Untreated wastewater from cities expose 885 million people to severe health risks globally

Advanced modelling methods helped determining that 65 per cent of all irrigated areas within 40 kilometres downstream from urban centres worldwide are affected by major wastewater flows

Next time when you are lured by the healthy looking fresh vegetables being sold in urban limits or periphery areas, 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 out 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 out 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 a maximum of 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 wastewater 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, and proportion of wastewater treated.

The study finds out that 65% of downstream irrigated croplands were located in catchments where the return flow ratio (of wastewater) exceeded 20%. 41% of downstream irrigated croplands were located in catchments where wastewater return flows constituted the majority of available blue water (more than 50%), while 28% were located in catchments where the return flow ratios exceeded 80%—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.  This study seems to have confined its analysis to faecal contamination. Further, even though this particular study has not dwelled into the impacts on 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 out 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, 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 out that in most developing countries direct wastewater use projects are normally centred near large metropolitan areas. These schemes often only use a small percentage of the wastewater generated. The result is that indirect use of wastewater prevails in most developing countries.

Indirect use, that 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.

According to the World Health Organisation (WHO), the health hazards associated with direct and indirect wastewater use are of two kinds: the rural health and safety problem for those working on the land or living on or near the land where the water is being used, and the risk that contaminated products from the wastewater use area may subsequently infect humans or animals through consumption or handling of the foodstuff or through secondary human contamination by consuming foodstuffs from animals that used the area.

The FAO further notes that wastewater or natural water supplies into which wastewater has been discharged, are likely to contain pathogenic organisms similar to those in the original human excreta. Disease prevention programmes have centred upon four groups of pathogens potentially present in such wastes: bacteria, viruses, protozoa and helminths. There have been extensive reviews published on the range of these pathogenic organisms normally found in human excreta and wastewater.

Pollutants go beyond organic contamination

Urban wastewater do not only contain organic pollutants.  In fact, a lot of chemical contaminants too come with them, pollute our water sources, crops and ultimately health.  Inputs of metals and organic contaminants to the urban wastewater system occur from 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 industrial premises. The other principal sources of metals in domestic wastewater 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 load and polybutylene inputs equivalent to 25 per cent of the total load of this element have been reported in districts with extensive networks of polybutylene plastic pipework for water conveyance. The study finds out presence of several other dangerous chemicals including Mercury in urban wastewater that is being discharged and being reused for agriculture.

Treatments reduce risk but not enough

The above study of the European cities also finds out 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 in sludge 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.

This article was first published in Urban Update