Abstract
Yearly discharge of antibiotic-resistant bacteria (ARB) from combined sewer overflow (CSO) was estimated. The volume of CSO was estimated from operating data of the pumping station. In the target sewer catchment, 23% of the total of the volume of combined sewage was discharged untreated as CSO. Combined sewage contained 3-log larger E. coli than secondary treatment effluent although the abundance of antibiotic-resistant E. coli was not significantly different. In the target-combined sewer catchment, a yearly total of 4.8 × 1016 CFU of E. coli was discharged from 6.1 × 106 m3 of CSO, while 1.3 × 1012 CFU of E. coli from 2.1 × 107 m3 of effluent from the wastewater treatment plant (WWTP). This E. coli discharge was equivalent to 7.9 × 109 CFU/m3 from CSO, and 6.2 × 104 CFU/m3 from WWTP effluent. Consequently, a yearly total discharge of antibiotic-resistant E. coli from CSO was 3.7-log larger than the WWTP effluent. The small-flow CSO events, which had hourly flow rate smaller than five times of the average dry-weather flow, accounted for 43% of the total CSO volume, but 79% of the total discharge of antibiotic-resistant E. coli due to a small dilution factor with stormwater and frequent discharge. Reduction of small-flow CSO events would be important for effective reduction of ARB discharge from CSO.
Highlights
Increased emergence of antibiotic-resistant bacteria (ARB) is a great concern for future human health
When the sewage volume in a combined sewer system can exceed the capacity of wastewater treatment plant (WWTP) due to heavy rainfall or snowmelt, the combined sewage, which contains both wastewater and stormwater, is designed to overflow and discharge directly to nearby water bodies without treatment
The objective of this study is to estimate ARB discharge from combined sewer overflow (CSO) and wastewater treatment effluents of an urban-combined sewer system, in order to investigate the quantitative impact of CSO on ARB discharge into water environment
Summary
Increased emergence of antibiotic-resistant bacteria (ARB) is a great concern for future human health. Municipal wastewater is a major source of ARB in urban water environment in both developed and developing countries[3–8]. Combined sewer overflow (CSO) is a possible significant pathway of ARB discharge from municipal wastewater. When the sewage volume in a combined sewer system can exceed the capacity of WWTP due to heavy rainfall or snowmelt, the combined sewage, which contains both wastewater and stormwater, is designed to overflow and discharge directly to nearby water bodies without treatment. These overflows, called combined sewer overflow (CSO), are a significant source of fecal pollution in water bodies in many urban areas[15–19]. According to USEPA20, discharge of CSO was estimated to be 20% of the total annual discharge of combined sewage in the United States
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