Waste not want not

Use of outflows from urban sewage systems as a source of irrigation water is both extremely common, and in some countries, notably China, a very long-established practice. Nor is wastewater always used only as a last resort; even in wet climates many farmers actually choose to use it in preference to cleaner water, because of its high nutrient content, which can reduce or even eliminate the need for artificial fertilizers. For urban and peri-urban farmers living in arid areas, or depending on dry-season cultivation, sewage outflows may be their only reliable water source. And for many landless people living on city fringes, rented plots along wastewater canals allow them to grow vegetables for sale, a crucial source of income.

Credit: IWMI

High risk farming?

However, despite its importance for countless livelihoods, the use of urban wastewater for irrigation is not without danger. In the short term, the health of farmers and consumers is obviously at risk Wastewater contains the full spectrum of pathogens found in the urban population, and many of these can survive for several weeks when discharged onto fields. The most common threat comes from high concentrations of bacteria and from intestinal worms, which pose a high risk to farming communities and their customers. Environmental impacts of wastewater irrigation also raise doubts about its long-term sustainability. Long-term use of saline and sodium-rich water tends to erode soil structure and reduce productivity. Moreover increasing industrialisation is changing the composition of wastewater, raising the level of heavy metals, acids and the like, which are known to have impacts on soils and crops. Groundwater contamination from nitrates and other pollutants are another potential danger, and many of these problems are irreversible.

With cities industrialising and expanding, the challenge for municipal authorities of how to manage their wastewater resources is a growing one. One organisation that is attempting to increase the body of knowledge on wastewater use in agriculture is the International Water Management Institute (IWMI). The institute is currently undertaking a global assessment of wastewater use, to clarify the risks and benefits, and develop affordable improvements for wastewater systems. In the past international perspectives on wastewater have tended to focus on protecting consumers, while giving little weight to the needs of the irrigators. The World Health Organisation, for example, has standards for the degree of contamination that is permissible for wastewater use in irrigation activities. For most developing countries these standards are too stringent and expensive to be implemented, and are ignored. IWMI's assessment will include a broader range of interests, including water management and environmental authorities, farmers and the wider public.

In Latin America, IWMI studies have initially focussed on Mexico, the country with the highest wastewater use in the region. Application of wastewater to agricultural land has long been practised in Mexico, as elsewhere, as a cheap alternative to water treatment plants. Supporters of the practice point out that de facto 'treatment' by cropland application is more environmentally sound than uncontrolled discharge of effluents into rivers and lakes. However, recent policy changes in Mexico have mandated that municipal authorities re-assess water treatment, and this has prompted IWMI researchers to make a more detailed appraisal of just how effective, and valuable, the existing practice is. Wastewater from the city of Guanajuato, for example, has been calculated to be worth around US$270,000 per year to the surrounding farmers, and this would be a significant loss if water were recycled to the urban area via a treatment plant. The researchers found that levels of chemical contamination in the irrigated fields and drainage channels were within international norms, and concluded that while continuing use of wastewater would demand an ongoing programme to monitor health and environmental impacts, the costs of such a programme would be less than the operating costs of a treatment plant.

Minimising risks

IWMI is keen to point out that while water treatment or use of clean water sources are undoubtedly 'better' options, for many countries they will be unaffordable, and in this context finding ways to minimise the risks from wastewater use is a priority. Obvious ones relate to health, and here education for farmers and consumers is the key. Farming communities are often unaware of the dangers from wastewater, but for those that can afford them, boots and gloves can provide high levels of protection against infections, and regular self-treatment for intestinal worms can have a major impact on their general health. Educating consumers about proper washing of vegetables hardly sounds ground-breaking, but can have a profound effect on the levels of coliform and worm-related disease, and is certainly a much cheaper approach than trying to improve water quality. Putting restrictions on the type of crop that can be grown is another way of reducing risks. In the irrigated districts fed by wastewater from Mexico City, farmers are not allowed to grow fruit and vegetables, and instead produce grains like maize, wheat and sorghum, and fodder crops such as alfalfa. This does of course entail a productivity cost for the farmers. Other simple methods for farmers include changes in irrigation methods, which can have very different levels of risk. Using flooded furrows to irrigate leafy vegetables, for example, can greatly reduce the risk of contamination compared to overhead use of watering cans.

The environmental impacts of wastewater use will vary considerably from city to city, depending partly on the extent of industrialisation, the nature of water distribution, and the degree of treatment or dilution, if any, that the water is given before it reaches the fields. IWMI's assessment will include systems in Vietnam, Ghana, Pakistan and India, and over the next two years the work aims to build a much clearer picture of the issues raised by wastewater use. The findings should benefit both urban and peri-urban communities, as well as the local authorities charged with managing and exploiting the rising volumes of urban wastewater.

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