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
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Water is withdrawn from the river by a community, treated for water supply and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.