Urban Stormwater Runoff
Stormwater is another issue in sewage treatment. During rainstorms, the water washing down the buildings, streets, and sidewalks is collected into the sewers. A portion of the stormwater can be processed by the sewage treatment plant, but once the plant reaches over-flow, the water is often released directly into the environment. Most systems are not designed to process more than a small percentage of the overflow from major storms.
Stormwater overflow is a major source of pollution for urban rivers and streams. It has a high percentage of heavy metals (cadmium, lead, nickel, zinc) and toxic organic pollutants, all of which constitute a health and environmental hazard. It can also contain grease, oil, and other automotive product pollution from street runoff, as well as trash, salt, sand, and dirt. Large amounts of runoff can flush so-called dry weather deposition from sewer systems, causing overflow to contain the same types of pathogens as raw sewage. The runoff is oxygen-demanding, meaning that if routed directly into rivers, streams, and oceans, it will rob the water of the oxygen needed to support life.
Historically, stormwater runoff has not been considered part of the sewage treatment plan. Most municipal sewage treatment facilities have only minimal space for storing runoff, after which it is routed directly into receiving waterways. Government and engineers are studying various ways of lessening the problem, including construction of catchbasins to hold runoff, flushing sewers regularly to reduce dry-weather deposition of sewage, implementing sewer flow control systems, and a number of strategies to reduce deposition of litter and chemicals on city streets. Economical methods of creating storage tanks and performing preliminary and secondary treatment of the runoff water are being developed. According to some estimates, it could cost the U.S. as much as $300 billion for combined sewer overflow and urban stormwater runoff control. It is left to be seen how much more the environmental effects of uncontrolled runoff will cost.
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