Control of indigenous quorum quenching bacteria on membrane biofouling in a short-period MBR
Graphical abstract
Introduction
Membrane bioreactor (MBR) is a wastewater treatment technology that combines membrane separation and biological treatment (Gu et al., 2018a, Huang et al., 2018). It has significant advantages of good effluent quality, high volumetric load, small foot-print, less residual sludge production, and convenient operation management (Boonnorat et al., 2014, Lotti et al., 2014, Zhang et al., 2018). However, significant decline of flux and rapidly increased transmembrane pressure (TMP) make membrane biofouling one of the most challenging problems in the further development of MBR (Bucs et al., 2018, Nagaraj et al., 2018). Plentiful methods have been attempted to reduce membrane fouling, such as backwashing, air sparging, chemical membrane cleaning and antibiotics (Gu et al., 2018b, Oh et al., 2012), while these methods inevitably have side-effects like higher energy consumption, secondary pollution and resistance to drugs for microorganism (Davies, 2003, Lade et al., 2014, Zeng et al., 2018a).
Quorum sensing (QS) refers to that bacteria could sense the number of themselves or other bacteria in the surrounding environment by sensing the concentration of specific signal molecules, thus adjust the expression of related genes to adapt to environmental changes (Oh and Lee, 2018, Yeon et al., 2009a, Zeng et al., 2018b). Biological wastewater treatment processes such as biofilm formation, nitrification, denitrification, contaminant removal, granulation of activated sludge and power generation were found to be associated with N-acyl-homoserine lactone (AHL)-based QS between Gram-negative bacteria (Hu et al., 2016, Tan et al., 2014).
Quorum quenching (QQ) refers to the process that autoinducer-mediated quorum sensing is interrupted (Iqbal et al., 2018, Lade et al., 2014). Quorum sensing helps bacteria coordinate group-based behavior, but it does not affect the survival or growth of bacteria. Thus, interference to quorum sensing can result in inhibition of the desired phenotype, such as biofilm formation (Lee et al., 2018). In recent years, many researches were devoted to study the mitigation of membrane biofouling by quorum quenching (Lee et al., 2016, Oh et al., 2012, Paul et al., 2009).
Jahangir et al. (2012) added a QQ bacteria Rhodococcus sp. BH4 in shape of microbial-vessel to an external submerged MBR. The experiment results showed that the microbial-vessel effectively interrupted cell-to-cell communications (quorum sensing), inhibited membrane biofouling through quorum quenching activity of Rhodococcus sp. BH4 and thereby exhibited energy saving potential by reducing the aeration rate in MBR. And the microbial-vessel maintained its quorum quenching activity steadily over 100 days of the MBR operation. Ham et al. (2018) isolated a QQ bacterium named HEMM-1 from a MBR plant. It was found that the HEMM-1 cell-free supernatant (CFS) showed higher QQ activities than the CFS of other QQ bacteria (including Pseudomonas sp. 1A1 and Rhodococcus sp. BH4), mostly by degrading AHLs with short acyl chains such as BHL (N-Butyryl-dl-homoserine lactone), and the instrumental analyses revealed that HEMM-1 CFS degraded AHLs via lactonase activity. Under static, flow, and shear conditions, the HEMM-1 CFS was effective in reducing bacterial and activated-sludge biofilms formed on membrane surfaces. Kampouris et al. (2018) isolated a novel QQ-bacterium, namely Lactobacillus sp. SBR04MA from municipal wastewater sludge. Lactobacillus sp. SBR04MA showed great potential for biofouling control in a lab-scale MBR system treating synthetic wastewater, which was evidenced by the ∼3-fold increase in critical flux (8.3–24.25 L/m2/h), as well as by reduction of the SMP and EPS production, which was lower during the QQ-period when compared against the control period.
Above all, it is of significant importance to characterize the effect of a new isolated quorum quenching bacteria on the efficiency of mitigating the membrane fouling problem in MBRs. In this paper, we investigated the application of indigenous QQ bacteria in a short-period lab scale MBR for membrane biofouling control; compared the rising trends of TMP of experimental MBR and control MBR; examined the effect of the QQ bacteria on the content of microbial products (extracellular polymeric substance (EPS) and soluble microbial products (SMP)); and detected the degradation of AHLs (N-Hexanoyl-L-homoserine lactone and N-Octanoyl-dl-homoserine lactone) by QQ bacteria. The linear correlation between AHLs and microbial products (SMP and EPS) were also discussed to study the mechanism of the reduction of biofilm on the membrane surface by QQ bacteria.
Section snippets
Lab-scale MBR and operational parameters
The schematic diagram of MBRs in this experiment is shown in Fig. 1. The influent was synthetic municipal wastewater pumped by a metering pump, start and pause of which was controlled by a liquid level relay installed in the reaction tank, which had an effective volume of 5 L. The Polyvinylidene Fluoride (PVDF) hollow fiber membranes (KOCH, USA) with effective area of 0.02 m2 and 0.03 μm nominal pore size were used for the MBRs. A peristaltic pump was connected to the membrane module through a
The effect of QQ bacteria on the performance of MBR
Samples were daily taken from the mixed liquor and effluent of the two MBRs, respectively, to measure a series of parameters (SV30 and SVI) and COD removal rate of MBR and evaluate the effect of QQ on activated sludge performance in MBR. The COD removal rates were also maintained above 95% throughout the experimental period. Though the SV30 value is a little higher than that of the conventional activated sludge treatment, this may be attributed to the higher MLSS and uninterrupted aeration in
Conclusion
QQ bacteria could effectively mitigate TMP rise in short-period MBR. TMP of control MBR reached 43 kPa on the fourth day, while at the same time, the TMP of QQ-MBR was only 18 kPa. EPS and SMP in QQ-MBR were significantly inhibited compared with that in control-MBR. QQ-MBR also exhibited effective degradation on AHLs in mixed liquor. Two AHLs both had good linear correlations with SMP and LB-EPS. It could be indicated that QQ bacteria reduced SMP and EPS by degrading the signal molecules, thus
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (51178172, 51578222, 51521006, 51308076 and 51378190), the Project of Chinese Ministry of Education (113049A), and the Research Fund for the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17).
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