Untreated waste water from cities is used to irrigate 50 per cent more farmland worldwide than previously thought, leaving some 885 million people exposed to the risk of diseases, including diarrhoea and cholera, a study revealed.
Crops covering almost 36 million hectares – an area roughly the size of Germany – are irrigated with water from rivers and lakes used by cities within 40km upstream to discharge sewage, according to an international team of researchers.
About 80 per cent of these crops – 29 million hectares – are in countries with very limited waste water treatment, such as China, India, Pakistan, Mexico and Iran, according to the paper published in the journal Environmental Research Letters.
The study was the first to use remote sensing and geographic information systems (GIS) for its data analysis, improving on earlier estimates based on case studies and guesswork, researchers said.
Large numbers of consumers eating raw produce will face heightened threats to food safety
Untreated waste water, even when diluted, poses health risks for both farmers and consumers, said Pay Drechsel, one of the authors.
“In waste water we have a lot of faecal contaminants from excrement,” Drechsel, a scientist at the International Water Management Institute (IWMI), said.
Farm workers can get skin infections from contact with contaminated water, while consumers are at risk of contracting worms, diarrhoea and even cholera from vegetables eaten raw, he said.
Pollution from human and animal waste affects nearly one in three rivers in Latin America, Asia and Africa, and some 3.4 million people die each year from diseases associated with pathogens in water, according to the UN.
The threat will get worse as the global population grows and informal settlements not supported by proper infrastructure spring up in rapidly expanding cities in developing countries.
“As long as investment in waste water treatment lags far behind population growth, large numbers of consumers eating raw produce will face heightened threats to food safety,” said the study’s lead author Anne Thebo, of the University of California, Berkeley.
In March, UN experts said governments should see treating waste water not as a costly problem but a valuable resource that could be used to meet growing demand for freshwater, energy and raw materials.
Waste water contains nutrients such as phosphorus and nitrates which can be turned into fertiliser while treated sludge can be turned into bio gas.