Quantification and Comparison of Risks Associated with Wastewater Use in Spray Irrigation

Abstract

In the U.S., spray irrigation is the most common method used in agriculture and supplementing with animal wastewater has the potential to reduce water demands. However, this could expose individuals to respiratory pathogens such as Legionella pneumophila and nontuberculosis Mycobacteria (NTM). Disinfection with methods like anaerobic digestion is an option but can increase concentrations of cytotoxic ammonia (personal communication). Our study aimed to model the annual risks of infection from these bacterial pathogens and the air concentrations of ammonia and determine if anaerobically digesting this wastewater is a safe option. Air dispersion modeling, conducted in AERMOD, generated air concentrations of water during the irrigation season (May-September) for the years 2013-2018. These values fed into the quantitative microbial risk assessments for the bacteria and allowed calculation of ammonia air concentrations. The outputs of these models were compared to the safety thresholds of 10 −4 infections/year and 0.5 mg/m 3 , respectively, to determine their potential for negative health outcomes. It was determined that infection from NTM was not a concern for individuals near active spray irrigators, but that infection with L. pneumophila could be a concern, with a maximum predicted annual risk of infection of 3.5 × 10 −3 infections/year and 25.2% of parameter combinations exceeding the established threshold. Ammonia posed a minor risk, with 1.5% of parameter combinations surpassing the risk threshold of 0.5 mg/m 3 . These findings suggest that animal wastewater should be anaerobically digested prior to use in irrigation to remove harmful pathogens.

Risk of Legionellosis in residential areas around farms irrigating with municipal wastewater

“…Irrigation can be modeled in several ways, but one of the most common is with a Gaussian plume model (Dungan, 2014; Lighthart & Mohr, 1987; Mori et al., 2020). This model type calculates the air concentrations of water emitted from a source in a specified area over time.…”

“…There exists a large body of work dedicated to characterizing various aspects of this type of irrigation, from droplet size distributions (Ferguson et al, 2020;Kincaid et al, 1996;Montero et al, 2003) to evaporation rates (Uddin et al, 2010;Yan et al, 2010) and spray transport (Molle et al, 2012(Molle et al, , 2016Tomas et al, 2019). Another area of research involves the potential health effects of exposure to this aerosolized irrigation water, either for farm workers or individuals living downwind of these farms (Dungan, 2014;Mori et al, 2020). Quantifying these health effects of these aerosols requires knowledge of their air concentrations over space and time and under various weather conditions.…”