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 ease, 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 increase in more undesirable species of aquatic life.
