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Wastewater for farming

Published Jun 25, 2012 12:33am

IN most developing countries including Pakistan, the use of sewage and wastewater for irrigation is a common practice. Small farmers often prefer wastewater because its high nutrient contents reduce or even eliminate the need for expensive chemical fertilisers. However, farmers using wastewater often fail to adopt safeguards required for human, animal and environmental health and to improve beneficial use of water and nutrients. While the risks do need to be carefully considered, the importance of this practice for the livelihoods of countless small landholders must also be fully recognised.

Applying wastewater to agricultural lands is considered an economical alternative and ecologically more sound than uncontrolled dumping of municipal and industrial effluents into lakes and streams.

In some cases up to 37 per cent increase in harvest is possible if raw wastewater is applied compared to freshwater and chemical fertiliser. Year-round availability of wastewater allows farmers to grow economically valuable crops out of season, thus increasing their value.

The value of wastewater is well illustrated in the country where market prices of farmland with access to a source of wastewater are 3.5 times more than those without it. Wastewater treatment is necessary to protect human health and prevent contamination of lakes and rivers.

If planners are to give the right incentives to local stakeholders, including farmers, for participation in wastewater management, there has to be a contextual understanding of how and why farmers use wastewater.

Wastewater, effluent and polluted water sources contain nutrients in a variety of chemical forms with diurnal and seasonal variations in concentration. Nitrogen and phosphorus occur at concerning levels in wastewater but levels of potassium are generally below crop requirements.

Irrigation with primary treated domestic wastewater typically applies more nitrogen to the soil than is required by crops, with the exception of perennial grasses. Overdose of nitrogen and phosphorus affects the quality of certain crops and, when leached from the soil, may lead to eutrophication of surface waters.

Nitrates in drinking water can be hazardous to human health and nitrite is extremely toxic to most fish and other aquatic species. Adequate information is often neither available nor is forwarded to farmers in a useful way to encourage appropriate nutrient management.

Crop selection can be used as a strategy to optimise the application of wastewater for irrigation as a natural treatment. In this case, the selection of perennial grasses allows a longer application season, higher hydraulic loadings, and greater nitrogen removal than possible with other agricultural crops. Vegetables are most controversial because of health risks when eaten uncooked and irrigated with poorly treated wastewater.

Obviously the short-term benefits of wastewater irrigation could be offset by health and environmental impacts. The first step is to scientifically evaluate these.

This means, for example, finding affordable ways of monitoring the presence of harmful contaminants in wastewater, such as heavy metals that can accumulate in soil and crops. It means looking at farming practices and crops grown to find ways of minimising risks of infection for farmers and consumers.