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Water is the most plentiful liquid on earth. It is a requirement for all life, whether it is used in metabolic processes, as a solvent of minerals, or in the disposal of waste. Indeed, it is difficult to name a natural phenomenon in which water, in one form or another, has not had a role. Nature has provided for the conservation of this abundant and valuable liquid by endowing it with a considerable capacity to rid itself of foreign substances. This process, known as self-purification, is an inherent property of water. The water may be moving in streams or be relatively static as in lakes and ponds, but the natural processes of purification are similar. Advantage is taken of this intrinsic ability of water to purify itself by using it as a final stage in the treatment of sewage and industrial wastes. Streams polluted by domestic wastes (those derived from human populations) or industrial wastes (the liquid wastes of wet-process industries and acid mine drainage) may be rendered stable by self-purification. In most instances, such polluted water becomes relatively innocuous biologically and again serves nature in one role or another. This purification is accomplished by a combination of physical, chemical, and biological forces. The extent to which each force operates is dependent upon the type of contamination. Physical forces dispose of suspended solids; chemical forces cause reactions which render unstable wastes innoxious; and biological forces contribute to the stability of the stream by enhancing both physical and chemical phenomena. The biological forces are the most active, and the most important, in the equilibration of a stream.