Agriculture, food and water - FAO.org

AGRICULTURE, FOOD AND WATER
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BOX 7 USE OF WASTEWATER FOR IRRIGATION
The cost of disposing of urban wastewater is all too often externalized against the aquatic environment
and downstream users in rivers, estuaries and coastal zones and hardly, if ever, appears in the benefit
and cost accounts. However, wastewater is recognized as a resource, particularly in water-scarce
regions. If the polluter actually pays, wastewater is free or has only a low cost, is reliable in time
and close to urban markets. In addition to direct benefits to farmers who would otherwise have
little or no water for irrigation, wastewater improves soil fertility and reduces water contamination
downstream. The total land irrigated with raw or partially diluted wastewater is estimated at 20
million hectares in fifty countries, somewhat below 10 percent of total irrigated land in developing
countries. For irrigation use, wastewater should be subject to primary and secondary treatment,
but in poor countries that is often not the case and raw sewage is applied. Disadvantages and risks
related to use of insufficiently treated wastewater concern the exposure of irrigation workers and
food consumers to bacterial, amoebic, viral and nematode parasites as well as organic, chemical
and heavy metal contaminants. In a context of prevailing poverty, such water is used in the informal,
unregulated sector, but sanitary concerns preclude the export of the products and, at least partially,
the access to local food markets. Governments and the development community promote efforts to
lead wastewater reuse into sustainable channels, but countries and municipalities short of resources
are slow in facing the cost of water treatment. Given water scarcity and the relatively high cost of
obtaining potable freshwater for municipal uses, the use of treated wastewater in the urban context
is projected to increase in the future.
Health and irrigation
The key irrigation-related vector-borne diseases are malaria, schistosomiasis and Japanese
encephalitis. Irrigation development has in the past sometimes been accompanied by adverse
impacts on the health status of local communities. The principal causes for these impacts are
rooted in ecosystem changes that created conditions conducive to the transmission of vectorborne
diseases, and drinking water supply and sanitation conditions leading to gastrointestinal
conditions. The attribution of the burden of each of these diseases to irrigation, or components
thereof in specific settings is complex. Only where irrigation is introduced in an arid region where
the diseases previously did not occur, is the association between the resulting dramatic landscape
changes and the explosive rise of disease incidence and prevalence clear-cut. In most cases,
there is a complex mixture of contextual determinants of the diseases combined with a number
of confounding factors. For example, in parts of Africa south of the Sahara, the transmission
of malaria is so intense throughout the year that the additional risk factors from irrigation
development will not add to the disease burden. Schistosomiasis, rightly equated with irrigation
in Africa, is also determined by human behaviour and by the state of sanitation.
Many vector-borne disease problems in irrigated areas can be traced to absent or inadequate
drainage. The various forms of surface irrigation all impose increased vector-borne disease
hazards, while overhead irrigation and drip irrigation are virtually free of such hazards. Crop
selection can be important. In that sense, flooded rice and sugarcane are crops that carry increased