Quantitative Health Risk Assessment of Wastewater Treatment Plant Worker Exposed to Staphylococcus Aureus Bioaerosol During Warm and Cold Periods: Disease Burden and Sensitivity Analysis

Biological treatment in wastewater treatment plants (WWTPs) releases high amounts of bioaerosols carrying a variety of pathogens. Quantitative microbial risk assessment (QMRA) is a framework prevalently intended for the quantitative estimation of health risks for occupational exposure scenarios (e.g. in WWTPs). However, the quantitative contributions of health-risk-estimate inputted variable parameters remain ambiguous. Therefore, this research aimed to study the disease burden of workers exposed to Staphylococcus aureus bioaerosol during warm and cold periods and to strictly quantify the contributions of the inputted parameters of disease burden by sensitivity analysis based on Monte Carlo simulation. The results showed that the disease health risk burden in the warm period was higher than in the cold period, disease health risk burden in the rotating-disc aeration mode was regularly higher than in the microporous aeration mode. The disease health risk burden of the workers with personal protective equipment (PPE) almost all satised the WHO benchmark ( ≤ 10E-6 DALYs pppy), and was consistently lower by one or two orders of magnitude than the workers without PPE in both warm and cold periods. Referring to the sensitivity analysis, exposure concentration and aerosol ingestion rate were the most and second predominant factor for the estimated risk in all exposure scenarios, respectively. The sensitivity of the removal fraction by employing PPE ranked third in the contribution to disease health risk burden. In addition, no remarkable differences were revealed in the sensitivity percentage ratio between warm and cold periods. This research can deepen the understanding of the QMRA framework and promote the development of sensitivity analysis, especially under various meteorological conditions (warm and cold periods). follow-up


Introduction
Biological treatment is the most predominant treatment process in wastewater treatment plants At present, no dedicated risk control standards or strategies are available for bioaerosols in WWTPs because unquanti able variable parameters are involved in the health risk assessment process.
According to prior studies, the health risks of workers in WWTPs can be signi cantly reduced by equipping them with personal protective equipment (PPE) ( Quantitative microbial risk assessment (QMRA) is a framework intended for the quantitative estimation of health risks in occupational exposure scenarios (Haas, 2015;Haas et al., 2017;Seis et al., 2020), and was carried out following: (i) hazard identi cation, (ii) exposure assessment, (iii) dose-response assessment, and (iv) risk characterization (Shi et al., 2018). Dose-response assessment is typically carried out through the exponential dose-response model, while risk characterization is performed to assess the health risk by disease burden (Haas, 2015;Esfahanian et al., 2019). Disease burden is evaluated by the acceptable disability-adjusted life years (DALYs), as proposed by World Health Organization, and is the most authoritative and widely-used health risk benchmark (≤10 −6 DALYs per person per year (pppy)) (WHO 2008; Shi et al., 2018).
Therefore, based on our previous research, we aimed to study the disease burden of workers exposed to Staphylococcus aureus bioaerosol by using the exponential dose-response model in a WWTP under mechanical aeration and microporous aeration modes during warm and cold periods. Then, the contributions of the inputted parameters of disease burden were strictly quanti ed to determine the most in uential variable parameter by Monte Carlo simulation. This research can further deepen the understanding of the dynamic uncertainty analysis under two extremely different meteorological conditions (warm and cold periods). The ndings can also contribute effort to the establishment of mitigation measures and control strategies for the management of public health risks including exposure to bioaerosols in local utilities.

WWTP description
This study was performed at a WWTP, which was built in 2014, located in China. The collected domestic wastewater is distributed into the WWTP by a series of variable-frequency pump stations. The WWTP maintains two independently operated parallel phases and is equipped with a rotating-disc aeration tank (mechanical aeration mode) for Phase I and a microporous aeration tank (microporous aeration mode) for Phase II (Fig. 1). The treatment capacity of Phases I and II is 50,000 tons per day. rotating-disc aeration tank and microporous aeration tank at the same time, and each sampling was repeated thrice. The sampling site was located in the middle of the corridor of each aeration tank ( Fig. 1).
At each sampling site, the Anderson six-stage impactor was mounted at 1.5 m above the ground level of the corridor, and the impactor was operated at a ow rate of 28. 3

Bioaerosol analysis
The media used in the Anderson six-stage impactor for sampling airborne Staphylococcus aureus was egg-yolk mannitol salt agar base (Qingdao Hope Bio-Technology Co., Ltd.). The preparation and cultivating methods followed standard procedures and previous publications ( Table 2 and Supplementary Material. In fact, the Staphylococcus aureus were popularly con rmed as the most frequently detected, widely distributed, and extensively studied potential hazard in most bioaerosol samples (Kozajda et al., 2019). Therefore, for hazard identi cation, the indicator pathogen of concern in this study was Staphylococcus aureus bioaerosol emitted from the two aeration tanks.
The workers in this study were equipped with a wide range of models of PPE, including N-95 and N-99, as well as other types which e cacy were presumed between N-95 and N-99.

Monte Carlo simulation and sensitivity analysis
All inputted variable parameters (exposure concentration, aerosol ingestion rate, removal fraction by employing PPE (FPPE), exposure time, and breathing rate) were randomly selected from their corresponding probability distributions (Table 1 and

Results And Discussion
The mean concentration and standard deviation of exposure bioaerosol under mechanical and microporous aeration mode during warm and cold periods were reported in the Table 1.

Characterization of disease health risk burden
Box plot in the Fig. 2 demonstrates the disease burden with the rst and third quartiles (25th and 75th percentiles), the mean value (general condition), the 2.5th percentile (optimistic estimate at the best situation), and the 97.5th percentile (conservative estimate at the worst situation). They were all compared with the World Health Organization (WHO) benchmark (≤10E-6 DALYs pppy).
The disease health risk burden of the workers (with or without PPE) in the warm period was constantly one order of magnitude higher than that in cold period under all estimate situations. This result can be attributed to the higher bioaerosol concentrations in the warm period (Table 1) For workers without PPE, the disease health risk burden in rotating-disc aeration tank in warm period exceeded the WHO benchmark for all condition; the disease health risk burden in cold period under the optimistic estimate satis ed the WHO benchmark. The disease health risk burden in microporous aeration tank in warm period under general condition and in cold period under the conservative estimate were both on the same order of magnitude as the benchmark. The disease health risk burden of the workers with PPE all satis ed the WHO benchmark in the two aeration tanks excepting for rotating-disc aeration tank in warm period at the worst situation, and was consistently lower by one or two orders of magnitude than the workers without PPE in both warm and cold periods. Therefore, disease health risk burden can be signi cantly mitigated by one or two orders of magnitude by equipping workers with PPE.
3.2 Sensitivity analysis of the results of disease health risk burden Figure 3 displays the sensitivity partitioning coe cient of parameters in the two aeration tanks in warm and cold periods. Fig. 4 demonstrates the sensitivity percentage ratio of each sensitivity parameter in each exposure scenario.
The sensitivity analysis indicated that the exposure concentration and aerosol ingestion rate were the most and second predominant factor for the estimated risk in all exposure scenarios, respectively (Fig. 3). The sensitivity partitioning coe cient of exposure concentration was 1.17 to 1.35 times the value of the aerosol ingestion rate (Fig. 3). The degree of dispersion (standard deviation) of bioaerosol concentration was more than 50% of its mean value in all exposure scenarios. This led to the highest sensitivity ranking of exposure concentration. Therefore, decreasing the exposure concentration is one of the most signi cant methods to lessen the disease health risk burden in theory. Besides, controlling the aerosol ingestion rate, which is highly affected by breathing pattern, is an effective way to mitigate the disease health risk burden (Stuart, 1984;Warren et al., 1988;Lim et al., 2015). Literature reported that nasal breathers had higher infection risks than habitual oral breathers by about two orders of magnitude (Shi et al., 2018). Therefore, health risk mitigation may be achieved by effective oral breathing.
However, the sensitivity of the FPPE ranked third in the contribution to disease health risk burden. The sensitivity partitioning coe cient of the FPPE was 0.03 lower (by absolute value) to aerosol ingestion rate in all exposure scenarios and they showed almost the same sensitivity partitioning coe cients (by absolute value) and sensitivity percentage ratios (Figs. 3 and 4b). This result further demonstrates that the wearing of PPE can largely reduce the risk (Haas et al., 2017;Carducci et al., 2018), and proves that it is another effective way to mitigate the disease health risk burden.
In addition, breathing rate and exposure time showed an alternating regularity in two periods. In rotatingdisc aeration tank, the breathing rate ranked ahead the exposure time in warm period while the exposure time ranked ahead the breathing rate in cold period. And it was on the contrary in microporous aeration tank.
Referring to the sensitivity percentage ratio in one particular aeration mode, no remarkable differences were revealed between warm and cold periods (Fig. 4). This nding may be due to the same inputted values and distribution patterns of these sensitivity parameters, except the exposure concentration (Table  1). Hence, the disease health risk burdens in warm and cold periods are equally important and thus deserve correspond research attention.

Conclusion
The disease health risk burden of workers exposed to the bioaerosol in the warm period was constantly one order of magnitude higher than that in cold period, the disease health risk burden in the rotating-disc aeration mode was regularly consistently higher by one order of magnitude than it in the microporous aeration mode.
Furthermore, the disease health risk burden of the workers with PPE all satis ed the WHO benchmark excepting for rotating-disc aeration tank in warm period at the worst situation, and was consistently lower by one or two orders of magnitude than the workers without PPE in both warm and cold periods. For workers without PPE, the disease burden values in rotating-disc aeration tank in warm period exceeded the WHO benchmark for all condition; the burden in cold period under the optimistic estimate satis ed the WHO benchmark. The burden in microporous aeration tank in warm period under general condition and in cold period under the conservative estimate were both on the same order of magnitude as the benchmark.
The sensitivity analysis indicated that the exposure concentration and aerosol ingestion rate were the most and second predominant factor for the estimated risk in all exposure scenarios, respectively. However, the sensitivity of the FPPE ranked third in the contribution to disease health risk burden.
Therefore, decreasing the exposure concentration is the most preferred in theory to lessen the health risk; effective oral breathing and equipping personal protective equipment are also feasible methods to reduce the risk. In addition, no notable differences were discovered in the sensitivity percentage ratio between warm and cold periods, revealing that seasonal variation equally contributed to the health risk.
This research systematically further delivered novel data on the sensitivity analysis of quantitative health risk assessment framework in one WWTP for comparing exposure to bioaerosols in warm and cold periods. This sensitivity analysis study can provide a theoretical basis for follow-up research on the mitigation measures and then assist local utilities understanding control strategies for bioaerosol exposure.

Declarations
Ethics approval and consent to participate Not applicable.

Consent for publication
Not applicable.

Availability of data and materials
All data generated or analyzed during this study are included in this published article and its supplementary information les. Box plot of the disease burden (DALYs pppy) of workers (with or without PPE) exposed to Staphylococcus aureus bioaerosol in the wastewater treatment plant in warm or cold period The bottom and top of the box represent the rst and third quartiles (25th and 75th percentiles), respectively. The band inside the box denotes the second quartile (median), and the tetragon inside refers to the mean value (general condition). The whiskers show the 2.5th percentile (optimistic estimate at the best situation) and 97.5th percentile (conservative estimate at the worst situation) from each end of the box. WHO = World Health Organization Figure 3 Tornado graphs of the ranking of sensitivity partitioning coe cient of input sensitivity parameters that affect the output value for workers exposed to Staphylococcus aureus bioaerosol in the wastewater treatment plant, referring to: (a) workers without PPE in rotating-disc aeration tank in warm period, (b) workers with PPE in rotating-disc aeration tank in warm period, (c) workers without PPE in microporous aeration tank in warm period, (d) workers with PPE in microporous aeration tank in warm period, (e) workers without PPE in rotating-disc aeration tank in cold period, and (f) workers with PPE in rotating-disc aeration tank in cold period, (g) workers without PPE in microporous aeration tank in cold period, and (h) workers with PPE in microporous aeration tank in cold period RD = Rotating-disc aeration tank M = Microporous aeration tank C = Exposure concentrations T = Exposure time AG = Aerosol ingestion rate BR = Breathing rate FPPE = Removal fraction by employing PPE PPE= Personal protective equipment Figure 4 Percentage stack chart of the sensitivity percentage ratio of the sensitivity partitioning coe cient of each sensitivity parameter of disease burden for workers exposed to Staphylococcus aureus bioaerosol in the two aeration tanks of the wastewater treatment plant in warm or cold period, referring to: (a) workers without PPE, (b) workers with PPE Warm = Warm period Cold = Cold period RD = Rotating-disc aeration tank M = Microporous aeration tank C = Exposure concentrations T = Exposure time AG = Aerosol ingestion rate BR = Breathing rate FPPE = Removal fraction by employing PPE PPE = Personal protective equipment

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download. 20211029SupplementaryMaterial.docx