Reduction of bacterial regrowth potential by membrane filtration for different water types and different pore sizes

Membrane filtration removes microorganisms and particles from water. If the filtrate enters a non-sterile environment, bacterial regrowth occurs. The question arises as to how much regrowth potential is removed by membrane filtration. This study investigated the extent of regrowth of autochthonous bacteria in water from a building with integrated ultrafilter and in treated wastewater that had undergone ultrafiltration in the context of water reuse. The 7-day regrowth of filtrates was reduced on average by approx. 69% and 20% compared to the non-filtered waters. To better understand the behaviour and kinetics of regrowth and how much of the nutrient pool is retained by stripping water of its biomass, three different water types (drinking water, river water and reservoir water) were subjected in a laboratory study to membrane filtration using pore sizes between 0.45 µm down to 0.05 µm. The reduction in the regrowth potential depended on the water type. Filtration reduced the 7-day regrowth potential most strongly in case of drinking water (≥70%), while the reduction was less pronounced for river water and reservoir water (approx. 40%). The kinetics of regrowth over 14 days differed significantly between the water types with peak regrowth reached fastest in case of filtered river water (4 days), followed by filtered reservoir water (4-7 days) and slowest in case of filtered drinking water (14 days). There were also distinct differences in the unfiltered waters, with positive regrowth in case of drinking water and negative regrowth in case of reservoir and river water.