To keep our used water from spoiling our water resources, we have to remove the pollutants before the water gets back into the environment. In urban areas in most developed countries, the wastewater from homes, businesses and factories is collected by a system of underground pipes-- sewers-- which carry it to one or more central treatment facilites. Most of these are located near bodies of water into which the treated wastewater is discharged.

In the U.S., all such facilites must have a permit issued by the federal and/or state government, describing limits on the amounts of various pollutants which may be discharged. The U.S. EPA (Environmental Protection Agency) refers to these as NPDES (National Pollutant Discharge Elimination System) permits. Industries located in areas where they are not connected to a sewer can discharge directly into a waterway, but will need a permit, and will probably have to have their own treatment plants. Even industries which are connected to sewers may have to pretreat their wastewaters before discharging them into the sewers, because they may contain materials which will harm the sewers or the treatment plants-- or may be a danger to the people who work in maintaining the sewer system.
[If you are some one who works in an industrial pretreatment program, you may be aware that the USEPA has recently (July 22, 1999) proposed a series of changes to the current regulations. I have prepared a summary of the proposals which you may find helpful before reading the entire document.]

Homes in non-urban areas that are not connected to a sewer are usually required by their town to have on-site treatment systems. Most common for single homes are septic systems, which consist of a buried tank connected to a set of perforated pipes, embedded in gravel, which distribute the water into the soil. (The "Links" page has several references on this subject.) Larger housing complexes may have treatment systems based on the principles used in full-scale sewage treatment plants.

A Word About Sewers: Besides having a set of sewer pipes-- called, strangely enough, sanitary sewers-- which carry wastewater to a treatment plant, cities and towns also need pipes to collect stormwater. These are needed to prevent street flooding and usually lead directly to a waterway without any treatment. The runoff of pollutants from streets and yards into these storm sewers contain oil and other automotive wastes, which may contain toxic metals and organic compounds-- as well as pesticides and nutrient-containing fertilizers from lawns and gardens, and pathogenic microorganisms from animal wastes. The problem of pollution from storm sewers is currently being addressed by the USEPA. Further complicating the situation is the fact that while some cities and towns have completely separate sanitary and storm sewer systems, many others have combined systems. During rainy periods, combined sewers cause two problems: overloading of the treatment plant with extra water and contaminating waterways with untreated sewage from overflows. Even in cities with separate sewer systems, the flows to the treatment plants often increase greatly when it rains because of cracks or separations in the pipes, which allow groundwater or stormwater from broken storm sewer pipes to infiltrate into the sanitary sewer-- or from direct inflow of stormwater into manholes and from illegal connections of roof drains and sump pumps in buildings

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