A fresh look at microbial guidelines for wastewater irrigation in agriculture: a risk-assessment and cost-effectiveness approach.

Abstract

AbstractThis study aimed to develop a risk-assessment/cost-effectiveness approach, to compare the risks of irrigating with wastewater treated to meet various recommended microbial guidelines - World Health Organization (WHO) versus United States Environmental Protection Agency (USEPA) - for unrestricted use in agriculture with the risk of irrigating with untreated wastewater. According to the authors' estimates, the annual risk of contracting infectious diseases including typhoid fever, rotavirus infection, cholera and hepatitis A from eating raw vegetables irrigated with untreated wastewater is in the range of 1.5×10-1 to 5×10-2, or 5-15% of consumers eating such vegetables will develop a case of disease compared to 10-6 (0.0001%) of those eating vegetables irrigated with treated wastewater effluent that meets the WHO guideline of 1000 faecal coliforms (FC)/100 ml. The USEPA considers a 10-4 (0.01%) annual risk of becoming ill with an infectious disease acceptable for drinking water. Cost-effectiveness analysis shows that, on average, in a city with a population of one million, the prevention of a single case (out of 61 cases/year) of the four diseases: hepatitis A, rotavirus infection, cholera and typhoid according to WHO guidelines versus USEPA guidelines would entail an extra annual expenditure of wastewater treatment of US$450 000/case. It is questionable if this is a cost-effective or reasonable public health expenditure. The authors estimate that if every one of a million people ate raw vegetables irrigated with untreated wastewater, there would be a 1 in 10 annual risk (100 000 cases/year) of contracting one of these four diseases. Thus, in the authors' view irrigating vegetables eaten with raw untreated wastewater presents an unreasonably high health risk. However, treatment to meet WHO guidelines would cost US$125/case prevented. This appears to be reasonably cost-effective, but, is a question that must be decided upon by each community. Evaluating health risks by disability adjusted life years (DALY) is also considered.