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Wastewater Reuse

Introduction
In many situations in developing countries, especially in arid and semi-arid areas, wastewater is simply too valuable to waste. It can be profitably reused to grow crops and/or fish. But first the wastewater should be treated to reduce pathogenic and faecal indicator micro-organisms to acceptable levels, to ensure there is no threat to human health.

Agricultural reuse
This is using treated wastewater for crop irrigation. Crop yields are higher than with freshwater irrigation as treated wastewater not only supplies water, but also plant nutrients (especially nitrogen and phosphorus).

Restricted irrigation is the irrigation of all crops except salad crops and vegetables that may be eaten uncooked. For restricted irrigation, WHO recommends the treated wastewater should contain no more than one human intestinal nematode egg per litre. These intestinal nematodes are:

Ascaris lumbricoides, the human roundworm,
Trichuris trichiura, the human whipworm, and
Ancylostoma duodenale and Necator americanus, the human hookworms.
For unrestricted irrigation, i.e. including salad crops and vegetables eaten raw, WHO recommends the same helminth egg value, and additionally no more than 1000 faecal coliform bacteria per 100 ml of treated wastewater.

See the article by Blumenthal et al. (1) on suggested revisions to these WHO guidelines, in the attached list of publications on agricultural reuse.

Aquacultural reuse
Aquaculture means “water farming”, just as agriculture means “field farming”, and so covers fish culture and aquatic vegetable culture. Fish and aquatic vegetables grow well in treated wastewater. WHO recommends that the treated wastewater used for aquaculture should not contain any viable human trematode eggs (zero per litre), and the fish or aquatic vegetable pond should not contain more than 1000 faecal coliforms per 100 ml. The major human trematodes are:

Schistosoma spp.,
Clonorchis sinensis, and
Fasciolopsis buski.
The schistosomes are widely distributed in the tropics (though not so much in South Asia). The last two are mainly found in the Far East.

Treatment
See the Sanitation Connection pages on Wastewater Treatment Technology.
It's worth noting that treatment requirements for unrestricted irrigation are much greater, and thus more costly, than those for restricted irrigation. Engineers and planners should talk to the local farmers to assess both their needs and the required quality of treated wastewater.

Environmental benefits
These accrue mainly because, when wastewater is treated and reused, it is not discharged (often untreated) into streams, rivers or lakes where it would cause aquatic pollution of varying severity - in extreme cases (which are not uncommon in developing countries) the receiving water can become seriously deoxygenated, even anaerobic, with a literally deadly effect on aquatic life, and on the lives of those who use the polluted water for domestic, including drinking, purposes.

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