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 x 10(16) CFU of E. coli was discharged from 6.1 x 10(6) m(3) of CSO, while 1.3 x 10(12) CFU of E. coli from 2.1 x 10(7) m(3) of effluent from the wastewater treatment plant (WWTP). This E. coli discharge was equivalent to 7.9 x 10(9) CFU/m(3) from CSO, and 6.2 x 10(4) CFU/m(3) 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.