Primary Colonizing Play a Key Role in the Growth of in Biofilms on Surfaces Exposed to Drinking Water Treated by Slow Sand Filtration.

Slow sand filtration with extensive pretreatment reduces the microbial growth potential of drinking water to a minimum level at four surface water supplies in The Netherlands. The potential of these slow sand filtrates (SSFs) to promote microbial growth in warm tap water installations was assessed by measuring biofilm formation and growth of bacteria on glass and chlorinated polyvinylchloride (CPVC) surfaces exposed to SSFs at 37 ± 2°C in a model system for up to six months. The steady-state biofilm concentration ranged from 230 to 3,980 pg ATP cm on glass and 1.4 (±0.3)-times-higher levels on CPVC. These concentrations correlated significantly with the assimilable organic carbon (AOC) concentrations of the warm water (8 to 24 µg acetate-C equivalents [ac-C eq] liter), which were raised about 2 times by mixing cold and heated (70°C) SSFs. All biofilms supported growth of with maximum concentrations ranging from 6 × 10 to 1.5 × 10 CFU cm Biofilms after ≤50 days of exposure were predominated by , mainly , , , and an uncultured bacterium. These rapidly growing primary colonizers most likely served as prey for the host amoebae of , mostly , e.g., , , and other amoeba-resistant bacteria, accounted for 37.5% of the clones retrieved. A conceptual model based on a quadratic relationship between the colony count and the biofilm concentration under steady-state conditions is used to explain the variations in the CFU pg ATP ratios in the biofilms. Proliferation of in premise plumbing poses a public health threat. Extended water treatment using physicochemical and biofiltration processes, including slow sand filtration, at four surface water supplies in The Netherlands reduces the microbial growth potential of the treated water to a minimum level, and the distributed drinking water complies with high quality standards. However, heating of the water in warm tap water installations increases the concentration of easily assimilable organic compounds, thereby promoting biofilm formation and growth of Prevention of biofilm formation in plumbing systems by maintenance of a disinfectant residual during distribution and/or further natural organic matter (NOM) removal is not feasible in the supplies studied. Temperature management in combination with optimized hydraulics and material selection are therefore essential to prevent growth of in premise plumbing systems. Still, reducing the concentration of biodegradable compounds in drinking water by appropriate water treatment is important for limiting the growth potential.