QUORUM QUENCHING FOR BIOFOULING CONTROL IN ANAEROBIC MEMBRANE BIOREACTORS

Abstract

Anaerobic membrane bioreactor (AnMBR) is a promising technology for domestic wastewater treatment and energy recovery. However, membrane biofouling (i.e., undesirable developed biofilm formed on membrane surface) is a challenging issue that leads to drastic permeability loss and tremendous energy consumption. It is one of the main obstacles restricting widespread adoption of AnMBRs for wastewater treatment. Membrane fouling control in field applications was commonly achieved through conventional chemical cleaning methods (e.g., NaOCl). However, these technologies are for the reduction or removal mature or developed biofilm, which might have both mechanical and chemical tolerance. Therefore, it is favorable to eliminate biofilm at an early stage rather than to diminish the biofilm which has already been developed to the matured ones (i.e., biofouling). Quorum sensing (QS), which is a cell-to-cell communication mechanism, could directly regulate the biofilm formation of microorganisms. Therefore, quorum quenching (QQ) via disruption of the QS systems of microorganisms in AnMBRs would be promising to reduce serious biofouling.

The overall aims of this thesis were to first isolate facultative quorum quenching consortia (FQQ) with preliminary investigation of their QQ performance. Key organic compounds in the extracellular polymeric substances (EPS) of suspended biomass and size-fractioned particles/organics in AnMBRs were further analyzed to better understand the mechanism of QQ governing the associated elusive fouling mitigation. Moreover, profound knowledge with regard to how QQ directly affects pioneering anaerobes attachment and biofilm (i.e., biofouling) development in AnMBR from the viewpoint of microbial community level was finally analyzed.

3 novel FQQ were isolated by enrichment culture and anaerobic screening, and their QQ performance in terms of growth rate utilizing N-acyl-L-homoserine lactones (AHLs), AHLs degradability, EPS retardation efficiency and membrane fouling management in a lab-scale AnMBR was investigated. Higher growth rates of both FQQ isolated by C6-HSL (i.e., FQQ-C6) and C10-HSL (i.e., FQQ-C10) when utilizing AHLs than those using acetate were observed, which suggested their high potential QQ performance. All 3 FQQs could efficiently degrade wide range of AHLs. FQQ-C6 was considered a promising QQ candidate for membrane fouling control in continuously-operated AnMBRs, as it could retard TMP increase by around 2 times. Bioinformatic analysis indicated that Comamonas sp. in FQQ-C6 and FQQ-C10 could be correlated to their high growth rates utilizing AHLs and high EPS retardation efficiencies.

Furthermore, long-term QQ effect on membrane fouling in an AnMBR treating domestic wastewater was investigated. The result showed that QQ extended membrane filtration period by an average of 75%, and decreased C4-HSL, 3-OXO-C6-HSL, and C6-HSL directly proved the QQ effect of FQQ-C6. QQ could cause a shift of dominant foulants from cake layer to pore blockage after a similar period of operation (i.e., 8d), compared with the control AnMBR. Moreover, QQ was demonstrated to reduce protein (p<0.005) and carbohydrates (p<0.005) yield in EPS, and reduced biopolymer with molecular weight larger than 20kDa (p<0.05) was correlated to the improved dewaterability of suspended biomass. QQ could also decreased 54.10%, 34.76%, 51.75%, and 31.13% of organics production within the colloidal particles having size range of 0.45-1 μm, 1-3 μm, 3-5 μm, and > 5 μm in the supernatant of suspended biomass, respectively. However, the small-size organics (~51nm) within SMP in the presence of QQ might lead to the increased occurrence of pore blockage.

The information regarding to the keystone bacteria governing the initial biofilm formation and the biofouling development process from suspended biomass, early biofilm to mature biofilm in AnMBRs with and without the effect of QQ was further investigated. Some low-abundance bacteria in suspended biomass, especially the AHL-based QS related taxa Rhodocyclaceae;g-, showed strong fouling potential in the initial biofilm formation. The delayed initial biofilm formation caused by QQ was not only attributed to the lower EPS content of suspended biomass, but also associated with the reduced attachment or retarded colonization of the pioneering keystone taxa Rhodocyclaceae;g- on membrane surface, which was probably due to the disruption of its QS activity. Nevertheless, the retarded initial biofilm of the AnMBR with presence of QQ was still mainly composed of the similar pioneering fouling-related taxa such as Sulfurovum and Rhodocyclaceae;g-, as compared to those of the control AnMBR. It suggested that AHL-based QQ only had a temporary anti-fouling effect in the AnMBR.