Estimated discharge of antibiotic-resistant bacteria from combined sewer overflows of urban sewage system

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.