Health risks in wastewater irrigation: Comparing estimates from quantitative microbial risk analyses and epidemiological studies

The combination of standard quantitative microbial risk analysis (QMRA) techniques and 10,000-trial Monte Carlo risk simulations was used to estimate the human health risks associated with the use of wastewater for unrestricted and restricted crop irrigation. A risk of rotavirus infection of 10^-2 per person per year (pppy) was used as the reference level of acceptable risk. Using the model scenario of involuntary soil ingestion for restricted irrigation, the risk of rotavirus infection is ~10^-2 pppy when the wastewater contains 10⁶ Escherichia coli per 100 ml and when local agricultural practices are highly mechanised. For labour-intensive agriculture the risk of rotavirus infection is ~10^-2 pppy when the wastewater contains 10⁵ E. coli per 100 ml; however, the wastewater quality should be 10⁴ E. coli per 100 ml when children under 15 are exposed. With the model scenario of lettuce consumption for unrestricted irrigation, the use of wastewaters containing 10⁴E. coli per 100 ml results in a rotavirus infection risk of ~10^-2 pppy; however, again based on epidemiological evidence from Mexico, the current WHO guideline level of 1,000 E. coli per 100 ml should be retained for root crops eaten raw.