Water quality and biofilm growth in drinking water distribution systems with the low-dose sodium hypochlorite disinfection after ultrafiltration pretreatment

In this study, the effects of low-dose sodium hypochlorite disinfection on water quality and biofilm growth in drinking water distribution systems (DWDS) after ultrafiltration pretreatment was investigated. The influence of pipeline hydraulic residence time (HRT) on disinfection efficiency, by-product formation, microbial activity, and biofilm growth were considered. The results show that both microbial activities and metabolite secretion were stimulated by increasing HRT, aggravating the potential risk of microbial pollution in DWDS. The enhanced microbial metabolism could further weaken disinfection efficiency by consuming extra residual Chlorine, after which the formation of disinfection by-products was facilitated. Residual Chlorine was found negatively correlated with HRT. With prolonging HRT from 5 h to 40 h, the concentration of disinfection by-products (Chlorate, Chlorite, and Trichloromethane) was on a continuously increasing trend by 37 %, 140 %, and 75 %, respectively. But the water kept in pipeline still reliably satisfied the Standards for drinking water quality in China (GB5749–2022). Besides, more biofilm with denser morphologies developed on rubber pipeline gaskets rather than the iron/plastic ones. Rubber material was inappropriate for DWDS due to its potential risk of secondary biological pollution. Prolonging HRT also enhanced the accumulation of dominant bacteria (e.g. Bradyrhizobium and Obscuribacter) and decreased microbial diversity.