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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

6. Wastewater and stormwater reuse (Topic d)

Human excreta and wastewater contains useful materials. These are water, organic carbon and nutrients. They should be regarded as a resource. In their natural cycles they are broken down by micro-organisms and become useful to plants and animals (Section 2 (2.2)), thus sustaining natural ecosystems. When improperly disposed these substances can cause pollution, because the organic materials exert oxygen demand, and the nutrients promote algal growth in lakes, rivers and near-shore marine environments.

Human excreta and wastewater contain pathogens. Reuse of the wastes must ensure that public health is maintained. Planned reuse is the key to wastewater reuse. Planning for reuse ensures that public health and protection of the environment are taken into account. Reuse of treated wastewater for irrigation of crops, for example, will need to meet (i) standards for indicator pathogens, and (ii) plant requirement for water, nitrogen and phosphorus. Standards for reuse of wastewater for various purposes have been developed by WHO and many states (see Regional Overviews). Plant requirements for water and nutrients is plant-specific and site-specific (dependent on soil type and climate) and information on these requirements need to be obtained from local sources of information.

Unplanned or unintentional wastewater reuse is already taking place widely when we have human settlements along a major river (e.g. the Mississippi River). Water is withdrawn from the river by a community, treated for water supply and distributed. After its use the water is collected, treated and discharged to the river. This process is repeated many times along the river. The only documented 'intended reuse' of this nature is in Windhoek, Namibia where treated wastewater is returned to the water reservoir supplying water to the town. This was initiated during a severe drought (See Regional Overview for Africa).

While reuse of wastewater for public water supply of drinking water quality standard is the exception, the technology exists to process wastewater to drinking water. A pilot plant at Denver demonstrated that 1 million US gallons per day (3.78 million L/day) of secondary effluent could be treated to produce water that is better in quality than water supplied to the city of Denver.

All water used for drinking purposes has in a sense been used, because in the water cycle (Section 2 (2.2)) water is continuously cycled.

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