Ultrafiltration of Water Has a Temporary Effect on Cell Numbers, but Profoundly Changes the Composition of Bacterial Populations—The ‘Reset’ Phenomenon

Ultrafiltration strips water of bacteria. The common misconception is that the filtrate is thus free of bacteria. This only applies, however, in the case that the filtrate compartment is sterile. In real-world applications, the filtrate is rapidly re-colonized, followed by regrowth. In extreme cases of low water usage, the cell numbers in the filtrate can even exceed those in the feed water, probably due to a combination of the microbial enrichment of the bulk water from surfaces, regrowth in the water body itself, and nutrient enrichment on the filter membrane. Regrowth is made possible because dissolved nutrients can freely pass through the membranes. This explains why the initial decrease in cell numbers in drinking water installation systems with ultrafiltration is often followed by an increase in the periphery of the plumbing system. The extent of actual regrowth hereby depends mostly on water usage behaviours. A shorter frequency of membrane wash cycles is beneficial for reducing cell numbers. Neither frequent wash cycles nor cleaning in place (CIP) in filtration units, however, seem to modulate the maximal regrowth potential. Although the effect of ultrafiltration on cell numbers is not sustainable, it causes profound changes in the bacterial communities, with highly distinct populations in the feed water and the filtrate. The microbiological “reset” is demonstrated using examples both from the fields of drinking water and water reuse. Overall, our results suggest that ultrafiltration has a profound impact on the microbiome, but the cell numbers in filtrates depend mostly on the water usage and operational conditions.