Enzyme measurements for risk evaluations in sewage treatment plants

Objectives: Work in sewage treatment plants may cause health problems in terms of airways symptoms, unusual tiredness, fever, and diarrhoea. The presence of such symptoms has been related to bacterial endotoxin. The project was undertaken to assess if risk evaluations could be performed by measuring the amounts of two different microbial enzymes and to evaluate their relation to endotoxin. Methods: Measurements were performed at 120 work sites in 10 different sewage treatment plants of varying age and capacity. The content of N-acetyl-hexosaminidase (NAHA), bacterial hydrolase (BH) and endotoxin was measured in sewage water and in airborne samples. Results: There was a close relationship between endotoxin and NAHA, both in the water itself and in air samples. Generally exposure levels were low but higher at work sites where the water or sludge was agitated or when repairing pumps and cleaning basins. Conclusions: Measurements of NAHA are suitable to determine risk exposures in sewage treatment plants. Such measurements are faster and cheaper than measures of endotoxin. A level of 50-70 U/m3 is suggested as a risk threshold. Further work is required to define more precise threshold limit values.