Conventional
Raji A.AL-Awadi
sewage treatment plants and their stages
Erciyes university -Turkey
The process of sewage disposal in healthy ways is one of the processes necessary to provide a healthy environment for communities according to international conditions and the latest engineering and scientific methods to preserve the surrounding environment free of sewage pollutants.
The public sewage network receives sewage water without interruption and discharges it to pumping and lifting stations, and then to purification stations, where this water contains quantities of solid materials that can be divided into suspended materials, biodegradable materials, pathogenic organisms, and others that cause pollution to natural water sources. The first important information should have for the design of wastewater treatment system is the strength and characteristics of the wastewater to be treated. The strength of wastewater is normally expressed in terms of pollution load, which is determined from the concentrations of significant physical, chemical and biological contents of the wastewater (Davis and Cornwell, 2008).
What is sewage?
It consists of wastewater discharged from residences and from commercial, institutional and public facilities that exist in the locality [Von Sperling, M. 2015)
Sewage is a mixture of water (from the community's water supply) human excreta (feces and urine), used water from bathrooms, food preparation wastes, laundry wastewater, and other waste products of normal living.
What is a wastewater treatment plant?!
Due to the variety of materials contained in waste water, the process of treating wastewater varies based on the purpose for which it will be used, for example: nitrates and phosphorous are very dangerous to remain in the treated water if the water after treatment is used in agriculture, where there are living organisms and this will lead polluting nutrients and affecting aquatic organisms. A wastewater treatment plant is a facility in which a combination of various processes (e.g., physical, chemical and biological) are used to treat industrial wastewater and remove pollutants (Hreiz et al., 2015)
Why should waste water be treated?
Sewage treatment is carried out to protect the environment, water resources and human health, where the solid biological waste water remaining from treatment can be reused in agricultural irrigation in order to reduce the consumption of fresh water, especially in dry areas.
Organic pollutants through bacteria naturally present in the organic matter found in sewage water where there are sewage treatment plants.
Levels of wastewater treatment
There are three broad levels of treatment: primary, secondary and tertiary. Sometimes, preliminary treatment precedes primary treatment. There are those who talk about two additional processes, one at the beginning and one at the end, and the number of treatments becomes five.
1-Initial treatment (physical treatment)
It is the first stage of treatment, as it aims to improve the specifications of the wastewater flowing into the plant by reducing the amount of polluting materials, which helps to raise the treatment efficiency of the wastewater in the following stages. The primary treatment is carried out through:
1. Separation of inorganic floating materials such as wood and plastic parts by mechanical screens.
2. Separation of sand and removal of surface suspended matter (including fat and grease. removes coarse suspended and grits. These can be removed by screening, and grit chambers respectively. This enhances the operation and maintenance of subsequent treatment units. Flow measurement devices, often standing-wave flumes, are necessary at this treatment stage (FAO, 2006). For the treatment processes, it starts with preliminary treatment units. This phase involves the removal of large solid objects through the use of nets to capture and remove them, as well as the deposition of sand and gravel by passing water when entering the station through a hole where heavy solids fall before proceeding to the next stage.
This stage is of great importance in terms of protecting the plant’s equipment from faults, especially pipes and pumps (Rungnapha, Hardy, Huub, & Karel, 2015)
2-Primary treatment
The main objective of this phase of treatment is to obtain a homogeneous liquid that can be biologically processed at later stage, on the one hand, and one the other hand, obtainable and around to be treated separately.
Sedimentation ponds are usually equipped with mechanical equipment that helps to collect the mud in the pelvic floor, and from there it is pumped to the treatment in subsequent stages. As well as to remove the floating material and dispose of the treated water stream. As well as, the mechanical equipment is to transfer homogeneous water to the next stages of treatment.
As this initial stage, some chemicals are used to help the materials float on the surface of the water, as well as to help the solids settle on the bottom. The mulch is derived from primary treatment, the primary mulch.
This process can reduce the BOD index, the level of contamination with biodegradable organic matter by more than 20–30 percent, and reduce the total TSS by more than 50–60%. Primary treatment is the first phase of treatment, followed by other processes. It is divided into: floating basins and deposition basins.
The settled solids are collected by mechanical scrapers into hopper, from which they are pumped to sludge processing area oil, grease and other floating materials are skimmed from the surface. The effluent is discharged over weirs into a collection trough. There are some types of clarifiers. Also, the common types of horizontal flow clarifiers are rectangular, square, or circular.
On the other hand, the types of include surface are tube settler and parallel plate settler. In general, the design of most of the clarifiers falls into three categories: (1) Horizontal flow, (2) solids contact and (3) inclined surface.We can summarize this stage as follows:
1-Filter:
Iron sieves with narrow openings are used, and in three stages, through which sewage passes, in order to get rid of all the large impurities stuck in them, such as: bags, stones, and some living organisms, such as: mice, which passed through the Initial treatment and are still
2-Sedimentation:
The goal of this step is to get rid of everything that is organic and contaminated with water, where solid pollutants are extracted from the water to be treated in the next stage anaerobic, and this is to produce biogas, and natural fertilizer from it.
3-Skimming:
In this stage, all floating oils or fatty substances floating on the surface of the water are removed.
3- Secondary treatment stage (biological treatment)
It is the basic stage in the treatment, as it provides the appropriate climate through ventilation for the proliferation of microorganisms and bacteria. It transforms the waste (organic and desirable materials) into inorganic materials (gases and cellular tissues) that are heavier than water and are deposited at the bottom and easy to dispose of. This stage is divided into two stages:
1- Aeration and mixing
After the primary treatment, the water is transferred to the aeration ponds and mixed with the active sludge returned from the sedimentation ponds to start the biological treatment in the aeration ponds and oxidation of organic materials by supplying these ponds with compressed air from the air compressor buildings.
2- sedimentation basins
From the aeration basins, the water flows to the collection points so that the mixture is distributed to the circular sedimentation basins, where it is collected and transferred to the tertiary treatment stage.
As for the sludge at this stage, the other part of the sludge is returned to the stage of treating the sludge.
The organic matter, when the air is reduced or (prevented from it), leads to the reduction of nitrite and produces nitrogen gas, which rises upwards. During its ascension, it pushes some of the desirable materials up. Therefore, it must be scraped from the surface of the basins daily, as the desired materials are collected in a special basin for disposal
In the field of wastewater management and treatment.
The purpose of secondary biological treatment is to remove dissolved and suspended organic material from wastewater to produce an environmentally-safe treated effluent and Bio solids /sludge.
4- (tertiary treatment)
Wastewater treated in the Previous stages contains other wastes such as sulfur, nitrates, heavy salts, difficult to decompose organic compounds, and other substances, according to the quality of the wastewater.
To protect water sources from entering these pollutants, some countries of the world treat water in different ways .
The choice of the appropriate method for treating wastewater in the this stage depends on the type of materials or pollutants remaining in the water to be disposed of.
Remove element phosphorous.
Nitrogen removal
sulfur removal
Wastewater treatment is a process in which wastewater is purified from impurities, suspended matter, pollutants, and organic matter, until it becomes suitable for non-human use, or it can be disposed of into sewers or water bodies without causing pollution
The purpose of tertiary treatment is to provide advanced wastewater treatment beyond secondary biological treatment. It results in a very high quality effluent, extremely low in BOD, suspended solids and nutrients.
Wastewater treatment plants that have discharge limits less than 10 mg/L of BOD and total suspended solids or have very low phosphorus limits, usually need to provide tertiary treatment.
Tertiary treatment usually consists of a type of physical and/or chemical process. Sand or mixed media filters, cloth discs, membranes, or other treatment units can remove total suspended solids and/or phosphorus to very low levels.
Chemicals can also be used to precipitate some pollutants in the wastewater. Air stripping or activated carbon is sometimes used to remove volatile organic chemicals from the wastewater.
This triple treatment eliminates over 99% of pollutants.
Sterilization is also called the chlorination stage. Chlorine or any other chlorine compound is added to kill a large amount of pathogenic germs. The treated water can be subjected to ultraviolet rays for sterilization. Ozone sterilization is one of the latest and most effective methods at present.
5-Sludge treatment
The sludge is transported and treated from the secondary resulting sedimentation ponds to ensure that it is free of polluting and harmful substances before being used. Initially, this sludge is condensed to increase its concentration and reduce its volume by sedimentation, then the condensed sludge is digested to remove harmful substances by feeding it with air.
With the end of the digestion process, the sludge becomes more dense and more concentrated in solids.
The remaining sludge is transported through a land line to the sludge drying basins, where this liquid sludge is spread on the surface of the basins, dried to a sufficient degree, and then transported for use or backfilling ( Peace
Amoatey and Richard Bani,2011)The sludge drying process depends on the evaporation of water from the surface layer of the sludge by weather factors (as with the passage of time the surface layer dries completely and then the rest of the sludge dries up relatively).
The drying process also depends on the process of filtering water through layers of sand and gravel at the bottom.
This water collects at the bottom of the basins by a network of pipes, where it is returned to the entrance to the basins by a network of pipes, where it is returned to the entrance to the aeration basins to be treated.
To kill microbes, chlorine is applied at a concentration of 100 mg / liter for 15-120 minutes, so that the sewage liquid becomes pure and free from toxicity and infection.
The sewage problem in Iraq
Approximately 30% of the Iraqi population has access to sewage system facilities, and out of 18 governorates, only 12 governorates have wastewater treatment facilities. Sanitation systems are concentrated in urban areas, where wastewater treatment facilities are available. healthy. This means that most rural areas do not have access to sanitation facilities and therefore resort to alternative means of sewage disposal (eg, cesspits, or untreated wastewater discharge into canals or rivers.
*Iraqi Ministry of Environment - Sewage Sector 2018
Summary
1-Pre-treatment or pre-treatment is a stage
Remove hard and large objects by using clips to pick them up.
Sand and gravel deposition, to protect pipes and pumps from holes, malfunctions and sediments.
2-Primary processing or mechanical processing
Sedimentation of solids in basins so that they float on the surface of the water, and are easy to remove.
Obtaining a process able liquid, obtaining a separately treatable material and solution from the sludge.
use of chemicals to help objects float on the surface of the water, and the deposition of solids at the bottom.
3-Secondary treatment or biological treatment
Remove more than 90% of organic matter.
Removal of dissolved organic matter, which was not removed in the initial processing stage.
use of some microorganisms (germs) as they consume organic materials as food for them, and convert them into carbon dioxide, water and energy, which are important materials for the growth of germs.
Aeration is an important part of supplying water with large amounts of oxygen to facilitate aerobic decomposition of organic matter.
The water is transferred to final basins for a final sedimentation process, prior to sterilization.
4- Tertiary treatment
Using complex and precise techniques such as: coagulation, sedimentation, filtration, and reverse osmosis.
This triple treatment eliminates over 99% of pollutants.
5- Sterilization
It called the chlorination stage.
Adding chlorine or any other chlorine compound to kill a large amount of pathogenic germs.
Expose the treated water to ultraviolet rays for sterilization. Ozone sterilization is one of the latest and most effective methods at present. Best