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Sewage
Disposal - Sewage constitutes the largest liquid, waste in cities and is heterogeneous with regard to both microbial and chemical composition. Generally, it contains about 1-2 per cent solids and 98-99 percent water. Domestic sewage contains human excreta and therefore, is a source of both pathogenic as well as nonpathogenic microorganisms.
Organisms commonly found in sewage include the enteric bacteria (Salmonella, E. coli, Shigella, Pseudomonas, streptococci, staphylococci etc). Sewage from larger cities is also mixed with industrial wastes containing non-biodegradable toxic materials. Because of a high organic matter content sewage has a high biological oxygen demand (BOD).
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The major purpose of sewage treatment is to reduce this BOD before it is disposed off. The term BOD is used to design ate the oxygen consuming capacity of a liquid which is a measure of the level of degradable organic matter present.
The BOD is measured by incubating a sample of the liquid in a sealed container and the amount of oxygen consumed in a definite interval of time (20°C/5 days) is measured. A high BOD represents a large amount of degradable organic matter in the sample. Unpolluted natural waters have little or no BOD.
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A decrease in the BOD during sewage treatment is a reflection of the effectiveness of stabilization of the organic to in organic materials. The method used for sewage purification depends on the BOD level, the presence of toxic materials and the natural water into which the treated sewage is to be discharged.
Normally, sewage treatment involves a primary and secondary treatment. In the primary treatment, large materials such as rags, sticks, trash etc., are removed, by passing the sewage through a series of screens and, is then allowed to settle for a period of 1.2 hours.
In some of the sewage treatment plants aluminium sulphate or ferrous sulphate is added to facilitate coagulation and quick settling. The fluid is then transferred for secondary treatment and the sediment is transferred to digesters for further treatment or burnt.
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In the secondary treatment, the organic matter is stabilized and the BOD is reduced aerobically and this is called as the activated sludge process. During this treatment the liquid is aerated vigorously and as a consequence the organic matter is converted into gases, and a very small amount of this is incorporated into cell biomass.
During the sludge treatment, different types of organisms function in sequence degrading the large molecular weight components into smaller compounds. Microorganisms isolated from the secondary stage are mostly bacteria and include those belonging to the genera, Pseudomonas, Alcaligenes, Flavobacterium, etc., and these are responsible for biodegradation.
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During the secondary treatment, the bacteria serve as food for the protozoa. A part of the sludge is removed as inoculum and the rest is retained in the digestor.
During activated sewage treatment pathogenic bacteria are generally eliminated. Most of them are inactivated because of their poor adaptability or because of severe competition for nutrients from other bacteria.
Aerobic oxidation is an efficient mechanism of removing biologically degradable organic materials. While the anaerobic process may reduce the BOD by 35-75 per cent, aerobic activated sludge treatment removes about 95 per cent of the BOD.
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In most of the sewage treatment plants, the effluent from the secondary settling tanks is generally dispresed into the receiving waters without any further treatment. This leads to considerable amount of pollution since the effluent may contain large amounts of inorganic phosphate and nitrates which cause serious pollution problems.
To remove these inorganic chemicals, a tertiary treatment has been used in some of the western countries. In this, lime is added to coagulate and precipitate phosphate and allowed to settle. Gaseous ammonia formed during this period is removed by passing the effluent over a stripping tower, while the water is broken into small droplets allowing the ammonia to escape.
The water is then neutralized, passed through filters of coal or sand and finally oxidized with, chlorine and recovered for use, summarizes the activated sludge process,
In small sewage plants, a process called the trickling filter method is also used. In this, the sewage is allowed to trickle over coarse sand or pebbles coated with microorganisms that oxidize the effluents. By adjusting the rate of flow over the filters, efficient oxidation of the sewage can be achieved.
The activated sludge process is suitable for large communities. For small communities or industries, a process called lagooning which involves storage of the sewage water or effluent in shallow ponds or lagoons for varying period to allow settling and stabilization by both aerobic and anaerobic organisms is, generally used. Pathogenic bacteria are eliminated during this process.
For disposal of wastes from isolated or from small communities, septic tanks are also used. The principle of a septic tank is that the sewage is collected in a large tank in which the solid material settles down by gravity and is degraded by anaerobic microorganisms. The liquid overflow from the tank with a high BOD is stabilized by either passing through trickling filters or by lagooning. In any case, the over flow is not allowed to drain directly into water supplies.
The treated waste water can be reutilized either for crop production or for industrial purposes but it is not desirable to allow this to run into drinking water. Treated sewage and the activated sludge, can be used as garden fertilizers. However, the presence of a high concentration of salts in these materials may interfere with seed germination and plant growth.
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