Effect of Permeate Flux and Backwashing on Quorum Sensing and Quenching within Biocake Layer in Membrane Bioreactor

Filtration and backwashing during the operation of a membrane bioreactor (MBR) physically induce the cyclic attachment and detachment of microbes on the membrane surface. Thus, permeate flux and backwashing affect the microbial quorum sensing (QS) and quorum quenching (QQ) processes taking place in a biocake layer on a membrane surface. From an analysis of the time to reach a transmembrane pressure (TMP) of 40 kPa (TTMP) under different water flux levels, 20 or 40 liter per square meter per hour (LMH), with QQ beads or vacant beads in MBRs, it was found that the accelerated deposition of microbial cells on membrane surfaces at 40 LMH significantly reduced the time for bacteria to initiate QS and, thus, allowed less room for QQ compared with the results at 20 LMH. On the application of backwashing, the TMP jump with QQ beads is more delayed than that with vacant beads in both the 20 and 40 LMH cases. However, the extracellular polymeric substance (EPS) level with backwashing decreased at 20 LMH but increased at 40 LMH regardless of bead type because the reduced fouling rate caused by backwashing provided the bacteria with more time to activate QS, facilitating active EPS production when backwashing is employed at 40 LMH. A biocake porosity analysis revealed that when backwashing was applied, the porosity increased significantly at 40 LMH but remained nearly unchanged at 20 LMH. It is suggested that, at the higher flux level, backwashing allowed enough time for bacteria to initiate QS and, thus, form biofilm with a porous mushroom-like structure. Similarly, the inhibition of QS through the addition of QQ beads indicated dependence on the permeate flux and backwashing. These results suggest that a hydrodynamic condition near the membrane surface could have a considerable influence on the activation of microbial QS, which in turn can affect membrane biofouling in an MBR.