Assessment and risk modeling of airborne enteric viruses emitted from wastewater reused for irrigation

Reclamation of wastewater (WW) for irrigation, after treatment represents a challenge that could alleviate pressure on water resources and address the increasing demand for agriculture. However, the risks to human health must be assessed, particularly those related to human enteric viruses that resist standard treatments in most wastewater treatment plants (WWTP). The risks associated with exposure to viral bioaerosols near WWTP and near agricultural plots irrigated with WW are poorly documented. The objectives of this study were to 1) better characterize human enteric viruses found in bioaerosols near a "standard WWTP" and over fields irrigated with treated WW and 2) propose a numeric model to assess the health risk to populations located close to the irrigated areas, with particular attention to norovirus, which is responsible for most viral gastroenteritis in France. Water and air samples were collected at various locations in the largest French WW-irrigated site near Clermont-Ferrand, at the WWTP entrance and after treatment, in the air above activated sludge basins, and above fields irrigated with WW. Various enteric viruses were found in the water samples collected both before and after treatment. Norovirus was the most abundant with >10e4 genome copies/l (GC/L) before treatment and ~10e3 GC/L after treatment. Low quantities (<10e3GC/m³) were detected in the air above active sludge pools and irrigated plots. Hepatitis E virus was detected in all sampled compartments. A quantitative microbial risk assessment (QMRA) approach, including a simplified atmospheric dispersion model, allowed assessment of norovirus infection risk. The Bayesian QMRA approach considered wind speed measurements over 21years, and the variability and uncertainty of all measurements throughout the chain up to the risk. The probability of infection within one year for the most exposed WWTP employees was >10e-4 for strong wind speed (≥3m/s) and a constant emission rate of 8e3 GC/m³/s. This probability decreases by 3 log when the distance to the emission source is doubled. This information can aid development of safe water reuse policies in terms of local setback distance and wind conditions for wastewater reuse.