Low affinity of heterotrophic bacteria to loose deposits in drinking water distribution systems

Loose deposits (LD) accumulate in drinking water distribution systems (DWDS) and may lead to tap water discoloration incidents upon resuspension. While inconvenient for the consumers and the water companies, discoloration may be accompanied by degradation of the microbiological quality of the water, and possibly to the transport of bacteria. The presence of heterotrophic bacteria towards LD with different characteristics was investigated. Bacterial loads in discoloured water samples and LD concentrated over different settling times were analysed. Total heterotrophic (TH) bacteria numbers did not relate to turbidity or to suspended solids in discoloured waters. Higher affinities of TH were observed for slower-settling LD (<24 h), which were also richer in microbial extracellular polymeric substances. The detection of viable iron-oxidizing bacteria in LD supported their possible roles in LD formation and may be related to microbial growth/regrowth in DWDS. Results suggest that LD may exhibit low affinities to hold and transport bacteria, at least to TH in chlorinated water supplies.