Wastewater, a resource for agriculture?

Wastewater should be recognised as a resource and not seen simply as sewage. That sentiment from Swedish researcher Therese Sjomander Magnusson, expressed at World Water Week in Stockholm, echoes the feelings of the many, usually poor, farmers who depend on wastewater from urban areas to grow crops and raise fish. The water also provides dissolved nitrogen and phosphate nutrients; indeed some prefer wastewater to 'clean' water, and farmers using wastewater in Pakistan earn approximately US$300 more than those using normal irrigation water. In fact, 25 per cent of Pakistan's vegetables, including salads, are irrigated with wastewater.

While wastewater use for food production is looked on with distaste by some, and there are undoubted risks attached, it is a practice centuries old in cultures such as China. In Europe, 'sewage farming' was accepted practice in the nineteenth century but was discontinued because of public health concerns. But, whereas Europe had ample access to alternative sources of water and crop nutrients, farmers in most developing countries do not and, with rapidly growing cities driving demand for fresh produce and other crops, and producing increasing flows of wastewater, it is not surprising that this available and affordable resource is being harnessed to meet a market demand. With cities predicted to use 150 per cent more water by 2025, and some 70 per cent of that returning as wastewater, there appears no foreseeable prospect of a wastewater shortage. Trying to ignore or even ban the practice, as some governments are doing, is unrealistic and a wasted opportunity to optimise use of the resource, with appropriate safeguards.

World Water Week saw the launch of a new book, Wastewater use in irrigated agriculture, by the International Water Management Institute (IWMI) and Canada's International Development Research Centre (IDRC). Lead editor, Christopher Scott of IWMI, suggests that as many as 20 million hectares are partially or wholly irrigated with wastewater and it is estimated that up to 10 per cent of the world's population eats food produced using wastewater. "The two organisations are not advocating the practice," he says, "but this is the reality and what we have put together are studies and examples to show how that water could be used, with a set of controls and management practices that take into account the health risks and the agronomic and soil quality issues to provide a sustainable system."

Risks and benefits
The risks from wastewater are primarily bacteriological and parasitic. Hookworms and tapeworms, present in human excreta, and amoebas and bacteria causing diarrhoea are the major hazards, particularly for the farmers themselves who cannot avoid constant contact with the water they are using. Consumers are also at risk from leafy vegetables contaminated with pathogens in the irrigation water, and they represent the larger number of people at risk. However, consumer risk can be greatly reduced by modifying irrigation techniques. "IWMI has done some work which shows that watering can and sprinkler irrigation is not the best way of applying wastewater", says Scott. "There is much less risk of contamination if water is applied to the root zone underneath the level of the edible portion of the crop using furrow irrigation." Unfortunately, micro-irrigation systems using drip-feed and seep-hoses are easily clogged with wastewater and so are not suitable. Contamination of produce can also occur at point-of-sale, when vendors use dirty water to 'freshen up' their produce and slow wilting. If clean water can be provided and used by vendors, contamination could be greatly reduced and wastewater-irrigated crops consumed with far less risk.

The extent of pathogenic contamination depends on whether the water used is 'grey' water, from bathing and washing clothes, or 'black' water, which includes excreta. But even 'grey' water may not be pathogen-free. Work by IDRC in Jordan found that where young children and nappies (diapers) are washed in the bath or basin, there is a significant risk of faecal contamination. Wastewater use in irrigated agriculture includes chapters on devising separation of wastewater sources and treatment in countries including Jordan, Mexico and Tunisia, and provides examples for other countries.

A third type of wastewater is industrial effluent. "This can contain a whole toxic brew of a number of different contaminants, including heavy metals, pharmaceutical by-products and organic solvents", says Scott. "Endocrine disruptors that are pharmaceutical by-products, solvents and other organic compounds pose very significant human health risks." A major concern is that we do not yet know the full effects of some of these materials on human health and development, and ideally such wastewater should only be used highly diluted with freshwater or not at all.

Separating different wastewater 'streams' and providing appropriate treatment to minimise risks to farmers and consumers is clearly a priority so that water, which is a finite resource, can be recycled to the benefit of farmers, consumers and the country as a whole. The challenge is that wastewater appears to fall into a 'no-man's land' of research, policy and management. Water researchers, for example, tend to focus on freshwater sources (lakes, rivers and aquifers), while urban research and planning views wastewater as a matter of waste disposal not water use. Benefits would accrue to rural and urban sectors if collaboration could bridge this divide and simultaneously help solve the problems of wastewater disposal and agricultural water demand.