Use of wastewater in Agriculture:The Nagging Dilemma

Next time you pass by a surface run-off, stream or polluted river with its heavy load of human, domestic, industrial and municipal waste, instead of covering your nose to ward off the stench you will do well to observe the kind of wastes in those waters, because a sizeable portion (20% global average) of what you eat come from plants irrigated with such wastewater – especially if you live in an urban area.

With an increasing global population, the ever-increasing demand for food and water has continued to place an increasing strain on the limited available freshwater resources. Although, over 70% of earth’s surface is water-covered, less than 3% is freshwater and two-thirds of this freshwater is locked up in glaciers and snow-cover. Furthermore, there are competing uses for this available freshwater; while drinking, sanitation, agriculture, and recreation feature high on the list; globally, agriculture takes about 70% of all freshwater withdrawals.

Thus, this increasing scarcity has led many resource-poor farmers, especially the urban and peri-urban farmers, to resort to the re-use of drainage water, effluents, and polluted streams and rivers for irrigating of their crops. The water from these sources is called wastewater.

Wastewater refers to water that has been polluted by of mixing with waste, industrial, agricultural, domestic or municipal drainage. It could be used directly when drainage water from the aforesaid sources is channeled to water a farm; or indirectly when untreated wastewater or drainage is discharged into rivers, streams or canals that supply irrigation water to farms.

The use of wastewater in agriculture is gaining tremendous popularity because of its wide range of benefits. Its agronomic, economic and environmental benefits cannot be overstated. These benefits include water conservation, provision of reliable water supply during dry seasons and periods of drought, and preservation/prevention of downstream rivers (and communities) from contamination with municipal and industrial drainages/wastewater and their environmental and health impacts.

Irrigation with wastewater increases available water supply and releases better quality water supplies for alternative uses (e.g. drinking and sanitation). In addition to these, its fertilizer value of is important. According to an FAO report in 1992, “a typical wastewater effluent from domestic source could supply all the nitrogen and much of the phosphorus and potassium that are normally required for agricultural crop production.”

This recycling of urban waste/nutrients helps in reducing farmers’ need to invest in chemical fertilizers which often have chronic negative impacts on the soil and water downstream. Consequently, many farmers using wastewater increase their income and are better able to support themselves and their families. Also, urban dwellers (especially the poor) have better access to fresher and cheaper farm produce. In all, wastewater use promotes urban food security.

However, unregulated use of wastewater poses risks to human (and environmental) health. This is because the composition of wastewater differs from one source to another. Wastewater may contain pathogens such as viruses, bacteria and protozoa; parasites such as helminthes; inorganic salts, toxic chemicals (or pesticides) and heavy metals in combination with its plant nutrients. Therefore, use of wastewater in farms could enrich soils with heavy metals which may bioaccumulates in plants to concentrations that may pose health and ecological risks, and can also contaminate farm produce with harmful microbes which transmit infectious diseases.

Examples of this abound over decades, including the “itai-itai disease” in Japan (1912-1946) caused by eating rice highly contaminated with cadmium. The rice paddies were irrigated with wastewater discharged from upstream zinc mines and many of the victims died of kidney failure and bone-softening. The fish in the river downstream also died as result of the heavy pollution of the surrounding water and soil – a situation that was not completely remedied until 2012. Another example in Japan is the “Minamata disease” caused by the heavy metal, mercury, in industrial wastewater which ended up in local fishery and caused both human and animal deaths between 1932 and 1968.

More recent happenings include a 1970 cholera outbreak in Jerusalem traced to the consumption of wastewater-irrigated vegetables, typhoid infections in the 1970s and 1980s and cholera outbreak in the early 1990s in Santiago, Chile also linked to the consumption of wastewater-irrigated salad crops; and the Mwea Rice irrigation project in Kenya where wastewater use was linked with a significant increase in schistosomiasis infection and death.

In many African countries, wastewater use is largely unregulated and often condoned by officials because of its benefits. Spore agricultural magazine (No 157) reported that, despite the health risks associated with untreated wastewater, “accessing raw wastewater through breaking mains and other means is a common practice in Dakar, while in Nairobi, thousands of families use the polluted Nairobi River to water their allotments”. It stated further that a study in Ghana reported “that typical microbiological and pesticide contamination levels of vegetables in Ghanaian markets pose a threat to human health.” In Nigeria the situation is not different, as a quick survey prior to a personal research work last year showed that wastewater is indiscriminately used by farmers (Oyo state) – especially the vegetable farmers.

Yet, experiences have shown that in spite of the multifarious benefits of wastewater use there is need for strict regulation of its use for public-health reasons. Already, many developed countries have strict wastewater guidelines and African countries will do well to adapt such guidelines for local use or enforce them for public health reasons wherever there are existing laws.

In conclusion, while the appeal of wastewater use will not go away because of its numerous benefits, the onus is on public officials to balance the benefits against the risks and draw a line through strict regulations in order to safeguard public health while reaping the benefits of wastewater use.