Impact of flushing procedures on drinking water biostability and invasion susceptibility in distribution systems

Abstract Maintaining microbial stability in drinking water distribution systems (DWDS) is a priority for water providers, yet operational events like flushing and the potential invasion of unwanted microorganisms pose significant challenges. This study utilized a pilot-scale DWDS to evaluate the susceptibility of drinking water networks after standard operational flushing procedures (with and without chlorination) as these procedures can alter biostability. Results showed that flushing with chlorination reduced bulk cell counts initially but led to subsequent microbial regrowth, highlighting the limitations of chlorination in long-term biostability. Biofilm cell densities and community composition were similar before and after flushing with and without chlorination. Additionally, the susceptibility of a flushed network against introduced bacterial indicators ( Aeromonas media , Pseudomonas putida , and Serratia fonticola ) was investigated. The invasion experiments revealed that the species decayed more rapidly in loops flushed with chlorination. This study emphasizes the trade-off between potential regrowth and microbial control, underscoring the need for strategies that enhance microbial resilience without fostering regrowth. These findings are critical for optimizing DWDS management practices to ensure drinking water safety and quality. Importance Flushing, with or without chlorination, is a standard procedure used by drinking water providers to address water quality issues such as brown water, as well as following waterworks, or contaminant detection. This process involves discharging large volumes of drinking water, often through fire hydrants. Depending on the contamination, additional disinfectants, such as free chlorine, are dosed during flushing. However, the longer-term effects of these processes on microbial dynamics are unknown. For example, while brown water issues can be solved, the overall biostability of drinking water may be compromised, potentially making the drinking water distribution systems more susceptible to the invasion of unwanted microorganisms. Studying these practices in full-scale distribution networks is challenging. Therefore, in this study, a unique drinking water distribution pilot system that mimics a real network with mature biofilm was used to investigate the changes in both bulk water and biofilm microbiomes following flushing procedures.