Biofilms on RO membranes treating tap water harbour diverse bacteria with opposing salinity optima

The escalating scarcity of clean freshwater, a crucial global economic asset, demands advancements in water purification technologies. Reverse osmosis (RO) stands out as an effective method for removing contaminants, yet biofouling of RO membranes poses a significant challenge, lowering the operational and energy efficiency. Through DNA sequencing, we identified a small set of abundant core bacterial species common to the biofilm in the three RO modules treating tap water. The core species are closely related to heterotrophic taxa from soil and groundwater systems and are likely important for the RO membrane biofouling. The community compositions of the RO-biofilms were system-specific. Within a given RO module, the biofilm community composition was uniform across the various sections of the membrane module. We established a method to efficiently extract live cells from the membranes and determined their salinity tolerance. Growth was mainly aerobic and was only inhibited at salinities above 3 to 5 %, i.e. exceeding those that can be reached in the specific RO modules. The species composition of the growing community changed discretely with the salinity level, indicating that salinity fluctuations may have an important role in shaping RO biofilm communities. Our findings highlight a complex interplay between microbial communities, salinity, and growth conditions in RO modules.