The activated sludge method and its derived improved processes were the mainstream technology for the purification and treatment of municipal and industrial wastewater worldwide (Yi et al., 2020). The method was to remove organic pollutants in wastewater by using biological coagulation, adsorption and oxidation of activated sludge under mechanical oxygenation conditions. Microorganisms in wastewater were highly susceptible to escape into the air to form bioaerosols due to mechanical perturbation (such as aeration and oxygenation, mechanical agitation)(Han et al., 2024; Liu et al., 2023a). Bioaerosols were aerosols with an aerodynamic diameter of less than 100 μm and containing microorganisms or biological material (Liu et al., 2025b).

Currently, a large number of studies had focused on the fugitive level, particle size distribution, community structure and exposure risk assessment of bioaerosols in municipal wastewater treatment plants (MWTPs). According to previous studies, bioaerosols were found to be fugitive to varying degrees in each wastewater treatment process section, with bacterial aerosol concentrations ranging from 1.05 × 102 CFU/m3 to 2.10 × 105 CFU/m3, and fungal aerosol concentrations ranging from 1.31 × 102 CFU/m3 to 1.05 × 105 CFU/m3 (Wang et al., 2024b). One of the main sources of bioaerosols was considered to be the outdoor treatment facility aeration basins (Gui et al., 2022; Han et al., 2021; Kowalski et al., 2017). The particle size of the bioaerosol produced by the aeration tank mainly consists of coarse particles of 3.3 μm to 4.7 μm. If microporous aeration was used, the bioaerosol particle size will be even smaller, distributed in 0.65 μm ∼3.3μm (Kim et al., 2012). Liu et al. also found that aeration basins using membrane modules also result in the retention of coarse-particle bioaerosols (Liu et al., 2022). Bioaerosols were composed of complex microorganisms and carried a large number of harmful components such as pathogenic bacteria, fungi and viruses (Kumar et al., 2020; Li et al., 2024; Wang et al., 2023, 2024a). Pathogenic bacteria identified in bioaerosols from MWTPs include Acinetobacter, Bacteroides, Enterococcus, Enterobacter, Pseudomonas, Shewanella oneidensis and Escherichia coli, which pose a potential health risk to people who were exposed to them for long periods of time (Liu et al., 2023a; Robertson et al., 2019). Bioaerosols can come into contact with the human body through various routes such as ingestion, dermal contact, inhalation and contact with contaminated surfaces, clothing or tools. Inhalation was the main route of exposure to bioaerosols, which is 105 times greater than dermal contact (Han et al., 2021; Liu et al., 2025a; Vishwakarma et al., 2024a; Wan et al., 2025). However, the concentration, particle size distribution and community structure of bioaerosols escaping from each MWTPs were not uniform, depending on the type of wastewater selected processes and meteorological parameters.

Compared to indoor bioaerosols, outdoor bioaerosols were susceptible to environmental factors such as temperature (T), relative humidity (RH), solar radiation (SR) and wind speed (WS)(Zhao et al., 2025). A high level of RH slows down the evaporation of bioaerosol droplets and also protects the microorganisms they were loaded with from UV radiation. Environmental conditions of high T and high RH were favorable for the growth of microorganisms, with some differences in the propensity of fungi and bacteria. Most bacteria had a significant positive correlation with T, and fungi had a significant positive correlation with RH (Dehghani et al., 2018; Han et al., 2019; Michałkiewicz, 2018).Wind speed (WS) favors the escape of bioaerosols and promotes diffusion of outdoor bioaerosols (Liu et al., 2023a). Bioaerosol concentrations vary seasonally depending on environmental factors. Warm and humid conditions prevailing in summer promote the survival of bacteria in the air, while cool and dry climates in spring and winter are not favorable for microbial movement and proliferation (Han et al., 2019; Jari et al., 2022). Wang et al. detected that bioaerosol concentrations in municipal wastewater treatment processes peak in the summer months and decrease in levels in the spring and winter.

Bioaerosols generated by outdoor treatment facilities suspended in the air, due to their small size and light weight, and the kinetic energy generated by the air movement process make them migratory, and they were easy to diffuse and spread in the air, polluting the MWTP and the surrounding air environment (Dong et al., 2024; Liu et al., 2023c; Vishwakarma et al., 2024b; Wang et al., 2024a). Diffusion modelling helps to predict the dispersion and propagation patterns of outdoor bioaerosols and provides strong support for the formulation of scientifically sound protective distances and environmental management policies. Gaussian plume modelling is a commonly used method for calculating downwind concentrations of airborne pollutants in the atmosphere, and is suitable for simulating and assessing the dispersion behavior of bioaerosols over relatively short transport distances (Gao et al., 2024). ZHANG et al. used a Gaussian plume model to predict the concentration changes of bioaerosols from a semi-subterranean wastewater treatment plant over a dispersion distance of 1000 m. The simulation results showed that the concentration decreased from 18.67 to 24.12 CFU/m3 at the source to 0.21–0.53 CFU/m3 at 500 m downwind, and 0.06–0.14 CFU/m3 at 1000 m (Zhang et al., 2023). Wang et al. used a Gaussian plume model to simulate the dispersion of bioaerosols from a hospital wastewater station and found that bioaerosols carrying pathogenic bacteria could travel up to 300 m from the source (Wang et al., 2024a).

This study focused on the transport and risk of bioaerosols escaping from MWTPs. Sampling points were set up in a MWTP in the north of China to collect bioaerosols in the air of the outdoor treatment facility and analyze the characteristics of their escaping. Gaussian plume models were used to simulate their dispersal behaviors in different seasons, and to assess the risk of the exposure of bioaerosols generated by the outdoor wastewater treatment facility to the operation workers of the MWTP and the residents in the vicinity. In order to provide scientific data for the effective control of bioaerosols in MWTPs and to reduce the health risk of nearby residents.