Research On Membrane Fouling Control In MBR Based On Microbial Quorum Quenching

From its development to wide application, membrane bioreactor(MBR) has experienced 25 years, but membrane fouling keeps on inpending all along the development of MBR. On the other hand, quorum sensing(QS) was proposed and then developed almost at same time as MBR. QS is defined as perception between bacterial, i.e. bacteria perceives its own cell population density using signal molecules(e.g. acyl homoserine lactone, AHL) and coordinates some quorum behaviors(e.g. pathogenicity, bioluminescence, antibiotic excretion, biofilm formation, etc.). QS was mainly studied in the field of microbiology before. Recently, some researches started to investigated the QS in water/wastewater treatment process. A few of studies even used quorum quenching(QQ) to suppress fouling in MBR, and this attempts provided a new paradigm for fouling control in MBR. However, study on QS and QQ in MBR is still at its initial stage, and many issues still remain to be studied. Effect of operational parameters of MBR(e.g. solid retention time, SRT) on QS and QQ in MBR has not been studies; the reported QQ approach is still too complex and not stable enough; impact of QQ on other comtaminants removal in MBR still remain unclear. This dissertation was amied to investigate the issues above, with an expectation of gaining a more complete understanding of QS and QQ in MBR, as well as proposing a simpler, more stable and more effective fouling control approach based on QQ.Firstly, effect of solid retention time(SRT) on quorum sensing and membrane fouling in MBR was investigated. It is indicatd the abundance of quorum quenching(AHL degrading) bacteria increased and that of quorum sensing(AHL producing) bacteria decreased with increased SRT. Activated sludge in MBR degraded AHL more efficiently under longer SRT, and thus, AHL concentration in MBR decreased as SRT increased. Extracellular polymeric substances(EPS) and soluble microbial products(SMP) excretion was associated with quorum sensing, so their concentrations decreased with longer SRT. Therefore, under longer SRT, much more weaken quorum sensing(thus less biofouling) and less EPS and SMP contents(thus less organic matter fouling) resulted in less membrane fouling. The results above also comfirmed the existence of quorum sensing in MBR and its significant effect on fouling.Inspired by the successful control of tuber soft rot of potato caused by Pectobacterium through quorum quenching(QQ) strategy, a new QQ approach based on biostimulation was proposed and its effect on fouling control in MBR was investigated. Gamma-caprolactone(GCL), a biostimulaiton reagent, effectively enriched QQ bacteria from activated sludge, in which Rhodococcus was the major QQ bacteria. The GCL consortia could effectively degrade AHL, which can be attrbited to qsd A, a lactonase, from Rhodococcus. When dosing GCL consortia and then continuously dosing GCL into MBR, the qsd A content in MBR was kept at high level, AHL concentration in MBR was decreased, EPS secretion was suppressed, biofilm formation on membrane surface was eliminated, and thus membrane fouling was effectively and enduringly controlled. Biostimulation approach for QQ proposed in this dissertation is much simpler and more effective than encapsulation of functional bacteria in published research.It was reported that ammonia oxidation bacteria(i.e. nitrosomonas) could secret AHL and its ammonia oxidation activity was coordinated by quorum sensing. However, in published studies and our research above, quorum quenching in MBR did not compromise the nitrification. In this thesis, effect of quorum quenching on nitrification in MBR was further investigated under some unfavorable conditions for nitrifaction(i.e. short hydrolic retention time(HRT), with presence of ammonia oxidation inhibitor(i.e. allylthiourea, methonal, and acetonitrile), and low tempareture(10℃)). It is indicated that quorum quenching strategy could also control membrane fouling under the unfavorable conditions above. However, quorum quenching did attenuate nitrification in MBR under those unfavorable conditions. Nitrification in MBR implemented quorum quenching was more susceptible, and permeate from that MBR hardly met the standard. Therefore, MBR adopted quorum quenching approach should be operated under rather longer HRT, with less nitrification inhibitor in influent, and more suitable temperature.