Risk assessment of Legionella and enteric pathogens in sewage treatment works

Exposure of sewage treatment workers to Legionella pneumophila and enteric pathogens through aerosols at different sewage treatment installations was quantified. The presence of Legionella sp. and L. pneumophila in water and air samples at different sewage installations was determined by cultivation, Direct Fluorescence Assay and PCR. Simultaneously, coliforms, F-RNA phages and spores of sulphite reducing clostridia (SSRC) were enumerated in water and air. For enteric pathogens, the aerosolisation efficiency of coliforms, F-RNA phages and SSRC were combined with published data on the concentration of Campylobacter, enteroviruses and Cryptosporidium parvum, respectively, in wastewater to calculate the concentration of these pathogens in the air. The frequency and duration of the presence of sewage workers at or in the different installations was determined by a questionnaire. The concentration of micro-organisms in the air and the data on the presence of workers at these locations were combined into an estimate of the exposure to Legionella and the enteric pathogens. Legionella was detected in all wastewater samples (n=7) with PCR, in concentrations of approximately 10⁵ per ml, but not with the culture method (<20–<200 /ml). Legionella spp. and L. pneumophila were detected in air samples of 10 m³ with PCR at 3 of the 5 sewage treatment plants tested. Samples of air above trickling filters (2/2), aeration tanks (2/2), the screen (1/1) and the belt press (1/1) were positive for Legionella. The concentration ranged from 0.56–56 per m³ of air with PCR, but again no Legionella were detected in any of the air samples with the culture method. The Legionella-concentration (determined by PCR) in the air were low compared to published data on Legionella concentrations in the air at outbreak sites (determined by culture). Hence, the risk of legionellosis to sewage workers was considered low. The calculated concentrations in air ranged from very low (<0.001/m³ up to 0.3–17/m³ at sites with high aerosolisation). During cleaning activities, the concentrations rose up to 100/m³. These concentrations were translated into the probability of infection using published dose-response models. The annual probability of infection for sewage workers was high (worker with average exposure 19%, high exposure worker (95-percentile) 100%). Therefore, measures to reduce aerosol formation or exposure to aerosols (both technical and protective) should be considered. Reduction of exposure to aerosols will not only reduce the risk due to enteric pathogens, but also to endotoxins, Legionella and other pathogens.