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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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4. Wastewater and stormwater treatment

The treatment of wastewater and stormwater means the reduction and removal of pollutants from the water. The first principle to bear in mind therefore is to prevent pollutants from entering the water in the first place. In the case of stormwater we need to ensure that surfaces through which stormwater run-off passes over should as far as possible be free from solids and other wastes. Thus the collection of solid wastes is an important part of stormwater treatment as is the separate collection of wastewater and stormwater. The treatment of industrial wastewaters before discharge to the sewer is also extremely important in preventing pollutants from entering the wastewater and stormwater systems.

In the case of wastewater, separating blackwater and greywater can mean less energy is required in treatment. This is because blackwater contains most of the solids, which during treatment have to be removed from the mixture. Further separating urine and faecal materials may also mean that the urine can be reused without much treatment and the faecal materials can be more simply treated. The use of water to convey toilet wastes may be questioned based on this principle, because treatment means separating these wastes from the water.

Besides preventing pollutants entering the water, water conservation means that a lower volume of water has to be treated. Since the size of treatment systems is primarily governed by the volume of water to be treated rather than the amount of pollutants in the water, a lower volume means smaller treatment plants and a corresponding capital cost. Use of less water to flush toilets belongs to this principle.

A range of wastewater treatment technology options is presented below. Treatment of wastes on-site is considered first (4.1), followed by off-site treatment of the wastewater (4.2). Each technology requires maintenance and proper operation. The demand of each technology for maintenance varies and this is also discussed, as well as the public health and environmental impacts of the technology. Treatment options for stormwater are presented in section 4.3.

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