Wastewater use in crop fields grows with urban growth and the Food and Agriculture Organisation (FAO) of the UN finds that in most developing countries direct wastewater use projects are normally centred near large metropolitan areas. However, this untreated wastewater from cities exposes 885 million people to severe health risks globally
Next time when you are lured by the healthy looking fresh vegetables being sold in urban limits or periphery area, don’t just jump into them. They could have been produced with wastewater, from the nearby city or town, with high levels of faecal contamination hence posing a lot of health risks for you and your family. A just published study by a team of researchers led by a recent graduate from University of California finds out that croplands irrigated globally by untreated urban wastewater is actually 50 per cent greater than previously thought.
The study, that has used advanced modelling methods, finds out that 65 per cent of all irrigated areas within 40 kilometres downstream from urban centres – amounting to about 35.9 million hectares worldwide – are affected by wastewater flows to a large degree. 29.3 million hectares of this are in countries with very limited wastewater treatment. This exposes 885 million urban population as well as farmers and food vendors to serious health risks, finds the study. These people are from inside the cities as well as within 40 kilometres from them. The health impacts may be spreading even beyond that.
China, India, Pakistan, Mexico and Iran account for most of this cropland. The study finds that only 22 per cent of India’s urban wastewater is receiving some sort of treatment. However, lack of data – both in India and other countries – makes it difficult to assess the exact level and extent of contamination. China with 71.2 per cent has the highest volume of urban wastewater treatment and Pakistan with just 1.2 per cent of treatment is the lowest. Iran treats 4.2 per cent while Mexico treats 53.9 per cent of its urban wastewater. The previous study that was so far being cited to be the most comprehensive study of such a nature was done in 70 countries and was published in 2004. That study had estimated the croplands irrigated by wastewater at 20 million hectares.
The study
The current study used GIS-based modelling methods to develop the first spatially-explicit estimate of the global extent of irrigated croplands influenced by urban wastewater flows, including indirect waste water use, said the International Water Management Institute (IWMI) that was also part of the study. Published this week in the journal Environmental Research Letters, the study classified these croplands further by their likelihood of using poor quality water based on the spatial proximity of croplands to urban areas, urban wastewater return flow ratios, andproportion of wastewater treated. The study finds that 65 per cent of downstream irrigated croplands were located in catchments where the return flow ratio (of wastewater) exceeded 20 per cent. 41 per cent of downstream irrigated croplands were located in catchments where wastewater return flows constituted the majority of available blue water (more than half) while 28 per cent were located in catchments where the return flow ratios exceeded 80 per cent—indicating that wastewater constituted an extremely high proportion of available surface water. Catchments with high values for these indicators were clustered around northern India, northern China, and Pakistan.
Health risks
Reuse of urban wastewater for irrigation of crops poses very high health risks especially for countries like India where most of the wastewater is discharged untreated into rivers, waterbodies, lands and everywhere else. This study seems to have confined its analysis to faecal contamination. Further, even though this particular study has not dwelled into the impactson the types of crops being grown in downstream irrigated croplands, several other studies have found out that vegetable crops are grown more often in these regions than in rural areas, particularly in low and middle income countries, There is every likelihood that this also holds true for India with the increasing urbanisation and vegetables going out of bounds for rural population. The study finds that the use of poor quality water in irrigated agriculture has the potential to impact farmers, labourers, market vendors, and consumers across the produce supply chain. However, the level to which these risks are realized depends upon irrigation and cultivation practices, crop type, post-harvest handling, and food safety and preparation practices (e.g. raw versus cooked).
Wastewater use in crop fields grows with urban growth and the Food and Agriculture Organisation (FAO) of the UN finds that in most developing countries direct wastewater use projects are normally centred near large metropolitan areas. Indirect use occurs when wastewater (treated, partially treated or untreated wastewater) is discharged to reservoirs, rivers and canals that supply irrigation water to agriculture. Indirect use poses the same health risks as planned wastewater use projects, but may have a greater potential for health problems because the water user is unaware of the wastewater being present, says the FAO and warns that indirect use is likely to expand rapidly in the future as urban population growth outstrips the financial resources to build adequate treatment works.
Pollutants go beyond organic contamination
Urban wastewater does not only contain organic pollutants. In fact, a lot of chemical contaminants come with it, pollute our water sources, crops and ultimately health. Inputs of metals and organic contaminants to the urban wastewater system occurfrom three generic sources: domestic, commercial and urban runoff, says a study by ICON in the UK. In fact, according to this study, in general, urban runoff is not a major contributor of potentially toxic elements to urban wastewater.
The above study done in European cities says that faeces contribute 60 to 70 per cent of the load of Cadmium, Zink, Copper and Nickel in domestic wastewater and less than 20 per cent of the input of these elements in mixed wastewater from domestic and industrialpremises. The other principal sources of metals in domestic waste water are body care products, pharmaceuticals, cleaning products and liquid wastes. Plumbing is the main source of Copper in hard water areas, contributing more than 50 per cent of the Copper loadand polybutylene inputs equivalent to 25 per cent of the total load of this element have been reported indistricts with extensive networks of polybutylene plastic pipework for water conveyance. The study finds presence of several other dangerous chemicals including Mercury in urban wastewater that is being discharged and being reused for agriculture. The above study of the European cities finds that large urban wastewater treatment plants have been less successful in treating pollutants in comparison to small plants. Potentially toxic element concentrations remain higher in sludge from large treatment plants compared to smaller plants and they are also greater in sludge from industrial catchments compared with rural locations. These patterns insludge metal content suggest that commercial sources may still contribute significantly to the total metal load entering wastewater treatment plants, asserts this study.
Way forward
There have been efforts to study nutrients from urban wastewater that in fact help agriculture, however the wastewater certainly do more harm than doing any benefit. India’s urban areas are growing fast and wastewater load in farming, especially vegetables – as can be gauged from the primary study in reference in this article. There is an urgent need to thoroughly investigate the urban wastewater-food production-health risk links and take appropriate measures to curb both pollution and health risks. A greater challenge in this would also be to ensure food security by ways in which the non-polluted crops take over the polluted crops in a systematic phased manner and the farmers and others involved in the agricultural operations don’t lose their livelihoods.