Quantification of diarrhea risk related to wastewater contact in Thailand

Wastewater reuse contributes to closing the nutrient recycling loop as a sustainable way of managing water resources. Bangkok has over a thousand man-made drainage and irrigation canals for such purposes. Its use for agricultural and recreational purposes has a long tradition in rural and peri-urban areas. However, the continuation of these practices is increasingly questioned since potential health risks are an issue if such practices are not appropriately managed. The microbial and chemical quality of canal water has considerably deteriorated over the last decade, mainly because of discharged, untreated domestic and industrial wastewater. It is important to understand the health risks of wastewater reuse and identify risky behaviors from the most highly exposed actors promote the safe use of wastewater. This study assessed diarrhea infection risks caused by the use of and contact with wastewater in Klong Luang municipality, a peri-urban setting in Northern Bangkok, using quantitative microbial risk assessment. Wastewater samples were collected from canals, sewers at household level, and vegetables grown in the canals for consumption. Samples were also collected from irrigation water from the agricultural fields. Two protozoa, Giardia lamblia and Entamoeba histolytica, were quantified and analyzed by real-time PCR, exposure assessment was conducted, and finally, the risk of infection due to contact with wastewater in different scenarios was calculated. The results showed that canal water and vegetables were heavily contaminated with G. lamblia and E. histolytica. Infection risk was high in tested scenarios and largely exceeded the acceptable risk given by WHO guidelines.