Development of a risk assessment approach for evaluating wastewater reuse standards for agriculture

A risk assessment approach was developed to arrive at a comparative risk analysis of the various recommended wastewater irrigation microbial health guidelines for unrestricted irrigation of vegetables normally eaten uncooked. The guidelines compared are those of the WHO and the USEPA/USAID. The laboratory phase of the study determined the degree of contamination of vegetables irrigated by wastewater. Based on these estimates of the risk of ingesting pathogens, it is possible to estimate the risk of infection/disease based on the risk of infection and disease model developed for drinking water by Haas et al. (1993). For example, the annual risk of infectious hepatitis from regularly eating vegetables irrigated with raw wastewater is shown to be as high as 10⁻³. The study indicates that the annual risk of succumbing to a virus disease from regularly eating vegetables irrigated with effluent meeting WHO guidelines (1,000 FC/100mL) is negligible and of the order of 10⁻⁶ to 10⁻⁷ The risk of the more infectious, but less serious, rotavirus is 10⁻⁵ to 10⁻⁶. The USEPA considers an annual risk of 10⁻⁴ to be acceptable for microbial contamination of drinking water. The additional health benefit that might result from a further reduction of risk gained by adhering to the USEPA/USAID Reuse Guidelines (1992), which require no detectable faecal coliforms/100mL, appears to be insignificant in relation to the major additional costs associated with the expensive technology required to treat effluent to such a rigorous standard. Our preliminary estimates show that meeting the USEPA Guidelines would result in an extra expenditure of $3–30millions/case of disease prevented.