The "Burn": water quality and microbiological impacts related to limited free chlorine disinfection periods in a chloramine system.

To control microbial proliferation and nitrification within distribution systems, utilities practicing secondary disinfection chloramination often discontinue their ammonia feed and provide a short, free chlorine disinfection period (FClP), commonly referred to as a "chlorine burn". However, the success and practicality of this approach is often criticized because of the return to nitrification; yet, previous studies conducted in full-scale distribution systems do not contain the sampling frequency to determine how quickly nitrification can return. In this research, a total of 15 hydrants distributed across hydraulically modeled water ages were sampled for 21 sampling events over a period spanning two annual FClPs (2018 and 2019) to investigate the water quality, planktonic community, and, using a new sampling technique, established biofilm community impacts within a single, distribution system pressure zone. Hydrants measured elevated nitrite only 10 weeks after the end of the FClP and live cell counts in the bulk and scour samples statistically significantly increased within two weeks after the FClP ended and chloramine disinfection resumed, indicating limited impacts from a FClP. Furthermore, the FClP significantly increased iron concentrations during the period of free chlorine disinfection creating a consumer water quality concern. Microbial fingerprint analysis using flow cytometry revealed that beta diversity did not significantly change for sampling locations that experienced even periodic low total chlorine concentrations. Only locations that maintained high chlorine residuals throughout both chloramine and free chlorine disinfection periods demonstrated significant changes in bulk water microbial community. Even for these locations, microbial communities of the scoured biofilms remained similar over the course of the study.