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  3. Municipal sewage is of relatively recent origin as a pollutant. It was first brought to public attention in the 19th century by

Municipal sewage is of relatively recent origin as a pollutant. It was first brought to public attention in the 19th century by

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问题    Municipal sewage is of relatively recent origin as a pollutant.  It was first brought to public attention in the 19th century by a London physician who showed that the city’s cholera outbreak had been caused by just one contaminated well. Even though the contamination of drinking water by disease germs has been nearly eliminated in this country, hundreds of communities are still discharging raw sewage into streams and rivers.
   The problem of municipal sewage disposal is complicated by the fact that, years ago, most cities combined their storm and waste disposal sewers. Many of these combined systems work well, but others cannot cope with sudden heavy rains. When such storms occur, water mixed with sewage may flood and disable treatment plants unless bypassed, untreated, into a stream.  In either case, the people may have little protection for several days from these wastes that may contain disease germs.
   One consequence of pollution, usually resulting from the discharge of either raw or treated sewage wastes into water sources, is an increase in nutrient levels in these waters. These higher nutrient levels result in a rapid increase in the biological population of the water. Excessive respiration and decomposition of aquatic plants deplete the oxygen content in these waters causing decay which, in turn, may produce an undesirable taste, odor, color and turbidity. Increasing nutrient contents may also result in an in- crease in more undesirable species of aquatic life. All these factors make the water un- fit for domestic, industrial and recreational purposes.
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答案地方污水相对来说是污染物的较新来源。它最初引起公众关注是在19世纪，英国一位物理学家发现该城市的霍乱是由一个被污染了的水井引起。即使该国会造成引用水污染的疾病细菌已经被清除了，但成百的社区仍然将未经处理的污水直接倾人溪水或河流中。 数年前，许多城市将风暴降水和污水处理管道合并，这一做法使得地方污水的处理问题更为复杂。许多组合系统运作很好，但有些组合系统不能应付突然的大幅降雨。当此类暴风雨发生时，雨水混合着污水，可能会泛滥，使污水处理厂无能为力，从而迂回流淌，未经处理就汇人溪流。出现这两种情况中的任何一种，人们就会数日受到这些包含着致病细菌的废物的威胁。 污染通常由未经处理或已处理过的污水混入水源引起。污染的一个后果是水中营养级别升高。而水中营养级别的增高又会导致水中生物数量的剧增。过量的呼吸和对水生植物的分解会耗尽水中的氧气从而导致腐烂，而腐烂又会产生一种令人不快的气味、颜色，使水变得浑浊。营养物质的增加也可能引起让人更为反感的水生物种的增加。所有这一切，使得水资源不能用做家庭用水、工业用水或者是出于娱乐目的的用水。
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