Sewage is anything carried by sewers. Waste water coming from domestic or industrial houses or garbage dumps is generally called sewage. Sometimes, it may even contain rain water and surface run off. The treatment of waste water involves the following six steps:
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Sedimentation
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Coagulation
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Filtration
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Disinfection
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Softening
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Aeration
If waste water is to be disposed off into a river, stream or any other receiving water body, it is treated only up to the first four steps, i.e, sedimentation, coagulation, filtration and disinfection. This is known as primary treatment. The wastes these days have become so complex, it is no longer permitted to drain sewage into receiving water body without treatment. In other words, waste water is given primary treatment to remove gross impurities and the recovered water is then disposed into stream. If, however, the recovered water is to be used for drinking purpose, it has to undergo further treatment, i.e., softening and aeration, these two are collectively known as secondary treatment.
The treatment of waste water/sewage is conducted in plants built specially for this purpose. We would advise you to visit a city sewage treatment plant situated near your locality, if you have not already done so. The treatment of waste water/sewage is done in order to restore its original purity and make it safe for drinking or disposal into the receiving water bodies. In the following passage we will discuss various steps involved in treatment of sewage:
Sedimentation occurs naturally in reservoirs and is accomplished in treatment plants by storing sewage or waste waters in basins or settling tanks. Silt, clay and other fine materials settle to bottom if water 4s allowed to stand or flow quietly at low velocity. Plain sedimentation will not remove extremely fine particles or colloidal material. This step is used principally as a prelude to other treatment methods.
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Fine particles and colloidal materials are combined into conglomerates by coagulation. These are called floe (plural: floes) and are large enough to settle in basins and to be caught on surface of filters. Coagulation is brought about by using special chemicals known as coagulants/ flocculants, such as potash alum. This step is also known as flocculation.
Suspended solids, colloidal material, bacteria and other organisms are filtered out by passing the waste water through a bed of sand or finely graded coal or through a matrix of fibrous material supported on a perforated core. Soluble materials such as salts and metals in ionic form are not-removed by filtration.
After filtration the water undergoes disinfection. There are several methods of treatment of water to kill living organisms particularly pathogenic bacteria. The application of chlorine or its compounds such as bleaching powder is commonly used for disinfection. Less frequently used methods of disinfection include the use of ultraviolet light, ozone, or silver ions. Boiling is the favorite household emergency measure for disinfection.
The waste treatment steps up to this stage are included in what is known as primary treatment and sewage or waste water treated in this manner can be safely disposed off after due checking for toxic wastes left over, in trace quantities. This water is however not fit for drinking, and has to undergo softening and aeration before it can be safely used for drinking purposes. This is known as secondary treatment.
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Softening is a process of treatment of water by which undesirable cations of calcium and magnesium are removed from hard waters. Two methods are used for softening : (1) the water is treated with lime and soda ash to precipitate calcium and magnesium ions as carbonates, after which the precipitate is filtered; (2) the water is passed through porous cation exchangers and is left cation free.
Aeration is a process of exposing water to air by forcing air through water in the form of bubbles to add oxygen and reduce carbon dioxide, hydrogen sulphide, and taste producing gases or vapors.
Characteristics of Treated Sewage
As a result of treatment, the sewage gets rid of silt, water becomes colorless, the number of pathogenic bacteria decline to almost zero, biological oxygen demand (BOD) and coli count (see glossary) go down, and the toxic wastes are neutralized. Water losses turbidity, becomes free of suspended particles, and gets loaded with life giving oxygen.
A curious pattern in Delhi and other major cities of India is that we take our drinking water from the same river that we use as dump for releasing human and chemical waste. To be sure we purify drinking water by giving it secondary treatment; but just as surely, we do not get absolutely everything out during the purification process. The communities located downstream are exposed to pollutants, pathogens and other inconveniences. Sometimes, careless discharge of partially treated water causes eruption of epidemics. Eruption of a cholera epidemic in J988 in Delhi took a toll of 300 people in the city.
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Interestingly, disinfection using chlorine kills the pathogens and also on combination with traces of organic matter may lead to formation of chlorinated hydrocarbons, that have cancer inducing (carcinogenic) potential. Chlorine is a halogen element. Most of our city water supplies test positive for halogenated compounds. Therefore, it is imperative to reduce the organic matter content of water, before chlorinating it. Preferably other methods of disinfection such as ultraviolet, ozone and silver ions, etc., should be used for this purpose. However, these methods are more expensive.
Scientists have developed various criteria for measurement of water quality. These are based on the knowledge of characteristics of polluted waters and the mechanisms operating in natural water bodies. You can, if you are interested, obtain standard water quality criteria for safe usage from booklets published and distributed free of cost by the Bureau of Indian Standards, New Delhi.
Careless release of toxic industrial wastes threatens groundwater supplies. You will read in the following few paragraphs how groundwater pollution has deteriorated the quality of rural drinking water supply and what can be the possible results of drinking such water.