Comparison of coliphage and bacterial aerosols at a wastewater spray irrigation site

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Abstract
Microbiological aerosols were measured on a spray irrigation site at Fort Huachuca, Arizona, USA. Indigenous bacteria and tracer bacteriophage were sampled from sprays of chlorinated and unchlorinated secondary-treatment wastewaters during day and night periods. Aerosol dispersal and downwind migration were determined. Bacterial and coliphage f2 aerosols were sampled by using Andersen viable type stacked-sieve and high-volume electrostatic precipitator samplers. Bacterial standard plate counts averaged 2.4 .times. 10.5 colony-forming units/ml in unchlorinated effluents. Bacterial aerosols reached 500 bacteria/m3 at 152 m downwind and 10,500 bacteria/m3 at 46 m. Seeded coliphage f2 averaged 4.0 .times. 105 plaque-forming units/ml in the effluent and were detected 563 m downwind. Downwind microbial aerosol levels were somewhat enhanced by nighttime conditions. The median aerodynamic particle size of the microbial aerosols was approximately 5.0 .mu.m. Chlorination reduced wastewater bacterial levels 99.97% and reduced aerosol concentrations to near background levels; coliphage f2 was reduced only 95.4% in the chlorinated effluent and was readily measured 137 m downwind. Microbiological source strength and meteorological data were used in conjunction with a dispersion model to generate mathematical predictions of aerosol strength at various sampler locations. The mean calculated survival of aerosolized bacteria (standard plate count) in the range 46-76 m downwind was 5.2% and that of coliphage f2 was 4.3%.