Jung-Kee Lee

Biofouling control with bead-entrapped quorum quenching bacteria in membrane bioreactors: physical and biological effects.

Recently, interspecies quorum quenching by bacterial cells encapsulated in a vessel was described and shown to be efficient and economically feasible for biofouling control in membrane bioreactors (MBRs). In this study, free-moving beads entrapped with quorum quenching bacteria were applied to the inhibition of biofouling in a MBR. Cell entrapping beads (CEBs) with a porous microstructure were prepared by entrapping quorum quenching bacteria ( Rhodococcus sp. BH4) into alginate beads. In MBRs provided with CEBs, the time to reach a transmembrane pressure (TMP) of 70 kPa was 10 times longer than without CEBs. The mitigation of biofouling was attributed to both physical (friction) and biological (quorum quenching) effects of CEBs, the latter being much more important. Because of the quorum quenching effect of CEBs, microbial cells in the biofilm generated fewer extracellular polymeric substances and thus formed a loosely bound biofilm, which enabled it to slough off from the membrane surface more easily. Furthermore, collisions between the moving CEBs and membranes gave rise to frictional forces that facilitated detachment of the biofilm from the membrane surface. CEBs bring bacterial quorum quenching closer to being a practical solution to the problem of biofouling in MBRs.

Microbial population dynamics and proteomics in membrane bioreactors with enzymatic quorum quenching

Quorum sensing gives rise to biofilm formation on the membrane surface, which in turn causes a loss of water permeability in membrane bioreactors (MBRs) for wastewater treatment. Enzymatic quorum quenching was reported to successfully inhibit the formation of biofilm in MBRs through the decomposition of signal molecules, N-acyl homoserine lactones (AHLs). The aim of this study was to elucidate the mechanisms of quorum quenching in more detail in terms of microbial population dynamics and proteomics. Microbial communities in MBRs with and without a quorum quenching enzyme (acylase) were analyzed using pyrosequencing and compared with each other. In the quorum quenching MBR, the rate of transmembrane pressure (TMP) rise-up was delayed substantially, and the proportion of quorum sensing bacteria with AHL-like autoinducers (such as Enterobacter, Pseudomonas, and Acinetobacter) also decreased in the entire microbial community of mature biofilm in comparison to that in the control MBR. These factors were attributed to the lower production of extracellular polymeric substances (EPS), which are known to play a key role in the formation of biofilm. Proteomic analysis using the Enterobacter cancerogenus strain ATCC 35316 demonstrates the possible depression of protein expression related to microbial attachments to solid surfaces (outer membrane protein, flagellin) and the agglomeration of microorganisms (ATP synthase beta subunit) with the enzymatic quorum quenching. It has been argued that changes in the microbial population, EPS and proteins via enzymatic quorum quenching could inhibit the formation of biofilm, resulting in less biofouling in the quorum quenching MBR.

Biofouling inhibition in MBR by Rhodococcus sp. BH4 isolated from real MBR plant

It has been reported that an indigenous quorum quenching bacterium, Rhodococcus sp. BH4, which was isolated from a real plant of membrane bioreactor (MBR) has promising potential to control biofouling in MBR. However, little is known about quorum quenching mechanisms by the strain BH4. In this study, various characteristics of strain BH4 were investigated to elucidate its behavior in more detail in the mixed liquor of MBR. The N-acyl homoserine lactone hydrolase (AHL–lactonase) gene of strain BH4 showed a high degree of identity to qsdA in Rhodococcus erythropolis W2. The LC-ESI-MS analysis of the degradation product by strain BH4 confirmed that it inactivated AHL activity by hydrolyzing the lactone bond of AHL. It degraded a wide range of N-acyl homoserine lactones (AHLs), but there was a large difference in the degradation rate of each AHL compared to other reported AHL–lactonase-producing strains belonging to Rhodococcus genus. Its quorum quenching activity was confirmed not only in the Luria-Bertani medium, but also in the synthetic wastewater. Furthermore, the amount of strain BH4 encapsulated in the vessel as well as the material of the vessel substantially affected the quorum quenching activity of strain BH4, which provides useful information, particularly for the biofouling control in a real MBR plant from an engineering point of view.

Effect of the Shape and Size of Quorum Quenching Media on Biofouling Control in Membrane Bioreactors for Wastewater Treatment.

Recently, spherical beads entrapping quorum quenching (QQ) bacteria have been reported as effective moving QQ-media for biofouling control in MBRs for wastewater treatment owing to their combined effects of biological (i.e., quorum quenching) and physical washing. Taking into account both the mass transfer of signal molecules through the QQ-medium and collision efficiencies of the QQ-medium against the filtration membranes in a bioreactor, a cylindrical medium (QQ-cylinder) was developed as a new shape of moving QQ-medium. The QQ-cylinders were compared with previous QQ-beads in terms of the QQ activity and the physical washing effect under identical loading volumes of each medium in batch tests. It was found that the QQ activity of a QQ-medium was highly dependent on its specific surface area, regardless of the shape of the medium. In contrast, the physical washing effect of a QQ-medium was greatly affected by its geometric structure. The enhanced anti-biofouling property of the QQ-cylinders relative to QQ-beads was confirmed in a continuous laboratory-scale MBR with a flat-sheet membrane module.