Membrane fouling mitigation in different biofilm membrane bioreactors with pre-anoxic tanks for treating mariculture wastewater
Graphical abstract
Introduction
The increased production of maricultural wastewater has had a negative impact on the environment in recent years. The high salinity of maricultural wastewater and fluctuation of salinity affect microbial metabolism and the water treatment process (Chen et al., 2018; Yogalakshmi and Joseph, 2010). Recently, membrane bioreactors (MBRs) have been widely adopted in the treatment of saline wastewater (Tan et al., 2019), due to the advantages of complete biomass retention, reduced footprint and high-quality effluent (Hong et al., 2013; Song et al., 2018; Zhang et al., 2014). Nevertheless, membrane fouling is an intractable problem in MBR applications, especially for saline wastewater. It is generally accepted that high salt stress reduces the metabolic enzyme activity, causes cell plasmolysis and inhibits the growth of microorganisms (He et al., 2017; Zhang et al., 2014). Specifically, high salinity breaks the multivalent cation bridges between extracellular polymeric substances (EPS) in sludge (De Temmerman et al., 2014), leading to poor sludge quality and deflocculation, and promotes the production of soluble microbial product (SMP) (Li et al., 2018) and EPS (Wang et al., 2013), which have been considered as primary causes of membrane fouling. Thus, it is urgently needed to develop an innovative and effective MBR process to mitigate membrane fouling in mariculture wastewater.
In recent years, biofilm membrane bioreactors (BF-MBRs) aimed at alleviating membrane fouling and improve the performance. Biofilm attached to bio-carriers not only promotes the removal of organics and nutrients but also improves the microbial diversity (Chen et al., 2016; Hazrati and Shayegan, 2016; Jamal Khan et al., 2011; Song et al., 2018). Moreover, bio-carriers could mitigate membrane fouling through the mechanical scouring of membrane surface (Huang et al., 2008) and biochemical effects of activated sludge (Hu et al., 2012). Sun et al. (2010) found that biofilm could reduce the release of SMP, leading to a lower fouling potential. Han et al. (2018) founded that protein (PN) and polysaccharides (PS) in loosely bound and tightly bound EPS (LB-EPS and TB-EPS, respectively) were reduced by sponge and PBS carrier. However, other studies obtained contradictory results regarding the bio-carriers' effect on membrane fouling. For instance, Hu et al. (2012) used two BF-MBRs—one of which involved mechanical effect on the membrane surface, and the other of which lacked mechanical effect through the use of baffles to prevent bio-carriers from contacting the membrane surface—to study the effect of mechanical scouring, and found that biochemical effects were more important factors in membrane fouling than mechanical scouring. Meanwhile, Chen et al. (2016) compared the effects of mechanical scouring on membrane fouling in two MB-MBRs, and found that, in the MB-MBR with mechanical scouring by bio-carriers, the time of membrane filtration operation was 5.3 times longer than that in the other reactor without mechanical scouring. Furthermore, Yang et al. (2009) found that the addition of suspended bio-carriers aggravated membrane fouling because of the overgrowth of filamentous bacteria. Thus, the mechanisms of membrane fouling in BF-MBR have not yet been adequately clarified. Moreover, few studies have investigated the effect on fouling behavior with different bio-carriers under the influence of salinity.
For better understanding the effect of the different bio-carriers on membrane fouling in saline wastewater, two different types of biofilm membrane bioreactor coupled with a pre-anoxic tank (BF-AO-MBR)—namely a fixed biofilm MBR (FB-MBR) with fiber bundle bio-carriers and a moving bed biofilm MBR (MB-MBR) with suspended bio-carriers—are designed to mitigate membrane fouling and improve the treatment of mariculture wastewater.
Thus, this study evaluates and compares these two BF-AO-MBRs and an anoxic/oxic MBR (AO-MBR) on membrane fouling behavior at salinities ranging from zero to 60 g/L. In this analysis, the membrane fouling behavior of the MBRs was analyzed by measuring the transmembrane pressure (TMP), membrane fouling rate (MFR), and distribution of filtration resistance. Then, mixed liquor properties (including the total biomass (suspended and attached), particle size distribution (PSD), SMP, LB-EPS, and TB-EPS), and the microbial community were analyzed to systematically investigate the membrane fouling behavior.
Section snippets
Configuration and operation of reactors
Schematic diagrams of the BF-AO-MBRs and the AO-MBR used in this study are shown in Fig. 1. The AO-MBR is hereafter denoted as MBRa, and the FB-MBR and MB-MBR are hereafter denoted as MBRb and MBRc, respectively. Each reactor consisted of a pre-anoxic tank (6 L) and an aerobic tank (12 L), giving a total effective volume of 18 L. A baffle was added to reduce mixing between the anoxic tank and the aerobic tank. The details and packing density of the fiber bundle bio-carrier (Jiangsu Yulong,
Comparison of the membrane fouling behavior under different salinities
The development of TMP, as an important parameter of the filtration performance, was measured during the operation, and the results are presented in Fig. 2. MBRa, MBRb and MBRc initially took about 29, 54, and 38 days to reach a TMP of 30 kPa, respectively, and reached the threshold level a total of 22, three, and nine times during the operation, respectively. This suggests that both the fiber bundle bio-carriers in MBRb and the suspended bio-carriers in MBRc alleviated membrane fouling, with
Conclusion
The FB-MBR exhibited the lowest fouling development, with three membrane filtration cycles, while the AO-MBR and MB-MBR had 22 and nine cycles, respectively. The key fouling factor was RC in all reactors, while Rp (24.63%) was also an important cause of fouling in the FB-MBR. The fiber bundle bio-carriers and suspended bio-carriers reduced the RT by 37.68 and 21.24% (mainly the RC) compared to the AO-MBR, respectively. Furthermore, FB-MBR and MB-MBR reduced the amount of suspended biomass (by
CRediT authorship contribution statement
Hanqing Wang: Methodology, Writing-original draft. Huining Zhang: Conceptualization, Data curation. Kefeng Zhang: Writing-review & editing. Yongxing Qian: Software. Xin Yuan: Methodology. Bixiao Ji: Investigation. Wanling Han: Visualization.
Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Membrane fouling mitigation in different biofilm membrane bioreactors with pre-anoxic tanks for treating mariculture wastewater”.
Acknowledgements
This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LGF20E080003, LQ20E080002), National Natural Science Foundation of China (21808200), Major Social Development Project of Ningbo (2017C510006) and the Ningbo Natural Science Foundation of China (2018A61028).
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