Bacterial survival and dispersal in spray irrigation aerosols

Abstract

New Zealand environmental authorities favour land-based treatment for wastewaters of faecal origin. However, microbial aerosols from spray irrigation could be a health risk. This study investigated bacterial survival in irrigation aerosols in 105 field trials in spring and summer. Serratia entomophila and tracer spores of Bacillus subtilis var. niger were added to well water at about 105 ml−1 and sprayed onto a flat paddock through either a high-pressure horticultural sprayer (1400 kPa) to maximise aerosol production or a low-pressure hammer-head rotary sprayer (300 kPa), commonly used for irrigation. Aerosol particles were collected in six-stage Andersen air samplers. Recovery of viable B. subtilis spores decreased with increasing distance from the sprayers, due to dispersion. There was a greater decrease for S. entomophila. Assuming similar dispersion of both micro-organisms, the relative decrease in S. entomophila was due to inactivation and this inactivation correlated with decreasing relative humidity. Under the prevailing meteorological conditions, viable S. entomophila were dispersed to at least 100 m from the low-pressure sprayer and 200 m from the high-pressure sprayer. Irrespective of sprayer type, sampling distance or micro-organism, the majority of viable organisms were collected on those Andersen sampler stages corresponding to the respirable fraction of inhaled air. © 2004, Taylor & Francis Group, LLC.