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6. Wastewater and stormwater reuse
Human excreta and wastewater contains useful materials. These are water,
organic carbon and nutrients and should be regarded as a resource. In their
natural cycles they are broken down by micro-organisms and become useful to
plants and animals, 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 also 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. WHO and others
have developed standards for reuse of wastewater for various purposes. Further
details of these standards can be found in the Regional Overviews in the Source
Book, published by IWA and IETC. Plant requirements for water and nutrients are
plant-specific and site-specific (dependent on soil type and climate) and
information on these requirements need to be obtained from local information
sources.
6.1 Wastewater reuse from off-site treatment plants
6.1.1 Wastewater reuse for agriculture
Treated wastewater from off-site treatment plants can be reused for
irrigation of parks and gardens, agriculture and horticulture, tree plantation
and aquaculture, if these exist or can be established not far from the
wastewater treatment plants. For these purposes the wastewater should generally
be treated to secondary wastewater standard (< 20 mg/L BOD and < 30 mg/L SS).
Total coliforms should be < 1000 organisms per 100 mL for irrigation by
spraying. When sub-surface irrigation is used this requirement may not be
necessary. A period of non-entry to irrigated sites may need to be observed,
particularly for wastewater-irrigated parks and gardens. Irrigation of
vegetables for direct human consumption requires a much stricter guideline.
Because requirement of wastewater for plant growth is governed by climatic
conditions, soil and plant type, there may be a need for storage of the
wastewater. An alternative to storage, if land area is not available for this
purpose, is to dispose of wastewater that is excess to requirement. A
combination of wastewater for irrigation and aquaculture (see below) is also an
option that can be considered.
Land application for treatment of wastewater described in Section 4.2.4 (Slow
rate land application and grass filtration) when combined with growing of
grasses for grazing by sheep or cattle plus the “cut and carry” system can
properly be considered as treatment and reuse of wastewater.
6.1.2 Wastewater reuse for aquaculture
Wastewater reuse for aquaculture has been practised in many countries for a
considerable period of time. It has the potential of wider application in the
tropics. There is great diversity of systems involving cultivation of aquatic
species, (mainly fish) and plants (mainly aquatic vegetables such as water
spinach). The Source Book, published by IWA and IETC, contains a detailed
section on aquaculture and a case study is presented in the Regional Overview
for Central & South America.
Farmers and local communities have developed most reuse systems; the primary
motivating factor has been reuse of nutrients for food production rather than
wastewater treatment, and with scant attention to either waste treatment or to
public health. In most aquaculture systems, wastewater is not reused directly in
aquaculture and the nutrients contained in the wastewater are used as fertiliser
to produce natural food such as plankton for fish. These nutrients, mainly
nitrogen and phosphorus, are also taken up directly by large aquatic plants such
as duckweed which is cultivated for animal feed, and aquatic vegetables such as
water spinach and water mimosa cultivated for human food.
As wastewater provides a source of nutrients for aquaculture, it is
technically feasible to link it up with most sanitation technologies, providing
that land is available at reasonable cost. Farmers have learned by experience
how to culture fish, first in static-water nightsoil-fed ponds and more recently
in conventional wastewater-fed fishponds. Research has provided a scientific
basis for the key parameters in wastewater-fed aquaculture practice developed
earlier by farmers and these can be found in the Source Book, published by IWA
and IETC.
There are a number of constraints to wastewater-fed aquaculture and they need
to be considered where the practice is considered to be an option. They include:
- lack of knowledge of aquaculture as a technical option in wastewater
treatment and reuse.
- limited available sites in peri-urban areas where wastewater is available
for reuse
- rapid urbanisation in developing countries threatens the existing
wastewater-fed systems
- rapid eutrophication from both urbanisation and industrialisation
- improved sanitation reduces the availability of nightsoil for agriculture
and aquaculture.
- rapid industrialisation contaminates nutrient-rich domestic wastewater with
industrial wastewater.
- social and cultural acceptance of wastewater-fed
- climate - wastewater-fed aquaculture involves the farming of warmwater
organisms
 Despite the constraints
listed above, there is considerable potential for the reuse of wastewater in
managed aquaculture in the tropics. A correctly managed system would limit
public health risks and wastewater should never be reused without prior
treatment if the produce (fish or aquatic vegetables) is intended for direct
human consumption. Figure 36 presents strategies for the reuse of wastewater
through aquaculture.
There are a number of situations where wastewater-fed aquaculture has
significant potential for incorporation into existing and proposed improved
sanitation schemes:
- Developing countries that cannot afford mechanical wastewater treatment
schemes. Although aquaculture in stabilisation ponds requires more land, it
produces significant benefits such as increased employment for local people and
revenue from sale of produce which, in turn, can be used to subsidise the
wastewater treatment.
- Arid and semi-arid countries have an increasing need to reuse water as well
as nutrients contained in wastewater. Pilot projects on culture of fish in
treated stabilisation pond effluents have been successfully completed in arid
areas in Egypt, the Middle East, Peru and in Latin America.
6.1.3 Wastewater reuse for industry
Treated wastewater can also be used for industrial purposes if suitable
industries are not far from the treatment plant. Industry’s requirement for
water quality ranges widely, from very pure water for boilers of electricity
generation to lower water quality for cooling towers. Treated wastewater can
fulfil the lower range of this requirement, e.g. water for cooling towers.
Secondary-treated wastewater after chlorination may be adequate for this
purpose.
With off-site treatment plants reuse of wastewater may be limited by the need
to pipe treated wastewater to where it is needed. To implement wastewater reuse
in houses for toilet flushing, watering of gardens and other purposes which do
not need drinking quality water, a third pipe-reticulation system is required,
that is in addition to the reticulation to provide drinking water and the sewer
to collect the wastewater. Care is also needed to prevent cross-connection
between drinking water and treated wastewater.
'Sewer mining' is the term given to the withdrawal of wastewater from a sewer
for reuse near to the point of withdrawal. This provides an opportunity for
reuse without having to pipe treated wastewater from the centralised treatment
plant. Wastewater needs to be treated to the standard required for the reuse,
and may duplicate the function of the centralised treatment plant.
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