环境工程专业英语(钟理编)英语全文.docVIP

Unit 13 Text: Water Treatment Processes One of the great achievements of modern technology has been to drastically reduce the incidence of waterborne of diseases such as cholera and typhoid fever. These diseases are no longer the great risks to pubic health that they once were. The key to this advance was the recognition that contamination of pubic water supplies by human wastes was the main source ci infection, and that it could be eliminated by more effective water treatment and better waste disposal. Today’s water treatment plants are designed to provide water continuously that meets drinking water standards at the tap. There are four main considerations involved in accomplishing this1 source selection, protection of water quality, treatment methods to be used, and prevention of recontamination. Common precautions to prevent groundwater and surface water pollution include prohibiting the discharge of sanitary and storm sewers close to the water reservoir, installing fences to prevent pollution from recreational uses of water, and restrictions on the application of fertilizers and pesticides in areas that drain to the reservoir. Screening, coagulation/flocculation, sedimentation, filtration. and disinfection are the main unit operations involved in the treatment of surface water. Water treatment operationa fulfill one or more of three key tasks1 removal of particulate substances such aa sand and clay, organic matter, bacteria, and algae; removal of dissolved substances such as those causing color and hardness; and removal or destruction of pathogenic bacteria and viruses. The actual selection of treatment processes depends on the type of water source and the desired water quality. Occasionally, raw water with low turbidity can be treated by plain sedimentation (no chemicals) to remove larger particles and then filtration to remove the few particles that failed to settle out. Usually, however, particles in the raw water are too small to be removed in a reasonably s