al2(SO4)3) = 666.5 g/mol Mr (Al) = 26.98 g/mol

Therefore, Aluminium per Litre =

Next the weight of Aluminium required per weight of Phosphorus must be determined, given: Al3+ + PO43-

â&#x2020;&#x2019;

AlPO4

Therefore the theoretical ratio is 1 mol-Al : 1 mol-P, therefore assuming Mr(P) = 30.97 g/mol Aluminium Required =

The Volume of Aluminium solution per kg of Phosphorus removal required, or Aluminium dose can be calculated via: Aluminum Dose

= =

Therefore given the aforementioned operational throughput and wastewater phosphorus concentration, the amount of Aluminium solution required per day: =

Finally given that a 30 day supply of Aluminium salt solution will be stored on site at any time, the volume of the storage tank require is: Vtank

= =

Chemical Storage Aluminium salts can be delivered as either a liquid or a solid, in the liquid form transportation and handling costs are considerably greater in comparison to the solid form. Ferric chloride is only delivered as a liquid. The anionic polymer is available in solid or liquid form. The minimum storage volume must be assessed, which should take into account daily usage, peak usage, supply volume and delivery schedule, for example for the previous calculation the minimum storage volume was based on the assumption of a 30 day supply of Aluminium Salt solution (Hammer, 2003).

Dose Points A typical dose point for metal salt will occur as far upstream from the primary clarifier as possible, and especially away from any equipment that will provide a large degree of turbulence. A typical dose point for the metal salt is bothbefore and after the aeration basin in Figure 5, it is important that the effluent is sufficiently dosed with the coagulant prior to the clarification stage, as this will ensure adequate mixing time for the metal salt, avoiding extreme turbulence, resulting in the formation of an ideal precipitate (Hammer, 2003).

COMBINED BIOLOGICAL PHOSPHORUS AND NITRIFICATION-DENITRIFICATION The schematic diagram for the combined nutrient removal system (Figure 6), this system is a combination of all the aforementioned nutrient removal processes for, phosphorus and nitrogenous compounds. The key processes occurring are biological phosphorus removal, nitrification and denitrification, given the aforementioned descriptions for biological nutrient removal, the system contains in sequence anaerobic, anoxic and aerobic zones. First the anaerobic zone is primarily for the biological removal of phosphorus, which has previously been discussed in the Biological Phosphorus Removal section, the anaerobic zone is mixed with submersible propellers to ensure complete and homogeneous mixing of the effluent. Each anaerobic basin will remain closed to the atmosphere to ensure optimum anaerobic conditions are achieved. The treatment facility is made up of three separate anaerobic chambers each containing a propeller, to ensure a high efficiency of phosphate removal is obtained. Raw wastewater and Return Activated Sludge containing new bacteria enters the first anaerobic chamber, where it passes through the series of three anaerobic chambers, then into the anoxic zone(Hammer, 2003). The first anoxic zone contains a Nitrate recycle stream for denitrification, where fresh nitrates from the aerobic basin can flow into the anoxic chamber and pass through each 60

anoxic chamber, where denitrification occurs and nitrogen gas is released. Each anoxic zone, again contains a submersible propeller to ensure complete mixing. After the anoxic stage the effluent will flow into the aerobic basin, which is a large â&#x20AC;&#x2DC;swimming poolâ&#x20AC;&#x2122; like basin, which contains numerous fine bubble aeration systems for oxygen delivery. Propellor pumps within the aerobic basin create a circular velocity of up to 0.3 m/s, which circulates the effluent about the aerobic basin. Within the aerobic basin nitrification will occurs, oxidising ammonia nitrogen to nitrates, and the last stage of biological phosphorous removal will occur, where bacteria absorb the orthophosphates for ATP synthesis. The aerobic basin contains two outlets, one for the aforementioned recycle stream connected directly to the anoxic zone, the second outlet directs the effluent to the clarifier where treated effluent can be separated out from the activated sludge via sedimentation. The activated sludge is to the recycled to the first anaerobic zone. Conversely this combinational method of biological nutrient removal can lack in efficiency given the number of processes occurring simultaneously, however by controlling the recycle ratio of the Return Activated Sludge and detention times within all stages, a thoroughly treated effluent meeting all regulatory limits, can be obtained (Hammer, 2003).

Figure 6, Schematic diagram of a Combined Biological Phosphorus and NitrificationDenitrification plant, both cross and plan view, (Hammer, 2003)

REFERENCES Hammer M. (2003), Water and Wastewater Technology, 5 th Edition, Upper Saddle Hall NJ USA, Prentice Hall Kaschka E. (1999), Phostrip Handbook: Biological Elimination of Phosphorus from Domestic Sewage by applying the enhanced Phostrip Process, 4 th Edition, Innsbruck Austria, Posch & Partners Consulting Engineers Levin G. (1987), Photstrip Process â&#x20AC;&#x201C; A viable answer to the Eutrophication of Lakes and Coastal Sea Waters in Italy, Oxford UK, Pergamon Press Metcalf & Eddy (2002), Wastewater Engineering: Treatment and Reuse, 4 th Edition, New York City, NY, McGraw-Hill Higher Education Sedlak R. (1991), Phosphorus and Nitrogen Removal from Municipal Wastewater: Principles and Practice, 2 nd Edition, New York City NY USA, The Soap and Detergent Association Wiesmann U. (1994), Biological Nitrogen Removal from Wastewater, Advances in Biochemical Engineering/Biotechnology, Vol. 51 pg 113-154

5. SLUDGE TREATMENT Part of the excess sludge is recirculated back to the start of the process, to maintain the biomass population but another part of it is removed from the activated sludge process. The excess sludge has some uncomfortable characteristics (Van Haandel and van der Lubbe, 2007): -

It is instable: It rots easily when aeration stops due to the organic matter content (around 75 % of the total matter) according to Autonoma University (2012). - It contains bacteria, viruses and other microorganisms which make the sludge unhygienic. - The content of water is high so the sludge is little concentrated which means more difficulties to handle the volume of the sludge. Sludge treatment process is usually carried out in order to reduce the content of water in the sludge and to achieve a better degradation of organic matter and a reduction of the pathogenic agents (ibid). According to Autonoma University (2012), the main steps included in the sludge treatment are concentration, stabilization, conditioning, dehydration and management. 

Concentration: It is focused on the reduction of the sludge volume as much as possible. It can be either by thickening, flotation (this one requires oxygen) and the concentration of the sludge is increased from 1-2% (10-20 gr/l ) to 3-4 % (30 - 40 gr/l) (ibid). According to Autonoma university,2012 the residence time is between 2 and 4 hours. Although sometimes this process is not taken into account, it is very recommendable since it eases the performance in the dehydration units. Degremont (1973) establishes different methods:  By settling: The effluent is left to be compacted in a tank and it is drawn off from the bottom. Thickeners used can be either static or mechanized (ibid): -Static: Cylindrical shape with slopes in the floor surface of 50 - 70 ° and up to 5 meters in diameter. Typical depths are 3 - 4 meters. -Mechanized: There are some scrapers to gather the sludge in the centre of the tank. Hence the bonds between gases and liquid are reduced. The slopes are less inclined (10 - 15 °C) and the diameters can be much larger. Same depths as static settlers.  By elutriation: The sludge is washed with new clean water to improve the precipitation of the sludge by eliminating fine and colloidal particles. The alkalinity is also decreased reducing the consumption of agents in the dehydration process. The elutriation tanks differ from the settler tanks in the incorporation of a great amount of water at the entrance. Sometimes another tank is placed ahead of the elutriation tank. The levels of concentration are pretty similar to those achieved by thickening and the overflow effluent does not contain more than 2 g/L.  By flotation: This mechanism shows some advantages: the thickening units used are smaller and the sludge is more concentrated although 64

higher costs are invested. Air injected plays an important role since particles are adhered to the bubbles and their specific mass is lower than the water itself, remaining then in suspension. The solids form a blanket on the surface which is easily removed. The air flow required to float the sludge is between 10 and 20% of the sludge.  Other alternative could be the use of centrifugation (Mountain Empire,2000) which is a simple and flexible unit although it requires high power costs and it has a great dependence on some chemicals to achieve good solid captures.



Stabilization : Is based on the elimination of pathogen microorganisms and to reduce the organic content which would lead to putrefaction and disgusting odours. It can be achieved either by aerobic or anaerobic procedure. Aerobic stabilization The sludge goes on with the aeration process in which the microorganism keeps growing up but endogenous respiration starts to occur when the cell material is oxidized eliminating part of the cellular matter to obtain the energy needed to sustain the cellular growth. This results in a reduction of the organic matter (Mountain Empire ,2000). The reaction taking place are: C5H7O2N + 5O2 → 5CO2 + 2H2O + NH3 + energy According to Valladolid University (2010) the ammonia is further oxidized to nitrate which also reacts with the biomass: NH3 + 2O2 →NO3- + H2O + H+ C5H7O2N +4NO3- → 5CO2 + 2N2 + NH3 +4H2O C5H7O2N +3NO3- + 2O2 +3H+ → 5CO2 + 2N2 + 5H2O According to Autonoma University (2012), between 70 and 80 % of the organic matter is reduced. However, in the stabilization only a part of the organic matter is biodegraded which is called active organic matter. Degremont (1973) sets it is around 40 to 45 % of the total sludge mass, but taking into account that the total organic matter mentioned before (75 %) that makes an efficiency of : This efficiency increases with the aeration time (days), but after 15 - 20 days the reduction of volatile solids is no longer achieved (ibid).

In terms of the design some important parameters must be controlled: Sludge temperature: A variation of a few degrees can mean significant reductions of the organic matter. For instance, while at 20 °C the reduction is around 40 %, at 12°C drops to 30 % (Degremont,1973) 65

   



Sludge loading rate: For activated sludge should not exceed 2kg/ m 3 (ibid) Air requirement: The oxygen demand is 2.3 kg per kg of biomass (Autonoma University, 2012) Detention time: The hydraulic retention time ranges from 15 to 20 days at 20 °C for the waste activated sludge (ibid). It can be obtained by the coefficient between the volume of the tank and the flow rate: HRT = V/ Q The tanks are rarely covered or heated, what means lower costs than anaerobic tanks which need to be closed and heated (Mountain Empire, 2000) The consistence of the sludge purged from the aerobic stabilization is between 60 and 120 g/L. (Autonoma University, 2012) Some advantages can be pointed when aerobic stabilization is carried out, (Mountain Empire, 2000): The product obtained is stable with no odors and the performance process has little operational problems The capital cost is lower than anaerobic stabilization Volatile solid reduction is similar to those gained in anaerobic stabilization. The supernatant liquors have less BOD content (from 50 to 500 mg/l) than the achieved in the anaerobic process (from 500 to 3000 mg/l) according to Degremont (1973). The mud after the aerobic stabilization has good dewatering properties, but it is not suitable for a mechanical dewatering. The main disadvantages are:  The high power costs required due to the continuous air supply which limits the size of these units to maintain profitability of the process.  The high influence of the temperature in the stabilization efficiency.  The absence of methane production.

Anaerobic stabilization Is characterized by the production of a valuable by-product, that is methane (which is an important source of energy) as well as other byproducts such as CO2, NH3, H2 and H2S in lower quantities. The organic matter is reduced, although in less percentages than those in aerobic stabilization. Around 50 % is transformed (Autonoma university, 2012). There are many reactions taking place as it happens in the anaerobic digestion but they can be divided into two important steps: - Firstly, hydrolysis and acidogenesis processes occur, in which the sugars are transformed into smaller acids, alcohols and gases rich in CO2, H2 and H2S. 66

- Then the gasification process takes place and the small particles are attacked producing methane gas as a result. Great amounts of gases are produced with at least 65 % of CH4. It is important to ensure the maintenance of pH on the range of 6.8 to 7.4 to ensure the best throughputs (Mountain Empire, 2000). The optimum temperature for the greatest reaction rates is around 35 - 37 °C. The anaerobic stabilization can occur either by a single stage or twostages. In both performances, the consistence of the sludge is around 80 mg/l 



Single stage: There is only one tank, it usually has several points feed, a heater device is incorporated in connection with the tank to maintain the temperature, mechanical mixing is required in the tank and it requires that the sludge had been previously thickened (Valladolid university, 2010). Different layers can be distinguished in the tank: the supernatant liquor at the top, the sludge digested at the bottom and the digestion between them. Two-stages: It incorporates two tanks. The feed enters the first one where the mixing is applied and the heating is set up in this tank. Then the effluent is led to a second tank where the different layers are formed. The gas produced in the first tank is sent to the second one and it is in this second tank from where the gas is collected (Autonoma university, 2012). The single- stage performance is used when the flow rate is lower than 4000 m3/day (ibid). The solids retention time is also higher and oscillates considerably from 30 to 60 days, while the two -stage is rarely higher than 20 days (ibid). In the same way there are some parameters that must be carefully controlled to ensure optimum performance. - pH, in the range of 6.8 to 7.4 - Temperature: If temperature drops, the digestion requires a longer time to complete the digestion. At around 35 °C, the digestion is complete in 24 days while if the temperature decreases to 13 °C it would require around 55 days (Mountain Empire, 2000). - Volatile acids and alkalinity ratio: It is a good indicator of the digestion progress. The volatile acids are formed once the bacteria start breaking down the sludge. The bicarbonate alkalinity (in the form of CaCO3) shows information about the capacity to maintain the ph constant and to neutralize acids. If the coefficient between volatile acids and bicarbonate alkalinity concentration is lower than 0.25, it means that the process is going as it should (ibid). - Solids supply: Overload can occur if too much sludge is allowed to go into the digester, what would lead to an excess for the first reaction needs and the creation of an 67

inappropriate environmental for the microorganism involved in the gasification process. 0.64 -1.6 (kg/m3.d) for a single stage and between 1.6 - 3.2 (kg/m3.d) in the two-stage mechanism are the typical loads to be used. (Autonoma university, 2012)

ď&#x201A;ˇ

Conditioning: Is an important step to condition the sludge for the dewatering process. Conditioning can be developed either by chemical or thermal actions. -Chemical conditioning: facilitates a better performance later on in the centrifuges and vacuum filtration. There is a great range of chemicals that can be used but all of them aim to alter the pH of the effluent to create larger coagulant particles, so that it would then be easier to get rid of the water (Mountain Empire, 2000). The most common one is ferric chloride which can be used alone or combined with lime. Mountain Empire (2000) suggests using it alone for the conditioning of activated sludge. The higher the volatile matter is, the more product is demanded. In the latest years the use of organic polymers has emerged rapidly since the inorganic coagulants are more difficult to be handled and sometimes maintenance problems come up because of their corrosion tendency. Other flocculants that can be used are: ferrous sulphate, aluminum sulphate, sulphuric acid, etc (Degremont, 1973). He also affirms that flocculation process should be carried out in tanks where good mixing is continuously provided and with residence times of 10 to 15 minutes to ensure the growth of the flocculants. - Thermal conditioning: Some advantages can be pointed out according to Degremont, 1973: It shows a high effectiveness for any organic sludge, the sludge is no longer infected and the sludge obtained after the process can be easily thickened and dewatered. The fact of setting up a thermal conditioning enhances the filterability of the sludge which is furthermore improved as the temperature or heating time are increased (ibid). There is an extreme relation between these two parameters since the increase of one of them allows the decrease of the other one for same results. Degremont, (1973) states that shorter heating times improve the qualities for the filtration. Two different alternatives are mentioned by Mountain Empire (2000): - Wet air oxidation: It is carried out at 230-290 Â°C and around 80 atm of pressure - Heat treatment: The temperatures are lower (175-205) and so the pressure (between 10 and 20 atm) The second option is more used. The way it works is as follows: The sludge is pumped to a heat exchanger so its pressure increases up to 20 atm, and the sludge is heated to 175Â°C. Steam is injected and the mixture is led to a reactor which is connected to a boiler to achieve the desired temperature. The already treated sludge passes again in cross-flow through the 68

heat exchanger losing heat in favor of the inlet sludge. The gases released are taken away from the reactor and usually burnt (Mountain Empire 2000) Steam consumption is around 60 kg/m3 according to Degremont (1973). Wet air oxidation has a similar performance, although the most noticeable difference is the addition of air in the heat exchanger. The consistence of the sludge is higher with thermal conditioning rather than with chemical conditioners although fine organic particles are inevitably carried with the evaporated water. ď&#x201A;ˇ

Dewatering: Filtration is the most common way to eliminate water from the sludge. It can be developed either by mechanical ways (vacuum and pressure filtration) or by using drying beds to drain the sludge. A relevant coefficient in the filtration processes is the specific resistance of the sludge (cm/g). The lower it is, the more rapid the filtration process becomes. - Vacuum filtration: The most frequent type is an open drum where the filter is placed. The filter is a cloth made of cotton, nylon, fiber glass, plastic or stainless steel net or synthetic fibers (Mountain Empire, 2000). The drum is set in horizontal arrangement, it rotates and a part of it is submerged in the sludge tank (Degremont, 1973). Inside the drum, vacuum conditions are applied and the filter lets out the water but retains the sludge which is further drained as it keeps flipping being exposed to the atmosphere. Then before coming back to the tank the dewatered sludge is discharged by scrapers, an air blower or a pressure roller (Mountain Empire, 2000). The thickness of the cake accumulated inside the filter ranges from 5 to 20 mm, and the void fraction must have at least 150 microns. The usual speed of rotation is between 8 and 15 times per hour (Degremont 1973). The performance of a filter lies on the rate of dry solids filtered per hour and per unit area. According to Degremont (1973), for activated sludge is 10 kgm-2h-1. Mountain Empire states it is a bit more (around 12 kgm-2h-1). Autonoma university (2012) holds that the solids have a consistence of 220 250 g/L while Degremont says that the moisture content is between 72 and 80 % (200-280 g/L) in town sewages sludges. -Pressure filtration: According to Degremont (1973), the plate kind is the most used. It consists of several vertical tubes pressing each other and a cloth is placed in the gap between two plates. The sludge passes through the permeable material where is retained and driven out of the unit by different ducts. The filter material is usually made of synthetic fibers and is brushed after several hours (no more than 6 hours). It has to be substituted after around 3000 hours of operation (ibid). There can be hundreds of plates, each of them can have nearly 2 m2 in area (ibid) The pressure across the filter can reach 15 atm. This pressure is lower at first but then it increases as the sludge is deposited and the pores are closed. (ibid). Some chemicals such as lime, aluminum chloride and ferric salts are added to condition the sludge before being pressed (Mountain Empire, 2000) 69

Sometimes an additional membrane is placed between the cloths to improve the water removal. Extra air pressure is required (Degremont 1973) According to Degremont 1973 pressure filtration can achieve between 35 and 55 % of dry solids with town residues and the performance of the filter is of up to 10 kg/m-2h-1. The grade of dryness depends on the pressure, the thickness of the cake and the type of the conditioning but a reference value would be around 1000 minutes (Pg 453). These filters are more suited than the vacuum ones when the content of colloidal matter is no too high, otherwise it the filters clogged faster (ibid). The operating costs in filtration performance are higher than those invested in drying beds although less area is required (around 50 m 2) ,weather conditions do not have so much influence and the dewatered sludge is ready to be incinerated (Mountain Empire, 2000).

-Centrifugation: It consists of the separation of the sludge from the water by the use of a centrifugal force (between 1000 and 3000 G, where G = 9.81 m/s 2 according to Degremont 1973, and 6000 G according to Autonoma University) driven by an additional machine and then sedimentation. Thus, the gravitational values are much higher than those in normal settlers and the separation of the sludge-water is better and quicker. According to Autonoma University (2012), the consistence of the solids are between 25 and 30% while Degremont states it is 25% the average of two different layers which can be distinguished in the deposited sludge. These layers are attenuated by the addition of flocculants. Degremont (1973) suggests that horizontal continuous centrifuges are the best suited in most centrifugation cases. The sludge is introduced through a tube inside of a cylindrical bowl which rotates at high speed. There is a screw conveyor mounted inside the tube rotating at a slightly lower speed and it distributes the sludge forcing it to the walls by the action of centrifugal acceleration. The dewatering action carries on in the final part of the bowl which has conical shape and then it is finally discharged. The advantages of using centrifugation are: low capital and operating cost, the units are not disposed to the atmosphere so no odors are released, they can handle a great range of sludge and they can deal with some sludge which are unfeasible for vacuum filters (Mountain Empire, 2000). There are some disadvantages as well: chemicals are usually required in order to get good solids captures, and trash gets accumulated at the entrance of the unit so it has be removed periodically. - Drying beds: The disposition is the following one: the drains which are cement open no-jointed pipes are placed within a layer of gravel. Above it, a layer of sand is set and above this one, the sludge is spread over the entire surface and flows downwards through the different layers. The thickness of the sludge is usually around 30 cm and the water content is reduced by drainage to around 80 % (Degremont 1973). Autonoma university (2012) holds that the consistence of the solids is 150 g /L .Once it is dried, the removal of the sludge can be done either by hand (less satisfactory) or by motorized bridges with scrapers which allow the separation of the sludge by layers with the advantage 70

that the lowest layers are being dewatered a longer time and the efficiencies increase. However it requires the set up of the heavy equipment and reinforced walls. By the instauration of covers above the beds the efficiency is slightly increased as well. (ibid). The outputs established by Degremont (1973) in Mediterranean areas, are between 0.5 and 0.6 kg of dry solids per day and per unit of area depending whether it has previously been stabilized or not. These values are enhanced by chemical conditioning since the resistance of the sludge is reduced. 

Management: It is related to the handling once its water content has previously been reduced. - Incineration : It is the last possibility to remove the remaining moisture of the sludge. It requires great consumptions of energy producing a free-water, mineral product. Since Degremont (1973) suggests it should only be carried out if the product is going to be used as fertilizer or with a further application in industry, it is going to be included within procedure of treating the raw sewage by Green Country Industries. By going through incineration process, the weight and bulk of the sludge is highly reduced. Besides, its calorific value is recovered being a source of energy. It also prevents from the discomfort of having to be breathing unpleasant odors (Mountain Empire, 2000) Degremont 1973 states that in order to make the process profitable, mechanical dewatering is needed before the incineration, otherwise the process would be non-viable. According to Mountain Empire (2000) there are two main furnaces which can be used: - Multiple-health furnace: Its design is based on several plates placed one over each other and the sludge falls all the way down plate by plate being moved by rotary scrapers. The diameters of the furnace can measure up to 7m (Degremont 1973). The outlet gases are at around 250 °C and the sludge at the top of the incinerator is at 70 °C. (ibid) The water content is reduced to 50 - 60 % in the upper plates and the combustion takes places as it approaches the lowest levels. The combustion is completed when the temperature reaches to between 760 and 870 °C (ibid). This unit can also be used as a drier if the products are removed from intermediate plates, before the combustion begins. In order to homogenize optimize the consumption in the incinerator, the cake should be broken up after the filtration process. These units require great energy consumptions when they are made to run since they are large and it takes a long time to heat the bricks surrounded the unit. -

Fluidized bed incinerator: It is characterized by the fact that the hot gases are not in contact with the mechanical parts moving. All the smells are removed although the engineers have to face up to great heat losses. The combustion is likely to be always complete. According to Degremont 1973, the sludge is introduced at around 700 - 800 °C through from the top and it remains because it meets an air stream coming in cross-current. The residence time in the incinerator is short 71

but long enough to dry and incinerate the product. Complete combustion occurs once the product leaves the top part of the bed ( Between 900 and 950 °C according to Degremont (1973). Some important considerations must be mentioned: The polluted air has to be treated in a cyclone and then be washed. The washed current has to be then sent to a lagoon to be treated The injection of the air has to be at a pressure high enough to offset the pressure losses along the bed. Since the heat losses are important, in order to optimize the performing, the use of the outlet temperatures to pre-heat the feed is very profitable and most probably essential. The control of these units is based on measurements of pressure and temperature both in the bed and in the area above it.

REFERENCES OF AEROBIC AND ANAEROBIC BIOLOGICAL TREATMENT AND SLUDGE TREATMENT Autónoma university, Madrid (Spain),2012 Waste effluents treatment. Environmental engineering. Unpublished. Degrémont, Gilbert, 1973. Water treatment handbook. Fourth edition. Published in Great Britain by Austin, Stephan. Edyvean,2013. University of Sheffield. EPA. United States Environment Protection Agency. Bio solids Technology Sheet. Multistage Anaerobic Digestion. [Online]. Available http://water.epa.gov/scitech/wastetech/upload/2006_10_16_mtb_multi-stage.pdf Accessed: 10/03/2013

Fact at:

Henze et al, 2008. Biological wastewater treatment: principles, modelling and design. Published in London Mountain Empire, community college, 2000. Water/Wastewater distance learning website. Lesson 13. Advanced Wastewater treatment. Stabilization. Acceded the 30 th March. [Online]. Available at: http://water.me.vccs.edu/courses/ENV149/stabilization2.htm Valladolid university (Spain), 2010. Chemical engineering and environmental technology department. Chapter 8 - Sludge stabilization. Torre, R.M and Encina, P.G. Acceded the 30th March. [Online]. Available at: http://iqtma.uva.es/instrat/Presentaciones_pdf/Chapter%208-%20Sludge.pdf Van Haandel, Adrianus and Van der Lubbe , Jeroen 2007. Handbook Biological Wastewater treatment. Design and Optimization of activated sludge systems. Published in Netherlands. Winkler, M. Biological treatment of Waste-Water,1981. Department of Chemical Engineering. University of Surrey. Published by Horwood,E in England. 72

6. FERTILIZER PRODUCTION

Introduction Fertilizers are materials that have one or more of the chemical elements necessary for development and growth of plants. Fertilizer products are the most important type of fertilizers which are produced by industrial processes with the specific purpose of being used as fertilizer. Nutrients are the chemical elements that allow for the optimal growth of plants. In the following scheme, these nutrients are shown.

Figure 1.Soil nutrients and factors.

Fertilizers have three basic components: nitrogen (N), phosphate (P 2O5 or P) and potash (K2O or K).

Nitrogen is present in amino acids, the biological building blocks of proteins, enzymes, genetic material, etc. Therefore this is a basic element facilitates the growth of the plants on the macroscopic level. The availability of N to plants is largely controlled by soil microbial processes. In the next figure we can see the nitrogen cycle in the atmosphere-biosphere.

Figure 2. Nitrogen Cycle (Brody N. Macmillan, Pub.Co. 1984)

Phosphorus compounds allow for the transfer of energy and storage reactions, and are a key component within intracellular genetic material, such as nucleic acids. The function of the potassium is to remain as an ion, and to control turgidity of cells, and participate in transfer mechanisms of starches and sugars, as well as, acts in protein synthesis and activate enzymes.

The fertilizers can be firstly classified as follows: ď&#x201A;ˇ ď&#x201A;ˇ

Physical state: Solid, Liquid (less used), Gas Composition: Simple, Binary, Ternary, Mixed

In the next figure, the most important types of fertilizers are shown.

Figure 3.Fertilizers classification.

And in the following table we can see the different fertilizers related to a specific type of soil.

Neutral and alkaline soils, no limed

Alkaline and limed soils

Acid soils

Saline soils

Calcium ammonium nitrate

Ammonium sulfate

Calcium ammonium nitrate

Nitrogenous fertilizers Calcium nitrate Superphosphate of lime Phosphorus fertilizers

Potassic fertilizers

Potassium sulfate Potassium nitrate

Ammonium nitro-sulfate Ammonium nitrate Urea Monoammonium phosphate Diammonium phosphate

Potassium sulfate Potassium nitrate

Calcium nitrate Calcium nitrate Phosphorites

Potassium nitrate

Ammonium nitrate Urea Superphosphate of lime Monoammonium phosphate Diammonium phosphate Potassium nitrate

Table 1. Fertilizers related to a specific type of soil (Vian, 2 nded 2006).

Currently new fertilizers are being studied, such as liquid fertilizers (present in aqueous solutions or suspensions) that facilitate transport and can be incorporated into irrigation water. Other options are the gradual acting fertilizers, which release nutrients slowly, the combination between organic and inorganic fertilizers and the incorporation of microorganisms which inhibits or relaxes the transformations that occur in the soil. Therefore depending on the soil type, there should be a suitable fertilizer which can optimize both ion transfer within the soil (via active transport) and ultimately plant growth.

NITROGENOUS FERTILIZERS UREA See the portfolio’s page “Effluent Treatment”.

AMMONIUM NITRATE This is the most commonly used fertilizer within the group of the nitrogen fertilizers, but in the developing countries urea is the most employed. The nitrogen in the ammonium nitrate is more rapidly absorbed by the soil than the nitrogen in the urea or other ammonium salts. Some disadvantages are the hygroscopicity of the salt, the risk of fire and explosion, the low efficiency in flooded fields and the speed which it is absorbed, because in some cases is more interesting if the fertilizer is absorbed gradually. This type of fertilizer is supplied prilled and granulated. The quality of the product is improved with stabilizers, which depend on the type of soil. Because of its oxidizing behavior, special considerations have to be taken with the transport and handling of this material to prevent fires and explosions. These considerations also involve the storage, which should have the correct ventilation and a high-efficiency sprinkler system. The main reaction in the ammonium nitrate production is shown in the following equation:

Nitric acid used has a concentration between 45% and 60%, this concentration and the reaction temperature define the concentration of the product. The production of the ammonium nitrate consists of various processes:

Figure 4, Ammonium Nitrate production process

Neutralization In this section, the nitric acid, with a concentration of 50-60% is fed to a neutralizer (4-5 atm, 175-180ยบC) where reacts with gaseous ammonia (previously vaporized in a heat exchanger). This unit works with a low pH (3-4) and the product has to be neutralized with ammonia to maintain a pH of 7. This stream has a concentration between 80% and 87%, and then, using the steam produced in the process, the concentration increases to 94-98%. Upper concentrations can be achieved in evaporator-condensate units. The main industrial neutralization processes are shown below: -

Uhde Neutralization Under Vacuum or Atmospheric Pressure (Figure 5) Uhde Neutralization Under 0.2-0.25 MPa Pressure (Figure 6) Hydro Agri Neutralization Under 0.4-0.5 MPa Pressure (Figure 7) The CARNIT Neutralization at 0.7-0.8 MPa (Figure 8) The Pipe Reactor Neutralization (Figure 9)

Figure 5.Uhde Neutralization Process Under Vacuum or Atmospheric Pressure (Figure 8.8, pg 228, UNIDO) The ammonia gas and nitric acid streams are preheated with vapor, produced in the flash evaporation units (E-5, E-9), before entering the reactor (R-1). The reaction products are concentrated in a flash evaporator and the neutralized with ammonia gas (R-6). This final stream is preheated and distillated, obtaining the ammonium nitrate solution at the bottom of the column. 78

Figure 6.Uhde Neutralization Process with 0.4 MPa Steam Production (Figure 8.9, pg 229, UNIDO)

Figure 7. Hydro Agri Medium-Pressure Neutralization Process (Figure 8.10, pg 230, UNIDO) 79

Figure 8. CARNIT Process Flowsheet (Figure 8.11, pg 231, UNIDO)

Figure 9. AZF Pipe Reactor Neutralization (Figure 8.12, pg 232, UNIDO)

Concentration This part of the production process (Figure 4) consists of the concentration of the ammonium nitrate, vapor treatment and the addition of stabilizing agents. The units used in the section are scrubbing columns, packed columns, ion exchange process, etc. To prevent from the rapid crystallization and to improve the storage characteristics, inorganic stabilizers are added to the ammonium nitrate. A common additive is the magnesium nitrate. Because of the hygroscopicity of this fertilizer, it is necessary to coat the granules and prills with oil-amine mixtures, which work as anticaking materials.

Finishing The product has to be transformed to the final form, that can be grains, flakes, granules, crystals or prills.

CALCIUM AMMONIUM NITRATE Calcium ammonium nitrate is used in acidic soils and is an alternative to ammonium nitrate in some countries. The production of this fertilizer consists on the mixture between the ammonium nitrate concentrated with calcitic or dolomitic limestone, chalk marl or calcium carbonate from the production of nitrophosphate. The reaction of the process is the described below:

PHOSPHORUS FERTILIZERS TRIPLE SUPERPHOSPHATE Triple superphosphate (TSP) and ammonium phosphate are the two most important phosphorus fertilizers. The production of the TSP decreased at the end of the XX century with the development of the ammonium phosphate industry. ADVANTAGES OF THE TSP -

DISADVANTAGES OF THE TSP

Greater phosphate concentration Important part of the phosphate comes from phosphate rock â&#x2020;&#x2019; lower costs Simple production Small capital investment

Less nutrients than other phosphorus fertilizers Acid properties In the presence of urea can cause deterioration

Table 2.Advantages and disadvantages of the triple superphosphate.

The production of TSP is based in this reaction:

The TSP is produced in the industry following four operations: reaction, denning, storage (and curing) and granulation.

AMMONIUM PHOSPHATES This type of phosphorus fertilizer is the most popular, specially the diammonium phosphate (DAP). The monoammonium phosphate (MAP) and DAP, in a wet-process acid can get good physical properties. The impurity level affects the storage and the granulation process, and in the case of the pure ammonium phosphates, there is a difficulty in the granulation process and a trend for the formation of cakes in the storage, which make essential the addition of conditioners. An early technology for the manufacture of MAP and DAP is the TVA Basic Process, showed in the Figure 10.

Figure 10. TVA DAP Process (Figure 12.5, pg 362, UNIDO) The phosphoric acid (54% P 2O5) is preneutralized (R-2) with ammonia. Another stream of less concentrated phosphoric acid is also going to pass through this preneutralizer after being treated in a scrubber with products from the granulation section. The granulated ammonium phosphate is the final product of the process.

POTASH FERTILIZERS POTASSIUM SULFATE Potassium sulfates and nitrates are more used in zones where chloride presence can be harmful to the soil, like in arid areas or areas with an intensive agriculture. The production of the potassium sulfate can follows two processes. The Mannhein Process, where sulfuric acid reacts with KCl in a two-stage reaction:

This reaction takes place in a furnace, where HCl and K 2SO4 are produced. After this section, the K2SO4 passes through a cooling drum and then can be crushed and

finished or compacted and granulated. In Figure 11, this process, as operated by Kemira Oy, is showed:

Figure 11.Kemira Oy Potassium Sulfate Process (Figure 15.4, pg 425, UNIDO)

The other process to produce potassium sulfate is based on the recovery of potassium sulfate from natural complex salts, such as Kainitine (KCl路MgSO 4路3H2O), Langbeinite (K2SO4路2MgSO4) and Carpathian polymineral ores.

POTASSIUM NITRATE Potassium nitrate is the third most used fertilizer salt. It is produced from a Chilean mineral containing sodium nitrate, potassium nitrate, some chlorides and sulfates. Sodium nitrate is added and then sodium chloride is separated. Potassium nitrate crystals are obtained after cooling. There are some different industrial processes for the production of this salt. In Israel, nitric acid and potassium chloride react at low temperature following this equation:

Figure 12. Israel Mining Industries (IMI) Potassium Nitrate Production Process (Figure 15.9, pg 429, UNIDO) Nitric acid and potassium chloride are put in contact and in two reactors in series (R-3, R-4). The first reactor receives also a brine recycle from the centrifuge (S-6) where the crystals of potassium nitrate are washed, and a diluted stream of HNO 3. After the reactor, the product stream is led to a decanter (S-5), where the main product is centrifuged, dried (D-7) and stored, and the solvent stream is treated in a set of settlers and evaporators.

Another process consists on working at higher temperature, taking the advantage of the oxidizing power of nitric acid.

Figure 13. High Temperature Potassium Nitrate Process (Figure 15.10, pg 429, UNIDO)

REFERENCES Angel Vian Ortuño (1994), Introducción a la Química Industrial, 2 nded, Spain, Reverté UNIDO, IFDC (1979), Fertilizer Manual, Kluwer Academic Pub.

7. EFFLUENT TREATMENT UREA MANUFACTURE & TREATMENT Introduction Urea is a nitrogenous compound that can be used for fertiliser as its nitrogenous properties allow plants to attain the basic molecular building blocks to synthesize amino acids and ultimately biomass, for growth and repair. It is of particular interest to the agricultural industry as farmers can introduce urea, via fertiliser granules to their soil which, increases soil fertility and ultimately allows for optimal soil condition for crop growth. Urea is formed via the reaction of ammonia and carbon dioxide, in the industrial manufacture of urea, the feed streams of the reactants are exposed to relatively high pressures and temperatures in a two-step reaction. However before the aforementioned urea synthesis can occur, ammonia must be synthesised from basic chemical elements, nitrogen and hydrogen. Therefore a typical urea manufacturing plant will operate continuous processes for ammonia and urea synthesis. Commercial manufacture of urea produces substantial amounts of liquid effluent usually in the form of water, typically for every one mole of urea formed there is also a mole of water formed as well. Therefore a considerable amount of effluent is produced via the reactions, however water must also be introduced into the process for cooling requirements which adds to the volume of the wastewater discharge stream. For example a typical 1900 ton/day urea production plant produces about 22,500 kg/hr of water from the reactions and an additional 9000 kg/hr is produced from condensing steam from the vacuum jets, which is mixed with the water obtained from the urea formation reactions, to give an average hourly effluent production of 31,500 kg/hr or 756 ton/day of process wastewater. However this effluent cannot be directly discharged to a surface body of water, the stream will contain considerable amounts of unused raw materials (ammonia and carbon dioxide) as well as product (urea) (Stokes, 1985). It is economically viable to treat this wastewater to remove any trace of raw materials and product, also the discharge of water to the environment containing both urea and ammonia can have severely negative implications to the environment, because urea promotes growth of plants, the discharge of wastewater effluents from a urea manufacturing facility has to be carefully monitored to ensure the effluent treatment facility removes sufficient amount of both urea and trace raw materials, as this can be particularly harmful and toxic to aquatic life. The presence of urea in surface bodies of water such as ponds or rivers can promote the phenomena of eutrophication. The nitrogenous properties from the excess urea within the aquatic based ecosystem are taken advantage of by water-based autotrophs, such as algae, therefore the additional nutrients present in the water cause the algae to respond by multiplying in an exponential manner, called an algal bloom. This consequently induces a stage of hypoxia to the surface body of water, which resultantly causes a reduction in aquatic life as there is insufficient oxygen for 87

organisms to respire, hence survive. In addition to eutrophication, ammonia present within the wastewater either from unutilised raw materials or urea hydrolysis can be toxic to fish, which results in a declining populations, but ammonia can also be harmful to other animals, including humans, which in small doses can cause skin irritation or burning, and in significant doses or ingestion can cause immediate death. For this reason environment agencies implement harsh regulation for the removal of urea and ammonia from the wastewater, with current widely accepted regulation calling for a maximum urea-wastewater concentration of 10ppm, which means that state of the art treatment facilities must be installed, to meet the regulation whilst efficiently treating a continuous stream of wastewater (Barmaki, 2009). Due to the large volumes of water handled during the urea manufacturing process, not all urea plants will have adequate access to readily abundant fresh water sources which can provide such a throughput of water, therefore the treatment and recycling of water for reuse as cooling water or boiler feed water is important to plant operation. Conversely the treatment of wastewater from the urea plant has two main advantages, the first being the recovery of raw materials which can be recycled for use as feedstock during the urea synthesis stage, and secondly for the reuse of process water as a heating or cooling fluid, which if contains any trace of urea can corrode process equipment resulting in a loss of production.

WASTEWATER TREATMENT PROCESS

Prior to wastewater entering the treatment process, the wastewater is collected in large storage tanks, Rahimpour (2010), speculates that such a wastewater stream prior to treatment contains typically; 0.3-15wt% urea, 0.8-6wt% carbon dioxide and 2-9wt% ammonia. Usual procedure to remove and recover urea and ammonia from the waste effluent stream can be broken down into four distinct steps: 1. Desorb any pre-existing ammonia and carbon dioxide from the waste water stream via a stripping column, leaving urea as the primary component within the waste effluent stream. 2. Hydrolyse the urea containing wastewater stream, to its constituent components, ammonia and carbon dioxide via thermal hydrolysis. 3. Total desorption of ammonia and carbon dioxide formed in the urea thermal hydrolyser. 4. Condense the off gases which contain the ammonia and carbon dioxide via a reflux condenser, which can then be used as feed material for the urea synthesis stage. The wastewater treatment section within a typical urea manufacturing plant will consist of a combination of desorber column and a hydrolyser, to ensure the aforementioned steps are carried out efficiently. Wastewater will be stored in a large storage tank, prior to effluent treatment, due to the large volume of process water produced on a continuous urea production plant, the treatment operation is usually continuous, and 88

therefore the wastewater feed is continually pumped from the storage tank into the treatment process. The whole reaction scheme is given by Figure 1.

Figure 1, Schematic diagram of a common urea treatment facility

First Desorber Column During the first stage of the treatment process wastewater enters a heat exchanger which the untreated wastewater is used as a cooling fluid, it absorbs heat from the outlet effluent from the second desorber, bringing about an overall temperature decrease in the treated effluent which then exits the treatment facility. Once the untreated wastewater exits the heat exchanger it then enters the top of the first desorber column, which operates at low pressure. The first desorber column aims to complete the first step out of the aforementioned four steps, hence it reduces the concentrations of both ammonia and carbon dioxide from the wastewater, leaving only a significant urea concentration present within the effluent. The desorber column works by introducing the wastewater stream at the top of the column and introducing off-gases from the second desorber, mainly steam containing carbon dioxide and ammonia, which is used for stripping within the first desorber column. The steam then flows upwards and the wastewater effluent flow downwards through the column in a counter-current regime, which in turn strips the remaining ammonia and carbon dioxide from the wastewater. The gas then exits from an outlet at the top of the column and the outlet gases go straight to a condenser to be condensed and reintroduced as feedstock to the urea manufacturing process as ammonia and carbon dioxide feedstock, the fourth step of 89

the aforementioned treatment steps. The wastewater effluent then exits the column at the bottom of the column with the ammonia and carbon dioxide concentrations significantly reduced. Therefore at the next urea hydrolysis stage, the system is maintained far from equilibrium, optimising the hydrolysis of urea (Barmaki, 2009)(Rahimpour, 2010). Hydrolyser Column Wastewater from the first desorber is pumped into the bottom of the hydrolyser column, which it immediately meets high pressure steam (25 bar) injected at the bottom of the column, this is to maintain a temperature of 185°C for thermal hydrolysis, therefore resulting in the almost instantaneous vaporization of the liquid effluent, which travels up through the hydrolyser column. The overall hydrolyser column is operated at medium pressure, and the hydrolysis reactions are shifted so that they proceed towards the production of ammonia and carbon dioxide (hydrolysis of urea), which are present in the outlet steam stream from the hydrolyser. After the hydrolyser the urea concentration will be significantly reduced to around 20ppm (Rahimpour, 2010). The hydrolysis stage is essential to the effluent treatment plant, for this reason control of the effluent residence time is important to ensure sufficient hydrolysis, a typical effluent residence time of the hydrolyser is about 1 hour (Stokes, 1985).

Hydrolysis Equations The liquid effluent solution from the manufacture of urea consists of a range of dissolved molecules and ions, mainly; CO(NH2)2 (urea), CO2, NH3, H2NCOO- and NH4+, this range of molecules and ions are representative of both stages of the overall urea hydrolysis, which is a two-step process (Rahimpour, 2010). The overall reaction for the hydrolysis of urea to its constituent components is: CO(NH2)2 + H2O

→

2NH3 + CO2

However this can be broken down further into two distinct steps, the first step which is exothermic by the magnitude of ∆H = -23 kJ/mol and involves the production of ammonium carbamate: CO(NH2)2 + H2O

→

H2NCOO- NH4+

Ammonium carbamate then reacts and rearranges atoms to produce the two initial reactants for urea production, ammonia and carbon dioxide. H2NCOO- NH4+

→

2NH3 + CO2

The second stage reaction of the overall urea hydrolysis is endothermic by the magnitude of ∆H = +84 kJ/mol, meaning that when the reaction enthalpies from the two stages are combined the overall urea hydrolysis reaction is endothermic ∆H = +61

kJ/mol, therefore heat is required to ensure optimal hydrolysis of urea, which is provided by the injection of high pressure steam, hence thermal hydrolysis occurs. However the initial reaction of urea with water molecules is slow in comparison to the rate of conversion of ammonium carbamate to ammonia and carbon dioxide, which is considerably faster therefore could be considered to be at equilibrium. For this reason, the first step in the hydrolysis of urea is considered as the rate determining step (Rahimpour, 2010) (Barmaki, 2009).

Second Desorber Column The second desorber column is operated at low pressure, with the injection of low pressure steam as a stripping agent to remove carbon dioxide and ammonia in the gaseous form from the effluent, which then exits out the top of the desorber column. The treated effluent is discharged from the bottom of the desorber column. The outlet vapour from the column that contains, carbon dioxide and ammonia, is used as a stripping agent in the aforementioned first desorber column, whereas for the liquid phase the treatment process is complete, so the treated effluent passes out of the treatment plant for discharge or further storage and reuse.

Case Study: Treatment Plant Operating Conditions Examples of industrial urea treatment plants can be found by within literature for example, Rahimpour (2010), has analysed specific stages within a treatment plant with a throughput of approximately 32m3/hr of wastewater. Within this in depth analysis the author calculated various effluent stream conditions and throughput, for a conventional urea industrial treatment plant:

Temperature (oC)

Wastewater Feed

Treated Outlet

Effluent 1st Desorber Vapour Outlet 124

Component Flow Rate (kg/hr) Water

30,200

33,040

3420

Urea

426

% Removal

99.3

CO2

649

959.2

% Removal

100

NH3

896

1132

% Removal

99.7

Total (kg/hr)

31,971

33,046

5511

Table 1, Specific stream (inlet & outlet) and unit op (1 st desorber) process conditions, (Rahimpour, 2010) Table 1, shows the outlet water flow rate is greater than the water input, which is can be accounted for by the additional water input to the process via the multiple high and low pressure steam inlets into the hydrolyser and second desorber respectively. The stream conditions of these inputs can be given by: High Pressure Steam

Low Pressure Steam

Temperature (oC)

380

145

Water (kg/hr)

1000

6600

Table 2, Specific process conditions for high and low pressure steam injection into the hydrolyser column and 2nd desorber respectively, (Rahimpour, 2010). Therefore this addition of water in the form of steam, will account for an overall increase in wastewater flow rate from the process, as within the desorber and hydrolyser steam and wastewater will come into direct contact with the steam, hence some of the steam will condense into the liquid phase. Table 1, also shows that relatively a relatively high percentage removal can obtained via the treatment process depicted in Figure 1. Removal of all wastewater components approaches essentially 100%, therefore it can be safely assumed that the treated effluent complies with the relevant agency regulation.

Role of Temperature in Thermal Hydrolysis Temperature plays an important role in the thermal hydrolysis of urea, the higher the temperature the more effective the thermal hydrolysis is, which is depicted in the following Figure 2. The Figure depicts that within the hydrolyser column, at effluent temperatures of about 200 oC, the outlet of stream contains no trace of urea, and therefore upto 100% hydrolysis of urea is obtained. Hence it is favourable to operate the hydrolyser column at a high temperature, 185 - 200oC, which is sustained by the use of high pressure steam injection (380 oC) within the process, thus an optimum effluent temperature can be maintained.

Figure 2, Outlet conversion and mass fraction of urea versus inlet temperature of wastewater in hydrolyser (Barmaki, 2009) As previously defined, the overall hydrolysis reaction for ammonia is endothermic, therefore heat addition is required to ensure optimum hydrolysis, meaning the more heat added the more efficient the hydrolysis process, which is evident in Figure 2. However given the sizeable amount of heat addition required, it may not be economical to operate at 200oC, therefore a compromise temperature of 185 oC, is the selected operating temperature by Barmaki(2009), which undoubtedly weighs up the economic viability vs the hydrolyser performance.

Design of the Desorber Column (or Gas Stripper)(Metcalf & Eddy, 2002 - Chapter 11) Stripping is the transfer of molecules from the liquid phase (wastewater effluent) to a gaseous phase (steam), the transfer occurs by contacting a liquid containing the dissolved gas to be removed, with the stripper usually either air or steam. When considering the fundamentals behind gas stripping, the key factors that must be considered are: 1. Characteristics of the compounds to be stripped. 2. The type of contactor and the number of steps required for efficient gaseous stripping. 93

3. Material mass balance within the stripping column. 4. Physical features and dimensions of the stripping column required. 1. Characteristics of the compounds to be stripped, the compound to be stripped will transfer from the liquid phase to the gas phase, therefore exit the strippping column via the gaseous outlet, and must satisfy Henryâ&#x20AC;&#x2122;s Equilibrium Law, the amount of gas the dissolves within a liquid is proportional to the partial pressure of the compound. For Ammonia the partial pressure is about 0.75 atm, hence stripping of ammonia involves removing it from the liquid effluent into the gaseous phase, which is showed by the following equation: NH4+(aq)

â&#x2020;&#x201D;

NH3 (g) + H+(aq)

It is advantageous to the process to raise the pH above 7 effectively creating an alkaline solution by adding lime to the wastewater. This will shift the above equilibrium to the left, producing ammonium ions, using Le Chatelierâ&#x20AC;&#x2122;s principle, the equilibrium will counteract this change, by converting the ammonium ions to ammonia gas which can then be removed by the gaseous stripper. When considering contacting the liquid and gaseous phases there are two common methods, continuous contact and staged contact. Once selecting the contact methods, the flow pattern must then be considered, there are commonly three effective flow patterns, co-current, counter-current and cross-current, and within in the industry the most widely utilised flow pattern is counter-current.

Figure 3. Possible flow patterns within the Desorber Column, (Metcalf & Eddy, 2002)

Mass Balance Considering the mass balance within a packed stripper column utilised for the removal of dissolved gas i.e. ammonia or carbon dioxide, with a counter-current flow arrangement, can be given by an overall mass balance:

Flow in = Flow out

Figure 4. Counter-current mass balance Adapting the terms to the previous above equation, flow in and flow out, can be given by:

Where, L - Flow rate incoming moles of wastewater (mole/hr), C â&#x20AC;&#x201C;Aqueous solute concentration within the column (mol-solute/mol-liquid), G - Flow rate of gas in (mole/hr), yo - Solute gas concentration in (mol-solute/mol-gas), CeAqueous solute concentration exiting the column (mol-solute/mol-liquid), y - Solute gas concentration within the column (mol-solute/mol-gas)

This can be rearranged to:

This equation considers the internal equilibria which governs the magnitude of mass transfer. The equation can be plotted, to give a straight line of slope L/G, which will pass through (Ce, yo) and (C, y), this is known as the operating line and can represent internal column conditions at any point. The operating line runs adjacent to the equilibrium line, which is based on Henryâ&#x20AC;&#x2122;s law and is temperature dependant, thus varies with temperature. It should be noted that when a gas is being stripped from solution the operating line will always lie below the equilibrium line. The operating line is depicted in Figure 5A.

Figure 5. Equilibrium and Operating Lines for a variety of conditions inside the gas desorption column, adapted from Metalfe & Eddy (2002).

If the gaseous inlet stream contains no dissolved gas solute (which can be assumed to be 100% present in the liquid phase), then y ocan be dropped, and the aforementioned equation rearranges to:

The changes in the operating line can be represented by Figure 5B.

Using Henryâ&#x20AC;&#x2122;s law:

Where, y - outlet concentration of the gaseous stream, k H - Henryâ&#x20AC;&#x2122;s law constant, PT Total Pressure (~1atm), C* - Outlet aqueous equilibrium solute concentration.

Therefore substituting this into the aforementioned equation:

Assuming the aqueous solute inlet concentration immediately assumes equilibrium with the gaseous outlet stream, the equation can be rearranged to:

The changes in the operating line can be represented by Figure 5C. This equation represents the minimum amount of gas required to strip a compound form the aqueous stream. Finally if it is further assumed that the liquid outlet contains no solute, therefore 100% solute mass transfer to the aqueous phase, then the C e term can be dropped and the two C terms cancel, meaning:

The apparent change in the operating line, depicted in Figure 5D, is that it approaches values equivalent to the equilibrium line, as the column now assumes equilibriumbased conditions.

Key Design Equations

Without deriving each equation individually the height of the stripper (desorber) column (Z) can be given by:

Where, HTU - Height of the transfer unit, NTU - Number of transfer units Alternatively the exact definition of the stripper column height can be given by:

Where

L - Liquid volumetric flow rate (m3/s), KLa - Mass transfer coefficient (1/s), A - Column cross-sectional area (m2), HTU is a measure of mass transfer characteristics of the column packing material. 97

and,

Assuming that the stripping factor S, can be given by:

For each value of S, if: S = 1, corresponds to the minimum amount of gas required for stripping S > 1, excess amount of gas present, however complete gas stripping possible S < 1, Insufficient gas present for stripping However a general rule of thumb is the stripping factors vary between 1.5 - 5.

Therefore a basic stripping column should be able to be designed from these expressions. However a stripping column in its simplest form consists of an outer column shell, a plate that supports the internal packing located at the top of the column for counter current flow, a liquid distribution system, and gas supply system located at the bottom of the column for counter current flow. Therefore key process variables that must be satisfied during the basic unit design are: 1. Packing material type 2. Stripping factor 3. Column cross-sectional area 4. Height of the stripping column, Z When considering the column cross-section, this will be dependent of the column pressure drop through the packing. The column head loss is dependent on the gas pressure drop. Therefore ultimately, an acceptable design procedure, for a stripping column or desorber: 1. Packing material selection, using the packing factor, 2. 3. 4. 5. 6. 7.

Select a suitable stripping facto Select a suitable pressure drop ( ), packing dependant Determine the cross-sectional area, based on the allowable pressure drop Determine the HTU Determine the NTU Determine the height of the stripper column, Z

Internal Desorption Column Structure A description of the theory behind the stripping or desorption of ammonia and carbon dioxide has been previously outlined. However considerations must be made for the internal structures within a typical desorption column. The first big consideration is whether the column should contain plates or packing material. For plates the same flow arrangement is maintained as for packing, i.e. gas introduced at the bottom, liquid at the top. However the liquid will flow down over the plates and the gas will flow upwards through the plates, via special holes within the plate, this ensures sufficient contact between the effluent and gaseous stripper. The other option is the use of spherical packing material within the column (Figure 6), the same aforementioned flow arrangement is maintained, with the liquid flowing down and gas flowing up, but however the fluids will come into contact between the voids in the packing material, where molecule stripping from the liquid phase can occur.

Figure 6, Cross-section through a typical â&#x20AC;&#x2DC;Packedâ&#x20AC;&#x2122; Gas Absorption Column Figure 6, depicts the typical arrangement and structure of a packed gas absorption column (or desorption in the current case), the stripper (steam) is introduced at the bottom of the column and flows up through the packing supports and up through the voids between the packing material. The effluent is introduced at the top of the column and is pumped into a liquid distribution system, where is it evenly sprayed across the entire diameter of the column. Subsequently the liquid will flow down through the packing material via gravity, and come into direct contact with the steam, where the dissolved gases (ammonia and carbon dioxide) can be transferred to the gaseous phase and exit through the top of the column. When the liquid reaches the bottom of 99

the column, it accumulates into a pool, before being discharged from the column. The effluent pool at the bottom of the column allows for further contact with the fresh steam inlet, where further molecular desorption can occur.

REFERENCES

Barmaki M. (2009), Treatment of wastewater polluted with urea by counter-current thermal hydrolysis in an industrial urea plant, Separation and Purification Technology, Vol. 66, Pg 492-503 Metcalf & Eddy (2002), Wastewater Engineering: Treatment and Reuse, 4 th Edition, New York City, NY, McGraw-Hill Higher Education Rahimpour M. (2010), Enhancement of urea, ammonia and carbon Dioxide removal from industrial wastewater using cascade of hydrolyser-desorber loops, Chemical Engineering Journal, Vol. 160, Pg 594-606 Stokes J. K (1985), US Patent Number: 4,552,979, Greenwich, Connecticut

100

8. LEGISLATION

Europe The following legislation applies only to member states of the European Union (EU), shown below in figure 1.

Water Framework Directive (WFD) (2000) The Water Framework directive establishes a legal framework to protect and restore clean water across Europe and ensure its long-term, sustainable use. It aims to achieve good chemical status of both surface water and groundwater bodies throughout the European Union. It offers a new legislative approach in the way it is not based on national or political boundaries, but on natural geographical and hydrological formations. This is a far more effective approach since rivers do not stop at national frontiers. It will coordinate many pre-existing EU policies and will follow a precise time for action. A unified and well-organised effort can achieve far more than a mix of different, isolated legislation from individual countries. It has been introduced due to inconsistencies in the previous legislation which covered different aspects of water management. It aims to introduce a simpler, more universal approach that will result in greater protection for European water networks.

Figure 1: EU Member States (http://upload.wikimedia.org/wikipedia/commons/5/59/Member_States_of_the_European_Union_%2 8polar_stereographic_projection%29_EN.svg)

101

Key points  Each member state of the EU must produce river basin management plans (RBMPs) for each river basin district under their control, every six years (the current cycle set to expire in 2015). These describe the river basin district, and the pressures that the water environment faces(identifying point and diffuse pollution sources). Furthermore they evaluate how the river basin district has been affected by these pressures and the actions that have been taken to address the pressures. Finally they set out the improvements that can be made before a new RBMP will be produced and evaluate how these will affect the local environment. A map of the RBDs of Europe is shown in figure 2.  It includes innovative principles for water management, including public participation in planning and economic approaches, including the recovery of the cost of water services.  Considers pollution in terms of what is related into the environment and the resilience of the receiving waters.  The framework aims to rank river basin districts based upon ecological and chemical status:

 

Ecological Status – defined as the abundance of aquatic flora and fish fauna, the availability of nutrients, and aspects such as salinity, temperature and pollution by chemical pollutants.

Chemical Status – environmental quality standards (EQS) have been established for 33 new and 8 previously regulated chemical pollutants of high concern across the EU. It is supported by the Directive for Integrated Pollution and Prevention Control (IPPC) for industrial installations.

The requirement to meet EQS for priority substances prevents the deterioration of waters by the accumulation of pollution from multiple sources. There are different rules and ranking systems for surface water and groundwater: o

Surface water must meet good ecological status, which provides a measure of healthy ecosystems.

Groundwater requires good chemical status. This is reinforced by the 2006 Groundwater Directive, which specifies measures to assess, monitor and control groundwater pollution.

102

Figure 2: River Basin Districts of Europe (http://fate-gis.jrc.ec.europa.eu/geohub/MapViewer.aspx?id=2)

Supporting Documents The WFD is supported by:   

  

REACH Regulation – controls chemicals in products to reduce the contaminants of water bodies. Directive on Plant Protection Products – controls pollution from agricultural chemicals by limiting substances that can be used in pesticides. Biocidal Products Directive – regulates pest-control and anti-microbial substances used in other sectors, limiting their placement in EU markets. This is set to be replaced in September 2013 by the EU Biocidal Products Regulation. Nitrates Directive – limits nitrogen pollution from fertilisers and manure. Directive on Integrated Pollution Prevention and Control (IPPC) – regulates pollution from factories and other facilities. Bathing Water Directive – establishes further controls to protect human health in recreational waters. All countries in the EU must ensure that their bathing waters meet the quality standards specified by the directive. 103

Priority Hazardous Substances As mentioned above the WFD sets out requirements for certain ‘priority substances’. There are 33 priority substances, of which 13 are described as being hazardous. This list is revised every 4 years by the European Commission. 

Key Characteristics – persistent, bio-accumulative and toxic (PBTs). Also of concern are endocrine disruptors (chemicals with the potential to interfere with the hormonal cycles of humans and wildlife). o PBTs –  Do not break down in the environment and collect in animal and plant tissues.  Concentrations increase as they ascend the food chain.  Pose long-term risks to human health and ecosystems.

 

Selection criteria relate to their behaviour in the environment. WFD seeks to progressively reduce emissions, discharges and losses of priority substances to waters. Priority substances must be reduced whilst hazardous substances must be phased out completely from European water networks within 20 years. The Environmental Quality Standards Directive sets out standards concerning the presence of pollutants in surface water. There are 2 environmental quality standards for priority substances: o (i) – Annual average concentration. o (ii) – Maximum allowable concentration.

 

List of priority substances:                    

Alachlor Atrazine Benzene Chlorfenvinphos Chlorpyrifos 1,2-Dichloroethane Dichloromethane Di(2-ethylhexyl)phthalate (DEHP) Diuron Fluoranthene Isoproturon Lead and its compounds Naphthalene Nickel and its compounds Octylphenols Pentachlorophenol Simazine Trichlorobenzenes Trichloromethane Trifluralin 104

List of priority hazardous substances:             

Anthracene Pentabromodiphenylether Cadmium and its compounds C10-13-chloroalkanes Endosulfan Hexachlorobenzene Hexachlorobutadiene Hexachlorocyclohexane Mercury and its compounds Nonylphenols Pentachlorobenzene Polyaromatic hydrocarbons Tributyltin compounds

Urban Wastewater Treatment Directive (1991) This legislation applies to domestic and industrial wastewater.  

Its aim is to protect the environment from the adverse effects urban wastewater discharges and discharges from certain industrial areas. Requires the collection and treatment of wastewater from agglomerations with a population equivalent of over 2000. Wastewater from agglomerations with a population equivalent of greater than 10,000 will require advanced treatment if located in a sensitive area. o Population equivalent (PE) – a number expressing the ratio of the sum of pollution loads produced during 24 hours by industrial facilities compared to an individual pollution load in household sewage produced by one person in the same time.

BOD – Biochemical oxygen demand is a measure of the quantity of oxygen used by microorganisms in the oxidation of organic matter. Sensitive areas are defined as : o (i) – freshwater bodies, estuaries, and coastal waters which are eutrophic or which may become eutrophic if protective action is not taken o (ii) – surface freshwaters intended for the abstraction of drinking water which contain or are likely to contain more and 50 mg/L of nitrates. o (iii) – areas where further treatment is necessary to comply with other directives e.g. for the conservation of wildlife or for bathing water protection. o



105

Nitrates Directive (1991) The Nitrates Directive is a key document within the WFD and is crucial for the protection of waters against agricultural pressures. It aims to protect water quality within the EU by preventing nitrates from agricultural sources polluting ground and surface waters and by promoting the use of good farming practices. Requires the: 

 



 

Identification of polluted water, or water at risk of pollution. o Surface freshwaters and groundwater, particularly those used or intended for the abstraction of drinking water, should not contain more than 50 mg/L of nitrates. o Polluted waters include all freshwater bodies, estuaries, coastal waters and marine waters found to be eutrophic. Designation of Nitrogen Vulnerable Zones (NVZs). o These are areas of land which drain into polluted waters. Establishment of codes of good practice for farmers. These code should include: o Measures that limit the periods when nitrogen fertilizers can be applied on land, to instead encourage application during periods when crops require more nitrogen and prevent the loss of nitrates to water bodies. o Measures that limit the conditions for fertilizer application (e.g. on steeply sloping land, frozen or snow covered ground, etc.) to prevent nitrate losses from leaching and run-off. o A requirement for a minimum storage requirement for livestock manure. The development of specific action programmes for compulsory implementation by farmers located in NVZs. These programmes must include: o Measures already laid out in the codes of good practice. o Limitations on fertilizer application (mineral and organic). National monitoring and reporting. Every four years Member States are required to report on: o Nitrates concentrations in groundwaters and surface waters. o Eutrophication of surface waters. o An assessment of the effect of action programmes on water quality and agricultural practices. o A revision of NVZs and action programmes. o An estimation of future trends in water quality. A map of the NVZs of the European Union is shown in figure 3. Figures 4 and 5 show eutrophic waters within Europe in 2000 and 2010 respectively. It can be observed that improvements have been made during this decade, as there were less eutrophic waters in 2010. This has presumably occurred as a consequence of the Nitrates Directive.

106

Figure 3: NVZs of Europe (http://fate-gis.jrc.ec.europa.eu/geohub/MapViewer.aspx?id=2)

REACH – Registration, Evaluation, Authorisation and Restriction of Chemicals (2007)  

Aims to ensure the protection of human health and the environment from the potential risks of chemicals. Requires that industries assess and manage the risks posed by chemicals and provide appropriate safety information to their users.

CLP – Classification, Labelling and Packaging (2009)  

Ensures that the hazards posed by chemicals are clearly communicated to workers and consumers through the classification and labelling of products. Industries must identify the potential risks to human health and the environment of the chemicals that they produce. Hazardous chemicals must be labelled according to a standardised system.

107

Figure 4: Exceedances of critical loads for eutrophication due to the deposition of nutrient nitrogen in 2000

Figure 5: Exceedances of critical loads for eutrophication due to the deposition of nutrient nitrogen in 2010 (Figures 4 and 5 taken from http://www.eea.europa.eu/soer/synthesis/synthesis/chapter3.xhtml) 108

))

IPPC – Integrated Pollution and Prevention Control (1996)  

Regulates pollution from larger industrial installations. Concerns itself with pollution water, air and land. Requires installations to have a permit containing emission limit values and other conditions based upon the application of Best Available Techniques (BATs). o Emission limit values – the mass, concentration and/or level of an emission, which may not be exceeded during one or more periods of time.

Dangerous Substances Directive (1976) 

 

Designed to control pollution caused by certain dangerous substances discharged to water bodies. These are classified into two lists: o (i) – List I is for substances regarded as being dangerous due their toxicity, persistence and bioaccumulation (PBTs). These must be eliminated from water bodies. o (ii) – List II is for substances which are less dangerous but still have a deleterious effect on aquatic environments. These must be reduced from water bodies. List I substances can be controlled using either uniform emissions standards (UESs) (also known as limit values) or environmental quality standards (EQSs). For list II substances member states are required to set EQSs developed on a national level.

List I  Organohalogen compounds and substances which may form such compounds in the aquatic environment.  Organophosphorus compounds.  Organotin compounds.  Substances in respect of which it has been proved that they possess carcinogenic in or via the aquatic environment.  Mercury and its compounds.  Cadmium and its compounds.  Persistent mineral oils and hydrocarbons of petroleum origin.  Persistent synthetic substances which may float, remain in suspension or sink and which interfere with any use of the water. List II  The following metalloids and metals and their compounds: o Zinc o Copper o Nickel o Chromium o Lead o Selenium o Arsenic o Antimony 109

o o o o o o o o o o o o  



   

Molybdenum Titanium Tin Barium Beryllium Boron Uranium Vanadium Cobalt Thallium Tellurium Silver

Biocides and their derivatives not appearing on list I (including pesticides). Substances which have a deleterious effect on the taste and/or smell of the products for human consumption derived from the aquatic environment and compounds liable to give rise to such substances in water. Toxic or persistent organic compounds of silicon, and substances which may give rise to such compounds in water, excluding those which are biologically harmless or which are rapidly converted in water into harmless substances. Inorganic compounds of phosphorus and elemental phosphorus. Non persistent mineral oils and hydrocarbons of petroleum origin. Cyanides, fluorides. Substances which have an adverse effect on the oxygen balance, particularly: ammonia, nitrites.

Groundwater Directive (1980)  This is an extension of the Dangerous Substances Directive to provide more protection to groundwater. It requires that: o List I substances must not enter groundwater. o List II substances must be restricted from entering groundwater.

United States of America Clean Water Act (CWA) (1972) The clean water act is a fundamental document with relation to water quality standards within the USA. It founds the basic structure for regulating discharges of pollutants into American water bodies and for regulating quality standards for surface waters. The CWA has made it illegal to discharge any pollutant from a point source into navigable waters, unless a permit is obtained. These are controlled by the National Pollutant Discharge Elimination System (NPDES) programme. As a result of the CWA the US Environmental Protection Agency (EPA) has instigatedpollution control programmes e.g. setting water quality standards for industry. In addition, water quality standards for all contaminants have been set by the EPA. Through the NPDES the EPA ensures that industries pre-treat pollutants in their wastes in order to protect local sanitary sewers and wastewater treatment plans. 110

The CWA is supported by the Safe Drinking Water Act (1974), which provides greater protection for public drinking water supplies.

References Anon., n.d. epa.gov. [Online] Available at: http://www2.epa.gov/laws-regulations/summary-clean-water-act [Accessed March 2013]. Anon., n.d. europa.eu. [Online] Available at: http://ec.europa.eu/environment/pubs/pdf/factsheets/water-frameworkdirective.pdf [Accessed March 2013]. Anon., n.d. europa.eu. [Online] Available at: http://ec.europa.eu/environment/pubs/pdf/factsheets/nitrates.pdf [Accessed March 2013]. Anon., n.d. euwfd.com. [Online] Available at: http://www.euwfd.com/html/wfd_-_a_summary.html [Accessed March 2013]. Anon., n.d. ukmarinesac.org.uk. [Online] Available at: http://www.ukmarinesac.org.uk/activities/water-quality/wq1_3.htm [Accessed March 2013].

111

9. CASE STUDY REVIEW Water Reuse in Europe Overview  



 

Average EU water consumption is around 32% of total water abstraction. Most abstracted water is returned to the water cycle. Around 75% of abstracted water in the EU comes from surface water and about 25% from groundwater. Reused treated wastewater and desalinated seawater make only small contributions. Figure 1 shows how water use varies significantly between countries. Mediterranean countries with warmer climates such as Greece, Italy, Portugal and Spain use a much larger proportion of water for use in agriculture. On average 30% of abstracted water in the EU is used in agriculture (see figure 2). There are currently over 700 water reuse projects in Europe, of which they are more commonly located in Southern European countries; however there are also examples of projects being implemented in Northern countries e.g. Belgium, UK and Sweden.

Figure 6: Sectoral Water Use in Europe (Bixio et al., 2006)

112

Figure 7: Sectoral Water Abstraction in Europe (Marecos Do Monte, 2007)

Belgium 



 

Belgium has a high Water Exploitation Index (WEI). This is the mean annual total demand for freshwater divided by the long-term average freshwater resources. This means that the water consumption is unsustainable and it is likely that there will be shortages in the future without intervention. A high strain on water supplies and the fact that Belgian waters are highly sensitive means that a large amount of water in Belgium is reused for other purposes. All urban wastewater in Belgium is treated for reuse. An important reuse project exists in Wulpen, where water is treated and reused for indirect potable water supply. This plant treats 2.5 million m 3/year of water by microfiltration and reverse osmosis. The effluent is stored for 1-2 months in an aquifer prior to abstraction.

Cyprus 

In Cyprus about 25 Mm3/year of wastewater is collected and used for irrigation after tertiary treatment. There are currently 25 wastewater recycling plants in operation. Water has a high transportation cost, therefore a majority of recycled water, about 50-60%, is used for amenity purposes in hotel gardens, parks and golf courses. About 10 Mm3 of wastewater is used for agricultural irrigation.

France 



There are currently over 17,500 wastewater treatment plants in operation in France, whilst the total volume of wastewater reused in 2004 was 19,178 m3/day Water reuse projects have grown in popularity in France due to a water deficit in some regions in addition to the need to reduce effluent discharges in recreational and sensitive areas. Most reuse projects are located on the Mediterranean coast, where the climate is warmer; however there is also interest in the historically wetter areas of Western and North-western France, which have suffered severe droughts in recent decades. 113

 



Treated wastewater is mainly used for agricultural irrigation, and currently over 3000 ha of land are irrigated in this way. Clermont-Ferrand in central France is the location of one of the largest water reuse projects in Europe. Over 700 ha of maize are irrigated with the effluent of an activated sludge plant followed by maturation ponds. Industrial wastewater is also widely used to supply cooling water, wash water or process water.

Greece 



Mainland Greece is a mountainous area that suffers from uneven water distribution, whilst the islands of the Aegean and Ionian seas are often devoid of significant water resources. This means that Greece suffers from high water stress levels in localised areas only. All towns with a population of over 2000 people are either served or planned to be served by wastewater treatment plants.

Italy 

    

There is an estimated total treated effluent flow of 2400 Mm 3/year of usable water in Italy produced by over 10,000 wastewater treatment plants mainly utilizing tertiary treatment. The reuse projects of Italy are mainly located in Sicily, Sardinia and Puglia. Water is mainly reused for agriculture. Over 4000 ha of land is irrigated with treated wastewater. Approximately 60% of urban wastewater is treated for reuse. There is a large project in Emilia-Romagna in Northern Italy where over 250 ha of land are irrigated with over 450,000 m3/year of treated effluents. Currently new legislation is being introduced to allow better management of water resources, particularly for the reuse of wastewater.

Portugal  



In Portugal water is mainly reused for agriculture and landscape irrigation. An increasing amount of projects are being dedicated to the use of wastewater for golf course irrigation, largely located in the Algarve region of Southern Portugal, where water shortages are more common. Portugal has recently introduced new guidelines for the safe use of wastewater.

Spain   



The total volume of wastewater reused in Spain in 2004 was 1,117,808 m3/day. Uneven distribution of water resources in Spain has led to serious deficit problems in some areas. Water reuse projects are mainly located on the arid Mediterranean coast as well as around Madrid and Vitoria-Gasteiz in the Basque Country. The regions of Valencia and Murcia account for 57% of the total wastewater reuse in Spain, whilst 23% is used on the Balearic and Canary islands. Spain has published a new ‘National Hydrological Plan’ which encourages the use of treated wastewater for irrigation. This is already a reality in many parts of Spain where wastewater is used for golf course irrigation, agricultural irrigation, 114

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groundwater recharge (which stops saltwater intrusion to coastal aquifers) and river flow augmentation. Of the present water demand of approximately 35,000 hm 3/year, the agricultural sector makes the largest contribution of approximately 24,000 hm 3/ year. It has been estimated that 1100 hm3 of wastewater were used for agricultural purposes in 2012.

Sweden 

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Despite possessing rich water resources, Sweden has plans for wastewater reuse for irrigation purposes. This is part of an effort to preserve groundwater for better uses. There are currently over 40 reuse projects, which consist of effluent storage of up to 9 months before being used for irrigation. Sometimes the effluent is blended with surface water. Projects are primarily located in South-eastern Sweden, which has low precipitation.

UK 

The UK is another country with a vast supply of freshwater that plans to reuse wastewater. Reuse projects mainly intend to use wastewater for indirect potable uses, and direct reuse in golf course irrigation, road verges irrigation, cooling, fish farming and car washing.

References Bixio, D. et al., 2006. Wastewater reuse in Europe. Desalination, Volume 187, pp. 89101. Kamizoulis, G., Bahri, A., Brissaud, F. & Angelakis, A. N., 2003. Wastewater recycling and reuse practices in Mediterranean region: Recommended Guidelines. [Online] Available at: www.a-angelakis.gr/files/pubrep/recycling_med.pdf [Accessed March 2013]. Kellis, M., Kalavrouziotis, I. K. & Gikas, P., 2013. Review of Wastewater Reuse in the Mediterranean Countries, Focusing on Regulations and Policies for Industrial Applications. Global NEST Journal. Marecos do Monte, M. H., 2007. ewaonline.de. [Online] Available at: www.ewaonline.de/journal/2007_07.pdf [Accessed March 2013]. Pedrero, F. et al., 2010. Use of treated municipal wastewater in irrigated agriculture Review of some practices in Spain and Greece. Agricultural Waste Management, Volume 97, pp. 1233-1241.

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