Flushing Temporarily Reduces Nontuberculous Mycobacteria and Nitrifiers in a Full-Scale Chloraminated Drinking Water Distribution System

Conventional flushing is widely used to improve drinking water quality, yet its effectiveness in controlling opportunistic pathogens and nitrifiers in full-scale chloraminated drinking water distribution systems remains unclear. This study evaluated microbial and physicochemical responses to weekly flushing over four months at four sites in a chloraminated distribution system. Start-of-flush (SOF) and end-of-flush (EOF) samples were analyzed using heterotrophic plate counts, 16S rRNA gene sequencing, and qPCR targeting nontuberculous mycobacteria (NTM), ammonia-oxidizing bacteria (AOB), Nitrospira spp., and complete ammonia oxidizers (comammox). EOF samples had lower mean alpha diversity than SOF samples, with the observed richness decreasing by 35.4%. Flushing reduced mean heterotrophic plate counts by 0.63 log 10 and mean NTM and nitrifier gene concentrations by 1.4–1.6 log 10 . These reductions coincided with lower mean turbidity levels (from 16.9 to 9.48 NTU), mean nitrite concentrations (from 0.14 to 0.050 mg/L NO 2 –N), and higher mean total chlorine levels (from 1.9 to 2.4 mg/L as Cl 2 ). However, these effects were temporary, and repeated weekly flushing did not produce clear shifts in bulk water microbial community structure. Overall, conventional flushing provided short-term water quality improvements but did not achieve sustained reductions in persistent microbial populations.