Xingran Zhang

A novel composite conductive microfiltration membrane and its anti-fouling performance with an external electric field in membrane bioreactors

Membrane fouling remains an obstacle to wide-spread applications of membrane bioreactors (MBRs) for wastewater treatment and reclamation. Herein, we report a simple method to prepare a composite conductive microfiltration (MF) membrane by introducing a stainless steel mesh into a polymeric MF membrane and to effectively control its fouling by applying an external electric field. Linear sweep voltammetry and electrochemical impedance spectroscopy analyses showed that this conductive membrane had very good electrochemical properties. Batch tests demonstrated its anti-fouling ability in filtration of bovine serum albumin, sodium alginate, humic acid and silicon dioxide particles as model foulants. The fouling rate in continuous-flow MBRs treating wastewater was also decreased by about 50% for this conductive membrane with 2 V/cm electric field compared to the control test during long-term operation. The enhanced electrostatic repulsive force between foulants and membrane, in-situ cleaning by H2O2 generated from oxygen reduction, and decreased production of soluble microbial products and extracellular polymeric substances contributed to fouling mitigation in this MBR. The results of this study shed light on the control strategy of membrane fouling for achieving a sustainable operation of MBRs.

Antibiofouling Polyvinylidene Fluoride Membrane Modified by Quaternary Ammonium Compound: Direct Contact-Killing versus Induced Indirect Contact-Killing

Widespread applications of membrane technology call for the development of antibiofouling membranes. For the traditional contact-killing strategy, the antibacterial action is restricted to the surface: the membrane loses its antibiofouling efficacy once its surface is completely covered with a fouling layer. However, in this study, polyvinylidene fluoride (PVDF) microfiltration membranes blended with quaternary ammonium compound (QAC) exhibited a surprisingly lasting antimicrobial activity in the vicinity of the membrane surface. The results indicated that QAC was capable of driving surface segregation with a high structural stability, and the QAC modified membrane shows clear antibacterial effects against both Gram-positive and Gram-negative bacteria. Covering the modified membrane surface by an abiotic alginate layer resulted in a loss of antibacterial efficiency by 86.2%. In contrast, the antibacterial efficiency was maintained after developing a biofilm of Staphylococcus aureus of 30 μm in thickness. The current study may suggest that bacteria affected by contact-killing might interact with other bacteria in the vicinity, resulting in retarded biofilm growth. The antibiofouling effect and associated mechanism of the QAC modified membrane were further validated in a membrane bioreactor during long-term operation.

Identification of microbial communities in open and closed circuit bioelectrochemical MBRs by high-throughput 454 pyrosequencing.

Two bioelectrochemical membrane bioreactors (MBRs) developed by integrating microbial fuel cell and MBR technology were operated under closed-circuit and open-circuit modes, and high-throughput 454 pyrosequencing was used to investigate the effects of the power generation on the microbial community of bio-anode and bio-cathode. Microbes on the anode under open-circuit operation (AO) were enriched and highly diverse when compared to those on the anode under closed-circuit operation (AC). However, among the cathodes the closed-circuit mode (CC) had richer and more diverse microbial community compared to the cathode under open-circuit mode (CO). On the anodes AO and AC, Proteobacteria and Bacteroidetes were the dominant phyla, while Firmicutes was enriched only on AC. Deltaproteobacteria affiliated to Proteobacteria were also more abundant on AC than AO. Furthermore, the relative abundance of Desulfuromonas, which are well-known electrogenic bacteria, were much higher on AC (10.2%) when compared to AO (0.11%), indicating that closed-circuit operation was more conducive for the growth of electrogenic bacteria on the anodes. On the cathodes, Protebacteria was robust on CC while Bacteroidetes was more abundant on CO. Rhodobacter and Hydrogenophaga were also enriched on CC than CO, suggesting that these genera play a role in electron transfer from the cathode surface to the terminal electron acceptors in the bioelectrochemical MBR under closed-circuit operation.

Acute Responses of Microorganisms from Membrane Bioreactors in the Presence of NaOCl: Protective Mechanisms of Extracellular Polymeric Substances

Extracellular polymeric substances (EPS) are key foulants in membrane bioreactors (MBRs). However, their positive functions of protecting microorganisms from environmental stresses, e.g., during in situ hypochlorite chemical cleaning of membranes, have not been adequately elucidated. In this work, we investigated the response of microorganisms in an MBR to various dosages of NaOCl, with a particular emphasis on the mechanistic roles of EPS. Results showed that functional groups in EPS such as the hydroxyl and amino groups were attacked by NaOCl, causing the oxidation of polysaccharides, denaturation of amino acids, damage to protein secondary structure, and transformation of tryptophan protein-like substances to condensed aromatic ring substances. The presence of EPS alleviated the negative impacts on catalase and superoxide dismutase, which in turn reduced the concentration of reactive oxygen species (ROS) in microbial cells. The direct extracellular reaction and the mitigated intracellular oxidative responses facilitated the maintenance of microbial metabolism, as indicated by the quantity of adenosine triphosphate and the activity of dehydrogenase. The reaction with NaOCl also led to the changes of cell integrity and adhesion properties of EPS, which promoted the release of organic matter into bulk solution. Our results systematically demonstrate the protective roles of EPS and the underlying mechanisms in resisting the environmental stress caused by NaOCl, which provides important implications for in situ chemical cleaning in MBRs.

Microbial responses to transient shock loads of quaternary ammonium compounds with different length of alkyl chain in a membrane bioreactor

Extensive applications of quaternary ammonium compounds (QACs) in household and industrial products inevitably lead to their release into wastewaters; however, little attention has been paid to the acute effects on activated sludge. In this work, we investigated the responses of microorganisms in a membrane bioreactor (MBR) to transient shock loads of QACs with different alkyl chain length and their impacts on MBR performance. Results showed that QACs affected microbial viability and caused damage to key enzymes (e.g., ammonium monooxygenase and nitrite oxidoreductase), inhibiting organic matter degradation and nitrogen removal. The presence of QACs also caused negative influences on dehydrogenase activity, catalase and superoxide dismutase, thus increasing the production of reactive oxygen species. Moreover, QACs with longer alkyl chains and/or benzyl groups bonded to the nitrogen atom could induce a more severe damage to cell integrity and microbial viability. The interaction with QACs also induced the release of organic matters and the changes of adhesion properties of microbial products, resulting in aggravated membrane fouling in MBRs. Our results demonstrate the acute negative effects of QACs on activated sludge, and special attention should be paid to the performance of biological wastewater treatment processes subject to the shock loads of QAC-bearing industrial streams.

Insights into iron induced fouling of ion-exchange membranes revealed by a quartz crystal microbalance with dissipation monitoring

Understanding the mechanisms of multivalent iron interacting with ion-exchange membranes (IEMs) is crucial for the prediction of membrane fouling as well as the development of control strategies. In this study, the adsorption and desorption behaviors of Fe(III) species on a typical IEM, Nafion, were investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). The Nafion thin film formed on the crystal sensor surface via a sedimentation method showed a non-rigid structure with a Voigt-based mass concentration of ∼500 ng cm−2 at a Nafion solution injection time of 10 min. Adsorption of Fe(III) species was first assessed using a 10 mM Fe(III) solution, followed by rinsing under different conditions to induce the structural transformation and/or release of Fe(III) from the Nafion film. The QCM-D results suggested that there was a rapid deposition of Fe(III) at the initial stage. It has been found that the ongoing adsorption process exhibiting pseudo-first-order kinetics was associated with the interaction between Fe(III) and the surface functional sites (–SO3H) of Nafion, which consequently retarded the proton transfer. Compared to the rinse with an acidic solution (HCl in ultrapure water, pH of 2.37), the QCM-D results of neutral elution (ultrapure water, pH of 6.50) indicated the hydrolysis of Fe(III) and/or structural transformation of the μ-oxo bridged, Fe–O–Fe.

Impacts of quaternary ammonium compounds on membrane bioreactor performance: Acute and chronic responses of microorganisms

Quaternary ammonium compounds (QACs) are emerging contaminants with the extensive applications in a variety of fields. However, little is known about their potential impacts on activated sludge and performance of biological wastewater treatment processes. In this work, the effects of benzalkonium chloride (BAC, a kind of QACs) on acute and chronic responses of microorganisms and on MBR performance were systematically investigated. The results showed that a low concentration (0.5-2.0 mg BAC/g SS) caused no significant effects on activated sludge property. In contrast, an elevated concentration of BAC led to severer inhibition on activated sludge and key enzyme activity (e.g., dehydrogenase activity) in both short-term and long-term exposure, thus deteriorating the pollutant removal efficiency. Compared with the control MBR (R1) and the reactor with 0.5 mg/L BAC (R2), the removal efficiency of ammonia in R3 with 5.0 mg/L BAC at identical hydraulic retention time (4.3 h) and sludge retention time (30 d) was decreased, i.e., ammonium removal efficiency in R1∼R3 was 95.4 ± 6.1, 93.4 ± 8.1 and 89.3 ± 17.6%, respectively. Moreover, MBR tests showed that membrane fouling was aggravated in the presence of high-concentration BAC. Long-term exposure to BAC reduced microbial community diversity and enriched the BAC-resistant microbes. For instance, the abundance of Pseudomonas genus in R3 was increased from 0.02% to 14.9% with the increase of operation time. Microbial community structure was changed to resist the environmental stress induced by BAC during long-term exposure, thus decreasing the inhibition effects.