Sewerage
The problem of protecting natural water resources has grown very pressing for many countries since the beginning of the second half of the 20th century. The development of human society, the growth of civilization and social and technical progress have resulted in the changing of the composition of natural water resources. The rivers, lakes and ground-waters contain today a considerable amount of the products of mechanical, chemical and biological pollution.The waste products that result from the, daily .activities in a community are of two general types: namely, the liquid waste, kriown as sewage and the solid wastes, known as refuse. The different wastes of which sewage is composed are the following:, the wastes from lavatories, baths, sinks, and laundry tanks in residences, institutions, and business buildings; certain liquid wastes from various types of manufaturing or industrial plants, and, in many communities, the surface run-off that results from storms or street-flushing operations.Sewage may also be divided according to its source in to the following three classes. The sewage from residences, institutions and business buildings is called domestic sewage,sanitary sewage or house sewage; that resulting from manufacturing or industrial processes is known as industrial waste;and that from run-off during or immediately following storms is called storm sewage. A combination of domestic sewage, industrial waste and storm water is called combined sewage.Both sewage and refuse must be removed promptly in order to avoid endangering the health of the community and also present decomposition of the materials of animal or vegetable origin and the subsequent production of nuisances and odours. The removal of all kinds of sewage is usually accomplished by means of sewers. The sewers are placed in the streets at several feet below the ground surface. The general process of removing sewage is designated as sewerage and the entire systems of sewers including a sewage treatment plant is known as a sewerage system.The method of sewage treatment to be adopted in a particular case will depend almost entirely on local conditions. It may consist only of the discharge of the raw sewage into a stream or a large body of water. The usual methods of sewage treatment consist either of preliminary treatment alone or of primary treatment followed by secondary treatment.During primary treatment the larger and heavier solid particles settle out from the liquid. These solid particles that settle out form a slimy paste which is known as sludge. The partly clarified sewage that has been given primary treatment generally contains much decomposable materials. Therefore, further treatment which is known as secondary treatment, is usually required.An auxiliary treatment which may be used with either primary or secondary treatment is disinfection or the killing of the most of the bacteria in the sewage by means of chemicals.
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