Quantitative risk assessment of norovirus and adenovirus for the use of reclaimed water to irrigate lettuce in Catalonia

Abstract

Wastewater is an important resource in water-scarce regions of the world, and its use in agriculture requires the guarantee of acceptable public health risks. The use of fecal indicator bacteria to evaluate safety does not represent viruses, the main potential health hazards. Viral pathogens could complement the use of fecal indicator bacteria in the evaluation of water quality. In this study, we characterized the concentration and removal of human adenovirus (HAdV) and norovirus genogroup II (NoV GII), highly abundant and important viral pathogens found in wastewater, in two wastewater treatment plants (WWTPs) that use different tertiary treatments (constructed wetland vs conventional UV, chlorination and Actiflo® treatments) for a year in Catalonia. The main objective of this study was to develop a Quantitative Microbial Risk Assessment for viral gastroenteritis caused by norovirus GII and adenovirus, associated with the ingestion of lettuce irrigated with tertiary effluents from these WWTPs. The results show that the disease burden of NoV GII and HAdV for the consumption of lettuce irrigated with tertiary effluent from either WWTP was higher than the WHO recommendation of 10−6 DALYs for both viruses. The WWTP with constructed wetland showed a higher viral reduction on average (3.9 and 2.8 logs for NoV GII and HAdV, respectively) than conventional treatment (1.9 and 2.5 logs) but a higher variability than the conventional WWTP. Sensitivity analysis demonstrated that the input parameters used to estimate the viral reduction by treatment and viral concentrations accounted for much of the model output variability. The estimated reductions required to reach the WHO recommended levels in tertiary effluent are influenced by the characteristics of the treatments developed in the WWTPs, and additional average reductions are necessary (in WWTP with a constructed wetland: A total of 6.7 and 5.1 logs for NoV GII and HAdV, respectively; and in the more conventional treatment: 7 and 5.6 logs). This recommendation would be achieved with an average quantification of 0.5 genome copies per 100 mL in reclaimed water for both viruses. The results suggest that the analyzed reclaimed water would require additional treatments to achieve acceptable risk in the irrigation of vegetables with reclaimed water.