Development of a semi-quantitative approach for the assessment of microbial health risk associated with wastewater reuse: A case study at the household level

Abstract This study proposes a modified methodology for semi-quantitative microbial risk assessment (SQMRA) based on the risk matrix presented in the World Health Organization (WHO) Sanitation Safety Planning manual adding the variable ranking for probability of infection in the estimations of risk scores, which composes the risk matrix. The approach is applied to cases where quantitative microbial risk assessment (QMRA) studies are hindered by the lack of information regarding pathogens in the (waste)water, collected as a set of primary data. The methodology is fully described and its application is demonstrated by comparing the microbial risk to which individuals are exposed within a residence in different scenarios of direct or indirect exposure to wastewater. Three current scenarios and one proposed scenario, with four sub-scenarios, focused on the use of a decentralized treatment system and greywater reuse in a single-family residence. Scenarios were selected for application of the SQMRA to elucidate if solutions considering source separation and reuse would bring more health-related risks, or attenuate the existing ones related to the conventional onsite wastewater treatment and soil disposal. None of the studied alternatives achieved risk score above 160 (very high). Considering the average score of each alternative by exposed group, the current scenarios, representing low-cost, informal solutions, posed the highest risks for children. All proposed reuse types reduced the risks from high to medium (score 43.9) for children and from medium to low (score 22.2) for adults. The adapted approach was demonstrated to be a useful tool, aiding the decision-making process and clarifying uncertainties raised when source separation and household reuse are proposed.