Sewage is semisolid domestic and industrial form of waste. Water is the main constituent of sewage and approximately 0.5% inorganic and organic solid matter is suspended in this water. The composition of sewage is very dynamic and fluctuates according to the input from different waste sources like domestic (kitchen garbage, human excreta), agricultural (pesticides, manure, animal excreta, rainwater runoff or field soil) or industries (paper and pulp, textile, slaughter house, oil refinery, sugar factory effluents, paint, brewing, canning, food processing, metal, mines). Depending upon the input sources, chemically sewage is composed of carbon, nitrogen, phosphorous, sugars, fatty acids, proteins, fats, alcohols, amino acids, pectin, cellulose, lignocelluloses, lignin, heavy metal residues and many other complex forms. Looking at the chemical composition, it seems that sewage is an ideal environment for growth microorganisms like protozoa, algae, fungi, yeasts, bacteria and viruses. Bacteria from sewage are pathogenic, nonpathogenic, saprophytes, autotrophic, heterotrophic, facultative, obligate, aerobic or anaerobic forms. The millions of bacteria have been enumerated in per milliliter of diluted sewage sample. The common sewage bacteria include species of coliforms, streptococci, clostridia, lactobacilli, micrococci, Proteus and Pseudomonas. Most of these bacteria re causative agents of fatal diseases like gastro, typhoid, cholera and food poisoning in humans.
The query arises regarding the presence of multiple groups of bacteria in sewage. Why do they grow in sewage and what are they for? The first most important reason is that sewage has peculiar composition that favors and supports the growth of almost all types of bacteria. The second important reason is that sewage bacteria carry out decomposition of organic matter present in the sewage. Bacterial sewage degradation is very prolonged but ecologically important process. The characteristic feature of sewage decomposition is the shift in types of bacteria during time course of decomposition process. Let's know about these changes in bacterial flora from sewage.
Aerobic and facultative bacteria: Coliforms like Escherichia and Enterobacter, micrococci, lactobacilli, pseudomonads, facultative clostridia and streptococci) predominate during first course of sewage decomposition. Diluted sewage provides aerobic conditions for the growth of these aerobic and facultatively aerobic bacteria. Such condition is obtained when sewage is discharged into the water body like a river. Under these conditions, organic matter containing protein, carbohydrates and fats is completely oxidized by aerobic bacteria. Proteins are converted to amino acids and nitrates and carbohydrates and fats to carbon dioxide and water during the process of bacterial oxidation. This results in stabilization of sewage and changes in chemical composition. Since the oxygen present is consumed for oxidation of organic matter, anaerobic conditions are introduced in sewage along with the generation of high BOD (biological oxygen demand) and offensive odor.
Strictly anaerobic bacteria: The second course of sewage degradation is taken up by strictly anaerobic bacteria like methanogens or methane producing bacteria (Methanobacterium, Methanococcus and Methanogenium). As their name indicates, they produce methane from hydrogen and carbon dioxide present in the sewage. Methanogens decrease load of organic solids by degrading them into soluble substances and gases. Under anaerobic conditions, protein and nitrogenous compounds are degraded to amino acids, nitrogen, ammonia, hydrogen sulfide, carbon dioxide, methane, indole, organic acids and alcohols; carbohydrates are converted to carbon dioxide, alcohols, hydrogen, fatty and neutral acids and fats and fatty acids are degraded into lower fatty acids, glycerol, carbon dioxide and hydrogen. The typical gaseous mixture from sewage contains methane (biogas), nitrogen, hydrogen and carbon dioxide. These gases have very high potential to be used for power generation in industries. At this stage, the sewage is termed as digested sludge. Dewatered sewage (after exposure to sunlight) can be used as soil conditioner or compost or for land filling. The process of sewage degradation occurs naturally and also forms the basis of functioning of artificial sewage treatment plants. Had not bacteria been actively present in the sewage, water bodies on the Earth would have been turned into sewers by now.
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