Impacts of long-term electric field applied on the membrane fouling mitigation and shifts of microbial communities in EMBR for treating phenol wastewater
Graphical abstract
Introduction
Membrane bioreactor (MBR), combines the activated sludge process with an effective solid-liquid separation of mixed liquor by membrane filtration, is preferred to traditional biological treatment technologies due to the production of high-quality permeate, reduce of footprint, and decrease of sludge yield (Yang et al., 2006; Judd, 2008). However, membrane fouling remains a primary challenge for the sustainable application of MBR. At present, electric field applied in MBR (EMBR) was proved effective in alleviating membrane fouling from several aspects. Firstly, the applied voltage could increase electric repulsive force between negatively charged foulants (extracellular polymeric substances (EPS)) and membranes (Liu et al., 2012). Secondly, the suitable applied voltage could promote the flocculation between sludge flocs, which inhibited sludge bulking and reduced specific resistance to filtration (Liu et al., 2019). Besides, the electrochemical oxidation could remove foulants on the membrane surface or even deeply inside the membrane pores (Yang et al., 2019). Wang et al. (2013) found the electro-generated H2O2 could contribute to in-situ membrane cleaning to some extent. Moreover, Our recent research further confirmed that appropriate applied voltage reduced EPS content around membrane and altered the charged functional groups in EPS around membrane, which increased absolute value of zeta potential and enhanced the electrostatic repulsion between foulants and membrane, thus alleviated the absorption/accumulation of EPS on membrane surface (Shi et al., 2019). Although the variation of EPS has been elucidated from many aspects in EMBR in these studies, its relationship to biotic factors in EMBR still remains largely unexplored.
Quorum sensing (QS), which was regulated by cell-to-cell signals (N-acyl-homoserine lactones (AHLs)), has been reported to play significant roles in biofouling in MBR (Yeon et al., 2008). It is strongly linked with EPS production and biofilm formation, and several studies have linked the presence of AHL signals with EPS production and biofilm formation and revealed the problem of membrane biofouling in MBRs (Stuckey, 2012; Ren et al., 2013). The means by which QS signals are targeted to control QS behavior is known as quorum quenching (QQ), which was confirmed to be effective in decreasing AHLs concentrations and mitigated membrane fouling (Yu et al., 2016). Many environmental factors, such as alkaline pH, temperature, and C/N ratio can profoundly influence AHLs concentration, thereby affecting the levels of AHL-based QS and QQ in the surrounding environment (Decho et al., 2009; Kimes et al., 2012; Wang et al., 2019). Nevertheless, the effect of voltage applied in EMBR on AHLs concentration remains unknown. It is reported that the AHLs molecules in MBR can be decomposed via reactive oxygen species (e.g., H2O2) generated through UV photolysis process (Zhang et al., 2019). However, whether the electro-generated H2O2 could degrade AHLs molecules have never been explored.
In addition, the microbial community evolution in a long period of operation played a vital role in the performance of EMBR (Hou et al., 2019). Since large amounts of QS and QQ bacteria are known to be included in the microbial community in activated sludge, and the exocellular AHLs concentrations are the balanced results of QS and QQ bacteria, the shift of microbial community structure and QS and QQ bacteria in EMBR is also necessary to be investigated (Tan et al., 2015; Li et al., 2016; Li et al., 2017).
Therefore, we operated simultaneously four laboratory-scale EMBRs for 100 days under 0, 0.1, 0.4, and 0.8 V/cm voltages, which were confirmed effective on membrane fouling mitigation in our previous study (Shi et al., 2019). This study systematically (1) evaluated the long-term antifouling performance; (2) analyzed the variation of EPS around membrane and on membrane surface; (3) explored the electrolysis of AHLs; (4) monitored shifts of microbial community structure and involved in EMBR and biofilm on membrane surface.
Section snippets
EMBR construction and operation
A laboratory scale EMBR, developed by Shi et al. (2019), was used in the study. A flat sheet polyester filter cloth membrane module, characterized by an effective membrane surface area of 2.4 × 10−3 m2 (length 6 cm and width 4 cm), was placed vertically at the centre of the rectangular bioreactor with a working volume of 1 L (length 15.5 cm, width 8 cm, and height 19 cm). Two pieces of stainless meshes were built inside the membrane module, used as the cathodes. The stainless-steel mesh anodes
The degradation performance and the activity of key enzymes in EMBR
Phenol degradation rate was monitored every 0.5 h on the first day of the two stage (Day 1 and Day 21). The phenol degradation rates in G4 reached 100.00% at 1.0 h on Day 1. Meanwhile, the phenol degradation rates in G3, G2, and G1 was 100.00%, 93.79%, and 84.93%, respectively. Similarly, at 2.5 h on Day 21, the phenol degradation rates in G4 reached 100.00%, followed by G3 (98.87%), G2 (92.67%), and G1 (75.29%). The phenol was completely degraded within 1.5 h and 3.5 h in all the groups on Day
Conclusion
The phenol wastewater treatment performance enhanced from 0 to 0.8 V/cm, which was significantly positively correlated with key enzymes. The membrane fouling was significantly mitigated with the voltage increased in EMBR. The decrease of PN/PS in EPS increased the negative charge and decreased the hydrophobicity of sludge, which abated its absorption on membrane surface. The electro-generated H2O2 degraded AHLs. Decreased AHLs concentration was significantly negatively correlated with QQ
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge the financial supports from the National Natural Science Foundation of China (No. 51508259) and Dalian Science and Technology Star Project Support Plan (2018RQ31).
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