There's still another step up the ladder of wastewater treatment sophistication: the wastewater treatment plant, or sewage plant. The wastewater treatment plant is like a huge, very sophisticated septic tank because it collects the waterborne excrement of large numbers of humans. Inevitably, when one defecates or urinates into water, one pollutes the water. In order to avoid environmental pollution, that "wastewater" must somehow be rendered fit to return to the environment. The wastewater entering the treatment plant is 99% liquid because all sink water, bath water and everything else that goes down one's drain ends up at the plant too, which is why it's called a water treatment plant. In some cases, storm water runoff also enters wastewater treatment plants via combined sewers. Industries, hospitals, gas stations and any place with a drain add to the contaminant blend in the wastewater stream.
Many modern wastewater plants use a process of activated sludge treatment whereby oxygen is vigorously bubbled through the wastewater in order to activate microbial digestion of the solids. This aeration stage is combined with a settling stage that allows the solids to be removed. The removed solids, known as sludge, are either used to reinoculate the incoming wastewater, or they're dewatered to the consistency of a dry mud and buried in landfills. Sometimes the sludge is applied to agricultural land, and now, sometimes, it's composted.
The microbes that digest the sludge consist of bacteria, fungi, protozoa, rotifers and nematodes.9 U.S. sewage treatment plants generated about 7.6 million dry tons of sludge in 1989.10 New York City alone produces 143,810 dry tons of sludge every year.11 In 1993, the amount of sewage sludge produced annually in the U.S. was 110-150 million wet metric tons. The water left behind is treated, usually with chlorine, and discharged into a stream, river or other body of water. Sewage treatment water releases to surface water in the United States in 1985 amounted to nearly 31 billion gallons per day.11 Incidentally, the amount of toilet paper used in 1991 to send all this waste to the sewers was 1.3 million tons.13 With each passing year, as the human population increases, these figures go up.
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