Microbial risks of reclaimed water uses in agriculture: Development of a QMRA model
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In arid and semi-arid areas, wastewater is a reliable source of water with valuable nutrients for agriculture. However, treated wastewater might contain pathogenic microorganisms that survived the water treatment. The irrigation of food crops with such water may lead to infections in humans ingesting irrigated vegetables. A wastewater treatment plant in Catalonia, Spain, reuses tertiary effluent for golf course and agricultural irrigation. In this thesis the health risks derived from the reuse of wastewater for agricultural irrigation from the plant in Catalonia are assessed by developing a quantitative microbial risk assessment (QMRA) model. The model input parameters are based on scientific literature. Specifically, the health risks derived from exposure to noroviruses and adenoviruses are assessed. These viruses are investigated due to their high infection rate and resistance in the environment. In addition, these viruses cause diarrheal illnesses following ingestion of contaminated food. The results show that the average risk of getting ill due to consumption of lettuce, which has been irrigated with tertiary effluent, is higher than for tomatoes. The highest risk was obtained for adenovirus on lettuce, followed by adenovirus on tomatoes, norovirus on lettuce and norovirus on tomatoes. According to the WHO guideline from 2010 a yearly risk of 10-6 DALYS is acceptable with regard to ingestion of wastewater irrigated crops. The calculated risks of the analyzed scenarios are within this limit, except for the average annual risk of adenovirus contaminated lettuce. Since most input values chosen for the model are conservative and may not represent the true value, the calculated risks are likely to be overestimated. Other input values may lead to underestimation of the risks. Irrigation of vegetables, as considered in this model, represents a high-risk reuse activity and may not be indicative of risks associated with other uses. A failure in the tertiary treatment system would lead to a higher concentration of pathogens in the effluent. The plant in Spain has introduced techniques to prevent too high pathogen concentrations in the tertiary effluent due to failures, allowing the high concentrations for only a limited period of time. The risk during the failure of the system has not been assessed in this study. Nevertheless, it is likely that the daily risks will increase for the time the failure occurs. It is recommended to use concentrations measured in the plant of investigation as irrigation source for the model- if available. It is also suggested to improve the model by choosing more accurate distributions and input values for a more precise estimation of the risk. This can also be achieved by running more diverse scenarios to evaluate the risk of getting ill due to other pathogens associated with ingestion of other raw vegetables from the investigated farms. Failure scenarios in the plant as well as additional contamination sources should be considered when improving the model.