Bioaerosolization across 15 wastewater treatment plants: Decoupling between airborne and wastewater microbial concentrations.

Wastewater treatment plants (WWTPs) are recognized sources of bioaerosol emissions, however the roles of process configuration, wastewater quality, and environmental conditions in shaping airborne microbial loads remain poorly documented. This study aimed to relate airborne bacterial concentrations to WWTP configuration, quantify process-related changes, and assess the occurrence of aerosolized opportunistic pathogens. Air and wastewater of 15 WWTPs in Québec, Canada, were sampled across 81 processes. Samples were quantified by qPCR for total bacteria, Escherichia coli, non-tuberculous mycobacteria (NTM), Mycobacterium avium, Pseudomonas aeruginosa, Legionella spp., and Legionella pneumophila, alongside onsite environmental measurements. In wastewater, bacterial targets co-varied significantly with particulate indicators. Airborne total bacterial profiles were driven by process configuration: open, turbulent units showed the greatest variability (up to 3.6-log), whereas confined and disinfection units were stable (< 2 log). Indoor relative humidity was the only ambient parameter positively correlated with 5/6 bacterial targets measured in air, suggesting that humid, poorly diluted process microenvironments may favor droplet persistence and worker exposure. Airborne bacterial loads decreased as water progressed through treatment stages: total bacteria declined from pre-treatment to tertiary treatment (p = 0.03), with similar decreases for NTM, Legionella spp. and P. aeruginosa (p ≤ 0.04). Detection in air was high for total bacteria (96 %, ∼10⁵ genome copies/m³) and NTM (95 %, ∼104 GC/m³), intermediate for E. coli (67 %) and P. aeruginosa (60 %), and low for M. avium (11 %) and L. pneumophila (2 %). These data support prioritizing exposure controls for open, turbulent pre-treatment, aerated biological, and sludge-handling units, especially in humid or poorly ventilated rooms.