Why and how biofilms cause biofouling-the “hair-in-sink”-effect

Biofilms can interfere strongly with membrane filtration performance both by increased transmembrane pressure drop and increase of feed-concentrate pressure drop. Intuitively, the transmembrane pressure drop effect seems proportional to biofilm thickness. However, only few data about the intrinsic hydraulic resistance of biofilms are available, with the surprising result that the contribution of the biofilm to the transmembrane pressure drop seems of relevance only for ultrafiltration, but much less for nanofiltration and negligible for reverse osmosis membranes. Using optical coherence tomography, it could be shown that variations in permeate flux lead to lower biofilm thickness at higher permeate flux, indicating that biofilms are compressible. As a consequence, it was observed that the thicker biofilm at low permeate flux (20 L m-2 h-1) exerted a lower hydraulic resistance than the thinner biofilm at high permeate flux (60 L m-2 h-1)-both at the same bacterial cell density. This was investigated in a transparent membrane biofouling monitor. It could be demonstrated that exclusively the biofilm extracellular polymeric substances (EPS) and not the biofilm bacterial cells cause the hydraulic resistance of biofilms. The thinner biofilm had the same EPS content as the thicker biofilm, but a higher EPS concentration due to biofilm compression. Therefore, not biofilm thickness but EPS concentration dominates the hydraulic biofilm resistance. The mechanism on which this effect is based can be explained by a model developed from the Hagen-Poiseuille law in which the boundary effects of EPS polymers during forced water passage are included. This phenomenon, which we graphically term the “hair-in-sink-effect”, depends not only on the concentration of the EPS but also on the nature of the EPS molecules which can vary greatly among biofilms from different feed waters and strongly influence interactions between these molecules. The biofilm compression may be caused by transport of water from the biofilm gel to the bulk water and a physical rearrangement of the biofilm strand structure. Membrane operational aspects can increase the impact of biofouling on membrane performance. To the author’s best knowledge, for the first time, a theoretical understanding of the hydraulic resistance of biofilms has been presented.