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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Esystem 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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