The significance of high-molecular weight organic carbon for the biological stability of drinking water assessed with ultrafiltration as posttreatment after conventional surface water treatment

Avoidance of unwanted, uncontrolled high levels of microbial regrowth in disinfectant-free drinking water warrants the attainment of biological stability conditions by the establishment of appropriately low levels of biodegradable organic compounds in the treated water. Our study showed that the presence of biopolymeric high-molecular weight organic carbon (High-MW OC) compounds affects biological stability and promotes regrowth as the technical bacterial parameters Aeromonas and heterotrophic plate counts. Reducing the concentrations of high-MW OC by the implementation of ultrafiltration as posttreatment after conventional surface water treatment resulted in significant lowering of regrowth in a full-scale distribution network. Target levels were not fully reached however, due to the remaining occurrence of sediments, pipe wall biofilm and invertebrates in the distribution network. The latter was most likely promoted by iron concentrations of only a few microgram-per-litre. Nanofiltration and reverse osmosis are suitable treatment technologies to further lower the levels of high-MW OC concentrations, regrowth, and concomitantly, undesirable in-/organic micropollutants.