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Sewage is the used water from domestic, industrial or agricultural communities, surface run off, etc. and contains 0.1-0.5% inorganic and organic (proteins, amino acids, fats, fatty acids, carbohydrates, etc.) matters in suspended and soluble forms. Domestic wastes include detergents, antiseptics, pesticides, human excretements, various types of microorganisms (aerobic and anaerobic) like, bacteria, fungi, viruses, protozoa, algae, etc. (many are pathogenic) to which a variety of industrial organic and inorganic wastes are added that depends upon the type of industries. The microorganisms found in wastewater help in decomposition of organic matter in the sewage. During the course of sewage decomposition there is shift in the types of microorganisms and initially aerobic microorganisms predominate and later anaerobics specially methanogenic bacteria, which convert H2 and CO2 to CH4 that can be entrapped and used as an energy source). Fermentation products and reduction of NO3 and SO42- give rise to noxious odours, tastes and colours. Replenishment of dissolved O2 from atmosphere and by photosynthetic O2 evolution can be slower than the utilization rate of O2 by
heterotrophic microorganisms in the presence of abundant organic substrates, thus, reducing the DO of natural waters. Small amounts of sewage discharged through drainage systems (sewers) into a water body (river, lake or ocean) gets diluted due to inherent self purification capacity without significant deterioration of water quality, but large amounts of liquid wastes produced due to increased human populations do not get diluted sufficiently, leading to anaerobic conditions (high biochemical oxygen demand or BOD) because of consumption of O2 by the aerobic microorganisms and unacceptable deterioration of water quality. This leads to the death of aerobic plants and animals and foul odour in water. Decomposition of these dead organisms causes an additional O2 demand. At much slower rates the sewage may also percolate to groundwater table.

The BOD determination (by incubating about 1 litre of polluted water for 5 days at 20°C in dark and assaying the amount of O2 used for oxidation of biodegradable organic material by an O2 electrode) provides the best information concerning the strength of sewage and the efficiency of
sewage treatment processes. Other methods of pollution level estimation are COD (chemical oxygen demand, that gives an idea of chemically oxidisable organic matter. The water sample is incubated with a known amount of K2Cr2O7 for about 2 hours and the unused K2Cr2O7 is estimated by titrating with ferrous ammonium sulphate), biological indicators (relative abundance of resistant or absence of certain sensitive organisms in the polluted water), PO43-level, NH4+, quantity of suspended solids by weighing the amount of filtered or settled solids/litre water sample, etc.

Efficient waste (sewage or industrial) collection through drainage system (sewers) and specific treatment processes are employed since early times to reduce the biologically degradable organic compounds before discharging the wastewaters into natural water bodies. It is the largest biotechnological industry and some of the important technologies (the choice of method depends largely upon the area), depend upon the metabolic activities of mixed microbial populations, thus, leading to complete elimination or reduced incidences of various infectious waterborne diseases (typhoid, cholera, dysentery, polio, hepatitis, etc.) due to elimination of pathogens (by competition and predation by other aquatic populations, adsorption and settling), involve physical, chemical or biological processes. Organic nutrients are utilized and mineralized by heterotrophic microorganisms. NH4+ is nitrified and along with other inorganic nutrients utilized by algae and aquatic plants. The usual operation in wastewater treatment is the reduction of BOD associated with suspended and dissolved organic matter. Only a small percentage of suspended and dissolved organic matter is actually mineralized during wastewater treatment and most of the organic material is removed by settling that may create disposal problem. Occasionally, it may be followed by the removal of inorganic nutrients and recalcitrant organics prior to discharging the effluent into natural water bodies. After a prolonged septic period, O2 diffusion eventually re-aerates the water and allows the mineralization of accumulated fermentation products that give rise to algal bloom. If there is no further disturbance, secondary succession may eventually restore the aquatic water body to its former state. The primary and secondary treatment processes aim to reduce BOD of sewage prior to its discharge and this complex and highly successful system with aerobic coupled with anaerobic decomposition processes involves three steps (Fig. 17.8):

(i)                 Primary Treatment: It is the physical treatment in which a portion of larger suspended and floating solids, grit, and grease from sewage is removed by various mechanical devices, like passing the sewage through screening devices (the screened solid is collected and disposed by incineration or burial), traps and skimming devices (for the removal of grease). After screening, the sewage is passed through grit chambers to allow heavier solids (e.g. gravel, grit) to settle down. After this, the sewage is passed through the sedimentation tanks at a slower rate to settle out the solid matters as much as possible. The settled solids are drawn off from the bottom. For typical domestic sewage, primary treatment removes 30–40% of BOD and secondary treatment is necessary for acceptable BOD reduction. If the liquid waste mainly contains solids and little dissolved organic matter, primary treatment may remove 70-80% of BOD that can be disposed off safely. (If only primary treatment of sewage is to be performed, the liquid after chlorination is discharged into the natural water body).

(ii)               Secondary Treatment: In secondary treatment, a small portion of the dissolved organic matter is mineralized, whereas, the larger portion is converted from a dissolved state to removable solids. The combination of primary and secondary treatments reduces the BOD by HO 90% to produce effluents with less than 20 ppm BOD. This is the biological treatment and several methods are employed. These are:


The microorganisms are either attached as surface films or they are homogeneously suspended in sewage water. Organic materials are oxidized to inorganic materials such as CO2, SO42-, NO3, PO43- and water by applying any of the technologies.