Agricultural Use of Untreated Urban Wastewater In Low Income Countries

The use of urban wastewater in agriculture is a widely established practice, particularly so in urban and peri-urban areas of arid and seasonally arid zones.
Wastewater is used as a source of irrigation water as well as a source of plant nutrients (such as nitrogen, phosphorus and potassium) and trace elements (K, Na, etc) allowing farmers to reduce or even eliminate the purchase of chemical fertilizer and organic matter that serves as a soil conditioner and humus replenisher.
Lunven (1992) estimated that one tenth or more of the world’s population currently eats food produced on wastewater (but not always in a safe way).

However, irrigating with untreated wastewater poses serious public health risks, as sewage is a major source of excreted pathogens - the bacteria, viruses, protozoa- and the helminths (worms) that cause gastro-intestinal infections in human beings.
Wastewater may also contain highly poisonous chemical toxins from industrial sources as well as hazardous material from hospital waste. Relevant groups of chemical contaminants are heavy metals, hormone active substances (HAS) and antibiotics. The risks associated with these substances may, in the long run, turn out to constitute a greater threat to public health and be more difficult to deal with than the risks from excreted pathogens. Unregulated and continuous irrigation with sewage water may also lead to problems such as soil structure deterioration (soil clogging), salinisation and phytotoxicity.
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These risks are not limited to ‘official’ wastewater but often also apply to rivers and other open water sources, as indicated by figures gathered by Westcott: 45% of 110 rivers tested carried faecal coliform levels higher than the WHO standard for unrestricted irrigation (FAO, unpublished, cited in Birley).

The ideal solution is to ensure full treatment of the wastewater to meet WHO guidelines prior to use, even though the appropriateness of these guidelines are still under discussion. However, in practice most cities in low income countries are not able to treat more than a modest percentage of the wastewater produced in the city, due to low financial, technical and/or managerial capacity. The rapid and unplanned growth of cities with multiple and dispersed wastewater sources makes the management more complex. In many cities a large part of the wastewater is disposed of untreated to rivers and seas, with all related environmental consequences and health risks. The perspectives regarding the increase in wastewater treatment capacity in these cities are bleak. It may safely be assumed that urban and peri-urban farmers increasingly will use wastewater for irrigation, irrespective of the municipal regulations and quality standards for irrigation water.