Assessment of Airborne Transport of Potential Contaminants in a Wind Tunnel

摘要

The reuse of treated wastewater (TWW) for sprinkler irrigation could potentially diffuse pathogen-containing droplets off the application area. Wind and other unfavorable climatic factors enhance irrigation drift and bioaerosol dispersion, exposing humans to potentially severe health risks including the spread of diseases. Few studies have quantified bioaerosols during both spraying and airborne transport phases. Studies of effective sampling strategies to better qualify the dispersion process are also required. This paper presents experiments conducted in a wind tunnel for a deeper understanding of the effects of wind and temperature on pathogen or contaminant airborne dispersal and transport. It is the first time that passive collectors [polyvinyl chloride (PVC) lines] and active samplers (AGI-4 impinger) have been compared under analogous wind conditions using a fluorescent tracer. Droplet-size distribution was also investigated at 12 m from the boom with a NanoMoudi 122-NR cascade impactor in increasing wind conditions from 1 to 3 m s-1. PVC lines return a detailed evolution of the sprayed volume within a short range from the boom and for concentrated fluxes. Transport assessment of PVC lines indicates that transport and permanently airborne condition of the spray notably grow with increasing wind, resulting in a more compact and concentrated plume; mean transport increases from 0.13 to 1.18 L h-1 m-2 at 7.7 m from the nozzle as the wind velocity increases from 1 to 3 m s-1. AGI-4 appears more suitable to assess finely aerosolized conditions because of its greater sensitivity compared to PVC lines as shown for sample values less than 1 L h-1 m-2. The comparison between the AGI-4 and PVC lines shows higher values of recovery for the active samplers compared to the PVC lines. The total volume collected by the impingers was 2.93% of the sprayed volume, approximately twice that collected by PVC lines under analogous conditions, even though their sampling surface was only 1.54% that of PVC lines. Droplet-size distributions from the cascade impactor denote a median volume diameter from 1.1 to 2 mu m, for the nozzle type used, and a relevant reduction in recovery at stronger wind velocities. An empirical relation time of flight is proposed as a first step in developing decision models that can be used to make sprinkler irrigation safe and to define standards for TWW reuse in agricultural practices (e.g., safe distance of application depending upon wind conditions and droplet-size distribution).