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Sewage
Sewage or wastewater is the waterborne human, domestic and farm wastes. It may include industrial effluents, subsoil or surface waters, human wastes include faecal material. Domestic wastes include food wastes and wash water. Industrial waterborne wastes are acids, oils, greases and animal and vegetable matter discharged by factories. The composition of sewage varies depending upon the source of waste water. This also causes variation in the microbial flora of sewage. Almost all groups of microbes, algae, fungi, protozoa, bacteria and viruses are present.
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The waste water discharged through the drainage system has to be properly disposed. They cannot be simply disposed off into water bodies or landscapes because of the oxygen demand they exert and also due to the presence of pathogenic microbes in them. So before disposal, waste water has to be properly treated.
Rain water and ground water are very pure. But human activities like discharge of noxious substances including biocides, oil and sewage 'into water bodies spoil the aquatic environments to the maximum. Increased growth of microorganisms also contribute to the deleterious effects on aquatic environments by affecting the ecological balance with complicated consequences, as in eutrophication. The heterotrophic activity of microorganisms in water depletes the dissolved oxygen content of water which has a negative influence on the self- purification of water. This lead to what is known as biological oxygen demand or BOD.
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Liquid wastes are produced by everyday human activities (domestic sewage) and by various agricultural and industrial operations. Following drainage patterns or sewers, liquid waste discharges enter natural bodies of surface water such as rivers, lakes and oceans. At much slower rates, they may also percolate to the ground water table, especially if it is high or if fissures are present in the unsaturated soil layer.
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Natural waters have an inherent self-purification capacity. Organic nutrients are utilised and mineralised by heterotrophic aquatic microorganisms. Allochthonous populations of enteric and other pathogens are reduced in number and eventually eliminated by competition and predation pressures exerted by the autochthonous aquatic populations.
Waste treatment, protection of drinking water sources, and disinfection of drinking water and sewage, gradually introduced in the early years of this century, largely eliminated the spread of waterborne pathogens. Waste treatment, protection of drinking water sources, and disinfection of drinking water and sewage, gradually introduced in the early years of this century, largely eliminated the spread of waterborne epidemics in developed countries.
There are various options available to convert solid waste to energy. Mainly, the following types of technologies are available: (1) sanitary landfill, (2) incineration, (3) gasification, (4) anaerobic digestion, and (5) other types.
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Sanitary landfill is the scientific dumping of municipal solid waste due to which the maturity of the waste material is achieved faster and hence gas collection starts even during the landfill procedure. Incineration technology is the controlled combustion of waste with the recovery of heat, to produce steam that in turn produces power through steam turbines. About 75% of weight reduction and 90% of volume reduction is achieved through burning.
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A gasification technology involves pyrolysis under limited air in the first stage, followed by higher temperature reactions of the pyrolysis products to generate low molecular weight gases with calorific value of 1000-1200 kcal/ nm.
These gases could be used in internal combustion engines for direct power generation or in boilers for steam generation to produce power. In biomethanation, the putrescible fraction of waste is digested anaerobically (in absence of air), in specially designed digesters. Under this active bacterial activity, the digested pulp produces the combustible gas methane and inert gas carbon dioxide. The remaining digestate is a good quality soil conditioner.
Other technologies available are pelletisation, pyro-plasma, and flashpyrolysis. All these technologies have merits and demerits. The choice of technology has to be made based on the waste, quality, and local conditions. The best compromise would be to choose the technology, which (1) has lowest life cycle cost, (2) needs least land area, (3) causes practically no air and land pollution, (4) produces more power with less waste, and (5) causes maximum volume reduction.
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