Quorum Sensing based Biofouling Control in Membrane Bioreactor for Advanced Water Treatment

Abstract

Bacteria perceive their population density using small signal molecules called autoinducer and regulate specific group behaviors such as biofilm formation on the surface in cell density dependent way (Quorum sensing, QS). In this study, the concept of bacterial QS was applied to membrane bioreactor (MBR) for advanced wastewater treatment as a new biofouling control paradigm. The overall research was conducted in the following phases: (1) Evidence of autoinducer signal in MBR (2) Correlation between QS and membrane biofouling (3) Control of membrane biofouling by quorum quenching (4) Preparation of magnetic enzyme carrier (MEC) (5) Application of MEC to the MBR in continuous operation A bioassay with Agrobacterium tumefaciens A136 reporter strain proved that N-acyl homoserine lactone (AHL) autoinducers of gram-negative bacteria were produced in the MBR. Furthermore, thin-layer chromatographic analysis identified at least three different AHLs in the biocake, of which N-octanoyl-homoserine lactone was the most abundant. During continuous MBR operation, the membrane-biocake showed strong AHL activity simultaneously with the abrupt increase in the transmembrane pressure (TMP). Furthermore, both autoinducer level per unit biomass and filtration resistance in the biocake increased under very similar tendency, which implies that QS was closely associated with the membrane biofouling. Porcine kidney acylase I (EC-number 3.5.1.14), which can inactivate the AHL molecule by amide bond cleavage, was confirmed to prevent membrane biofouling by quenching AHL autoinducers, which suggested that quorum quenching, enzymatic destruction of QS autoinducer, could be a novel biofouling control approach in MBR. Despite of its high potential as a novel biofouling control approach, the short catalytic lifetime and difficulty in recovering free enzyme hamper the successful application of the quorum quenching technique in the MBR under a long-term continuous operation. A magnetic enzyme carrier (MEC) was prepared by immobilizing the quorum quenching enzyme (acylase) on magnetic particles to overcome the technical limitations of free enzyme. The MEC showed no activity decrease under both continuous shaking for 14 days and 29 iterative cycles of reuse. Furthermore, the comparison of the MEC with free enzyme in a batch-type MBR showed that the MEC efficiently alleviated the membrane biofouling and showed a great advantage over free enzyme in terms of recycled use and stability in mixed liquor. When the MEC was applied to the lab scale MBR in a continuous operation, it also enhanced the membrane permeability to a large extent compared with a conventional MBR with no enzyme.