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Wastewater Treatment The effective treatment of wastewater to meet water quality objectives for water reuse applications and to protect public health is a critical element of water reuse systems. Municipal/Industrial wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids, organic matter, pathogens, metals, and sometimes nutrients from wastewater. 2
General terms used to describe different degrees of treatment, in order of increasing treatment level are, preliminary, primary, secondary, tertiary, and advanced treatment. A disinfection step for control of pathogenic organisms is often the final treatment step prior to distribution or storage of reclaimed wastewater. 3
Goal: to design an integrated cost-effective treatment scheme that is capable of reliably meeting water quality objectives. The degree of treatment varies according to: 1. Specific reuse application 2. Associated water quality requirements
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Typical Wastewater Treatment scheme
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Generalized flow diagram for municipal wastewater treatment 8
The objective of preliminary treatment is the removal of coarse solids and other large materials often found in raw wastewater. Removal of these materials is necessary to enhance the operation and maintenance of subsequent treatment units. Pretreatment may include screening, pre-sedimentation, chemical addition, grit removal, and aeration.
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Preliminary Treatment The screens are used to remove rocks, sticks, leaves, and
other debris. Very small screens can be used to screen out algae in the
water. There are two primary types of screens - bar screens and wire-
mesh screens.
A wire-mesh screen
A bar screen 10
Preliminary Treatment
Grit chamber In grit chambers, the velocity of water through the chamber is maintained sufficiently high, or air is used, so as to prevent the
settling of most organic solids.
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Primary Treatment Primary sedimentation tanks or clarifiers may be round or rectangular basins, typically 3 to 5 m deep, with hydraulic retention time between 2 and 3 hours. Settled solids (primary sludge) are normally removed from the bottom of tanks by sludge rakes that scrape the sludge to a central well from which it is pumped to sludge processing units. Scum is swept across the tank surface by water jets or mechanical means from which it is also pumped to sludge processing units. For Colloids, the primary treatment processes is not a good option to removal. By incorporating coagulation/flocculation upstream of gravity sedimentation colloids can be removed. 12
Primary Treatment
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What is Coagulation? Coagulation is the destabilization of colloids by addition of chemicals that neutralize the negative charges The chemicals are known as coagulants, usually higher valence cationic salts (Al3+, Fe3+ etc.) â–
â–
Colloids can be destabilized by charge neutralization. Positively charges ions (Na+, Mg2+, Al3+, Fe3+ etc.) neutralize the colloidal negative charges and thus destabilize them. With destabilization, colloids aggregate in size and start to coagulate
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Colloid Stability Colloid: Colloids have a net negative surface charge • Electrostatic force prevents them from agglomeration • Brownian motion and particles-particles interaction keeps the colloids in suspension and thus, impossible to remove colloids by gravity settling. This is known as colloidal stability.
-- -Colloid - A
Colloid
Repulsion
H2O
-- -Colloid - B
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Relative coagulating power â?‘ Na+ = 1; Al3+ > 1000;
Mg2+ = 30 Fe3+ > 1000
â?‘ Typical coagulants: Aluminum sulfate: Al2(SO4)3.14 H2O Iron salt- Ferric sulfate:
Fe2(SO4)3
Iron salt- Ferric chloride: Fe2Cl3 Polyaluminum chloride (PAC): Al2(OH)3Cl3 16
Aluminum Chemistry With alum addition, what happens to water pH? Al2(SO4)3.14 H2O ⇔ 2Al(OH)3↓+ 8H2O + 3H2SO4 1 mole of alum consumes 6 moles of bicarbonate (HCO3-) Al2(SO4)3.14 H2O + 6HCO3- ⇔ 2Al(OH)3↓+ 6CO2 + 14H2O + 3SO4-2 If alkalinity is not enough, pH will reduce greatly Lime or sodium carbonate may be needed to neutralize the acid.
Optimum pH: 5.5 – 6.5 But pH is maintained near 8. 17
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Alkalinity calculation If 200 mg/L of alum to be added to achieve complete coagulation. How much alkalinity is consumed in mg/L as CaCO3?
Al2(SO4)3.14 H2O + 6HCO3- ⇔ 2Al(OH)3↓+ 6CO2 + 14H2O + 3SO4-2 594 mg
366 mg
594 mg alum consumes
200 mg alum will consume
156 mg =366 mg HCO3-
=(366/594) x 200 mg HCO3= 123 mg HCO3-
Alkalinity in mg/L as CaCO3
= 123 x (50/61) = 101 mg/L as CaCO3 19
Electrical Double Layer
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As a result of this EDL there is a net electrostatic repulsion/attraction developed between colloids.
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Coagulation Theories Double layer compression Adsorption and Charge Neutralization Adsorption and Interparticle bridging
Enmeshment in a precipitate (sweep floc)
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Flocculation ď‚— Flocculation is the agglomeration of destabilized
particles into a large size particles known as flocs by slow mixing which can be effectively removed by sedimentation or flotation. ď‚— The flocculation process can be enhanced by adding organic polymers. These compounds consists of long carbon chain with active groups such as amine, nitrogen or sulfate groups along the chain.
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Metcalf & Eddy 2003 26
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Sedimentation Basins Settling Solid liquid separation process in which a suspension is separated into two phases – Clarified supernatant leaving the top of the sedimentation tank (overflow) Concentrated sludge leaving the bottom of the sedimentation tank (underflow) Types of Settling Tanks Rectangular Circular
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Rectangular settling basins Rectangular settling, basins or clarifiers, are basins that are rectangular in plans and cross sections. The length may vary from two to four times the width. The length may also vary from ten to 20 times the depth. The depth of the basin may vary from 2 to 6 m. The influent is introduced at one end and allowed to flow through the length of the clarifier toward the other end.
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Rectangular settling basins A long rectangular settling tank can be divided into four different functional zones: Inlet zone: Region in which the flow is uniformly distributed over the cross section such that the flow through settling zone follows horizontal path. Settling zone: Settling occurs under quiescent conditions. Outlet zone: Clarified effluent is collected and discharge through outlet weir. Sludge zone: For collection of sludge below settling zone.
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Rectangular settling basins
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Circular Basins Circular settling basins have the same functional zones as the long rectangular basin, but the flow regime is different. When the flow enters at the center and is baffled to flow radially towards the perimeter, the horizontal velocity of the water is continuously decreasing as the distance from the center increases. Thus, the particle path in a circular basin is a parabola as opposed to the straight line path in the long rectangular tank. Sludge removal mechanisms in circular tanks are simpler and require less maintenance.
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Table Typical Dimensions of Sedimentation Tanks
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Lamella Clarifier Self-cleaning baffles are achieved with an inclination of 50
to 60 degrees. The spacing between lamellas is generally between 5 to 10 cm in wastewater treatment. The effective horizontal surface is the horizontal projection of each plate multiplied by plates number. This total projected surface value is used to calculate the hydraulic loading rate. An important factor to take into consideration is the critical scour velocity. This is a more compact equipment and with surface requirements considerably lower than in conventional circular and rectangular clarifiers. 34
Lamella Clarifier
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Secondary/Biological Treatment Biological Treatment Process: a method of contact
between microbes and substrate. Suitable temperature, pH, nutrients etc. are required for microbial growth. Such a growth results into the ‘removal’ of substrate. Objective of biological treatment: To stabilize the organic content To remove nutrients such as nitrogen and phosphorus 36
Biological Treatment Types: Aerobic Processes Anoxic Processes Anaerobic Processes Combined Aerobic-AnoxicAnaerobic Processes Pond Processes
Attached Growth Suspended Growth Combined Systems Aerobic Maturation Facultative Anaerobic
Aeration Aeration is a process that occurs naturally, not just in an aerator. Two purposes: To keep biomass, food and oxygen in contact (mixing) Oxygen supplied to bugs
In aerobic biological reactors, adequate DO must be maintained. The typical concentration range for most reactors is: 1.0 to 4.0 mg/L Adding dissolved oxygen to the mixed liquor creates the highest single electrical demand at most activated sludge facilities 38
Surface Aerations In this case a mixing device is used to agitate the surface so that there is increased interfacial area between liquid and air. There are many different proprietary types of surface aerators . For surface aerators, the most common way to control the DO and mixing is through the use of variable-speed motors.
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Diffused aeration Providing
maximum water surface per unit volume of air. Air bubbles brought with water in a mixing or contact chamber. A common way to aerate water is via diffused air. Air is pumped through some sort of diffuser to generate small bubbles.
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diffusers would be arranged by a manifold on the bottom of an aeration tank.
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Turbine Aeration In this system coarse bubbles are injected into the bottom of the tank and then a turbine shears the bubbles for better oxygen transfer. Efficiency of turbine aerators is generally higher than diffused aeration.
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Perforated Tube Aeration
Cascade Aeration
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Tertiary Treatment Porous Media Filtration: Definition: Removal of colloidal (usually destabilized) and suspended material from water by passage through layers of porous media. Wastewater treatment: tertiary filtration (removal of very fine suspended particles) In all filters the primary design/operating parameters are:
-quality (SS concentration) of the effluent. -headloss through the filter. 44
Deep Granular Filters Deep granular filters are made of granular material (sand, anthracite, garnet) arranged in a bed to provide a porous media as shown in the figure below. Filter bed is supported by gravel bed as also shown below. Flow is typically in the downflow mode. Upflow mode is used but much less frequently.
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Typical Sand filtration process
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Breakthrough Curve
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Head loss patterns When depth removal is the primary mechanism for SS removal the head loss pattern is shown in the following
figure. Head loss increases with surface loading rate due to
higher solids loading rate as well as higher frictional head loss. 48
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Mechanisms of filtration
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Tertiary-Membrane Treatment ď‚— A membrane is a selective barrier that permits the
separation of certain species in a fluid by combination of sieving and diffusion mechanisms ď‚— Membranes can separate particles and molecules and over a wide particle size range and molecular weights
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Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis
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Typical Size Exclusion
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Microfiltration Typical pore size: 0.1
microns (10-7m) Very low pressure Removes bacteria, some large viruses Does not filter
Microfiltration water plant, Petrolia, PA
small viruses, protein
molecules, sugar, and salts
A microfilter membrane
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Sources: http://www.waterworksmw.com/rack%201%20&%202b.jpg http://www.imc.cas.cz/sympo/41micros/Image126.gif
Ultrafiltration Typical pore size: 0.01
microns (10-8m) Moderately low pressure Removes viruses, protein, and other organic molecules Does not filter ionic particles like lead, iron, chloride ions;
nitrates, nitrites; other charged particles
An ultrafiltration plant in Jachenhausen, Germany
Source: http://www.inge.ag/bilder/presse/bildmaterial/referenzen/jachenhausen.jpg
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Nanofiltration Typical pore size: 0.001
micron (10-9m) Moderate pressure Removes toxic or unwanted bivalent ions (ions with 2 or more charges), such as Lead Iron Nickel Mercury (II)
Nanofiltration water cleaning serving Mery-sur-Oise, a suburb of Paris, France
Source: http://www.wateronline.com/crlive/files/Images/10899070-E891-11D3-8C1F-009027DE0829/newwater1.gif
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Reverse Osmosis (RO) Typical pore size: 0.0001
micron (10-10m) Very high pressure Only economically feasible large scale method to remove salt from water Salty water cannot
support life Reverse osmosis (or desalination) People can’t drink it water treatment plants, like this one, and plants can’t use it are often located close to the ocean to grow Source: http://iclaro.com/users/18342/pictures/Desalination%20Plant.jpg 58
How RO Works? ď‚— Osmosis is a natural process
that moves water across a semipermeable membrane, from an area of greater concentration to an area of lesser concentration until the concentrations are equal
Osmosis
ď‚— To move water from a more
concentrated area to a less concentrated area requires high pressure to push the water in the opposite direction that it flows naturally
Reverse Osmosis 59
Modes of Flow
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Tubular Hollow Fiber
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Cleaning of Fouling
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AOP Advanced chemical oxidation typically involves the use of chemical oxidants (e.g. ozone or hydrogen peroxide) to generate hydroxyl radicals (i.e. •OH), one of the strongest oxidants known.
Hydroxyl radicals are reactive and non-selective, capable of rapidly degrading a number of organic compounds.
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Reactivity of hydroxyl radical (•OH)
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Advantages of AOP • • • • •
Effective in removing resistant organic compounds Capable of complete mineralization of organic compounds to CO2. Not susceptible to the presence of toxic chemicals Generally produce innocuous end products Can be used to pretreat toxic compounds so that they can be bio-treated 71
ď‚— O3/H2O2 has gained the widest acceptance because
of effectiveness and low cost. ď‚— H2O2/UV has the advantage of simplicity (only
chemical is H2O2, cheap and soluble). Suited to small, minimum maintenance or intermittent operation systems. Some problems if materials in water absorb UV.
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ď‚— O3/UV considered less favorable because of high
pH requirement (chemical costs) but okay for low flows. ď‚— Least used are the TiO2 systems although they
have some advantages such as photocatalysts made be used, natural light may be used as a UV source, additional radical initiators are not required.
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UV/H2O2 Process
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UV/H2O2 Process
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What is Adsorption? Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a molecular or atomic film (adsorbate)
ADSORBENT
ADSORBATE
SOLUTION 77
Why does Adsorption occur ? ď‚— Consequence of surface energy ď‚— Atoms on the surface experience a bond deficiency, because they are not wholly surrounded by other atoms
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Adsorption Isotherms Plot of the amount of adsorbate on the adsorbent as a function of its pressure (if gas) or concentration (if liquid) at constant
temperature. Langmuir isotherm (adsorbed layer one molecule thick) Freundlich isotherm (Heterogeneous adsorbent surface with different adsorption sites) Brunauer, Emmett and Teller (BET) isotherm (molecules can be adsorbed more than one layer thick) 79
Freundlich Isotherm Freundlich and Küster (1909) Empirical formula: Limitation: Fails at high pressures Q- Mass of adsorbate / mass of adsorbent p- equilibrium pressure of adsorbate c- equilibrium con. Of adsorbate in solution K,n- constants 80
Langmuir Isotherm Irving Langmuir (1916) Assumptions:
Uniformity of sites Common Mechanism
Non interaction Monolayer only
Semi-Empirical Formula:
Q- Mass of adsorbate / mass of adsorbent Qmax- Maximum Q to form a mono-layer c- equilibrium con. of adsorbate in solution K - constant 81
BET Isotherm ď‚— Stephen Brunauer, Paul Hugh Emmett
and Edward Teller (1938) ď‚— Assumptions: Multilayer No Transmigration Equal Energy Langmuir to each layer
CS - saturation (solubility limit) concentration of the solute(mg/liter) KB - a parameter related to the binding intensity for all layers. 82
Applications Activated Carbon
Hydrophobic Surface area-500-1000m2/g Waste water treatment Decontaminant in pharmacy Silica Gel and Zeolites
Hydrophilic Polar Drying of process air CO2 and Hydrocarbon removal from natural gas Vapor Adsorption Refrigeration Protein Adsorption on biomaterials(cells) 83
Activated Carbon
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Carbon Contactors Activated carbon reactors are usually called carbon contactors because the waste stream is “contacted” with the carbon. Many times the contactor is of the columnar fluidized or fixed-bed type. Sometimes (less often) the contactor is in a slurry form. Fixed or fluidized beds have the advantage of not having to separate the carbon from the liquid stream after the contact period. Slurry systems need some sort of activated carbon removal process to separate the AC from the liquid stream.
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A typical packed (fixed) bed contactor looks like :
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Break Through Curve
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Carbon Regeneration ď‚— Since activated carbon is relatively expensive,
adsorption would not be feasible unless the carbon can be regenerated after exhaustion. Spent carbon is usually regenerated at 500 oC under low oxygen conditions in the presence of steam. ď‚— Activated carbon loss is about 5-15% for each
regeneration. Adsorbed organics are volatilized and oxidized during the regeneration process. A regeneration scheme is shown below. 88
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