Worker exposure to bioaerosols from sludge dewatering facilities: risks and mitigations

Sludge dewatering process is a major source of bioaerosols in wastewater treatment plants (WWTPs) which can pose significant health risks to on-site workers. While quantitative microbial risk assessment (QMRA) is commonly performed to assess the human health risks, it lacks immediacy in determining on-site risk levels. Alternatively, comparing detected concentrations of bioaerosols with critical concentration standards provides a practical approach. This study investigates Escherichia coli bioaerosol emissions from a sludge dewatering process (or facility) in a municipal WWTP under different working postures, ventilation scenarios, and seasonal conditions. The objective was to estimate a ceiling-type exposure limit (EL) corresponding to acceptable risk levels using Monte Carlo simulation and reverse QMRA. Sensitivity analysis was also used to identify the key input parameters significantly affecting the risk exposure. Results showed that the bioaerosol concentration in poor ventilation was 1.6-1.7 times as large as that in natural ventilation. The respirable fractions (particle size < 4.7 µm) accounted for 78 %-100 % of the total bioaerosol emissions. Opening the windows increased the EL up to 1.6 times higher compared to closed-window conditions under the optimistic scenario. Wearing masks reduced significantly the fraction of bioaerosols inhaled by workers, resulting in derived EL that were one to two orders of magnitude higher (2.24 - 20.10 CFU/m³) than those estimated without masks (0.09 - 0.41 CFU/m³). Among all parameters, the removal of the respirable fraction by the masks was the dominant contributor to EL estimation. EL benchmarks also varied with seasonal conditions and working postures. Overall, this study provides novel quantitative insights for deriving occupational exposure limits for bioaerosol exposure in dewatering facilities.