Reverse flow of urban pollutants in sewage irrigated crops in developing countries

Abstract

Dwindling fresh water supplies in developing countries have led to increased interest in exploring alternative irrigation water sources including untreated or partially treated wastewater. Wastewater originates from a variety of sources including domestic sewage, mixed with industrial effluents, and stormwater. Wastewater irrigation benefits farmers and other stakeholders through better crop yields owing to reliable water and nutrient supply and pollution reduction of rivers and other surface water bodies. However, it poses health risks to farmers and consumers alike due to pathogens, build-up of chemical pollutants especially heavy metals in soils and groundwater contamination. A very small proportion of sewage water (SW) is treated in a few working sewage treatment plants (STPs) in developing countries. Many existing STPs are running above the capacity and only partially treat SW. Many studies on SW in developing countries show the presence of high metal and metalloid concentrations beyond maximum permissible limits. Likewise, several studies evidence increased concentrations of heavy metals in both groundwater and surface waters in developing countries. Repeated use of SW for irrigation may result in accumulation of heavy metals in soils, which can enter the food chain. Review of pertinent literature demonstrates heavy metal accumulation in soils and eventually in edible portion of crops grown in peri-urban areas. Site specific best management practices aimed at curtailing the bioavailability of pollutants in soil and eventual uptake and translocation to edible parts of the crops are required. New STPs and upgradation of the existing STPs are the need of hour. Developing countries need to develop pretreatment standards for industrial waste discharged to the STP collection systems.