The choice of appropriate biofilm control strategies in membrane systems for seawater desalination pretreatment relies on understanding the properties of the biofilm formed on the membrane. This study reveals how the biofilm composition, including both organic and inorganic, influenced the biofilm behavior under mechanical loading. The investigation was conducted on two Gravity-Driven Membrane reactors employing Microfiltration (MF) and Ultrafiltration (UF) membrane for the pretreatment of raw seawater. After a stabilization period of 20 days (Phase I), a biofilm behavior test was introduced (Phase II) to evaluate (i) biofilm deformation during the absence of permeation (i.e., relaxation) and (ii) biofilm resistance to detachment forces (i.e., air scouring). The in-situ monitoring investigation using Optical Coherence Tomography (OCT) revealed that the biofilms developed on MF and UF membrane presented a rigid structure in absence of filtration forces, limiting the application of relaxation and biofilm expansion necessary for cleaning. Moreover, under shear stress conditions, a higher reduction in biofilm thickness was observed for MF (−60%, from 84 to 34 µm) compared to UF (−30%, from 64 to 45 µm), leading to an increase of permeate flux (+60%, from 9.1 to 14.9 L/m2/h and +20 % from 7.8 to 9.5 L/m2/h, respectively). The rheometric analysis indicated that the biofilm developed on MF membrane had weaker mechanical strength, displaying lower storage modulus (−50 %) and lower loss modulus (−55 %) compared to UF. These differences in mechanical properties were linked to the lower concentration of polyvalent ions and the distribution of organic foulants (i.e., BB, LMW-N) found in the biofilm on the MF membrane. Moreover, in the presence of air scouring led to a slight difference in microbial community between UF and MF.

Our findings provide valuable insight for future investigations aimed at engineer biofilm composition to optimize biofilm control strategies in membrane systems for seawater desalination pretreatment.