A clear understanding of membrane aging process is essential for the optimization of chemical cleaning in membrane-based facilities. In this study, two-dimensional (2D) Fourier transformation infrared (FTIR) correlation spectroscopy (CoS) analysis was first used to decipher the sequential order of functional group changes of NaOCl-aged poly(ether sulfone)/polyvinylpyrrolidone (PES/PVP) membranes. The synchronous maps showed 12 major autopeaks in total. Based on the asynchronous maps, a similar aging sequence of membrane groups was clearly identified at three pHs (i.e., 6, 8, and 10): 1463, 1440, and 1410 (cyclic C–H structures) > 1662 (amide groups) > 1700 (succinimide groups) > 1320, 1292 (S═O asymmetric) > 1486, 1580 (aromatic structures) > 1241 (aromatic ether bands) > 1105, 1150 cm–1 (O═S═O symmetric). Among them, membrane chlorination occurred at 1241, 1410, and 1440 cm–1. Moreover, the initial degradation of PVP and the subsequent transformation of PES could be highly responsible for the increased water permeability and the enlargement of membrane pores, respectively, both leading to serious fouling with humic acid filtration. In summary, the 2D-FTIR-CoS analysis is a powerful approach to reveal the interaction mechanisms of NaOCl-membrane and could be also useful to probe the process of membrane fouling and chemical cleaning.

•-DSlope325–375 is sensitive to changes in HS samples during chlorination.•A strong correlation exists between -DSlope325–375 and Fmax obtained from EEM-PARAFAC.•XPS reveals the alteration in the hydrophilicity of HS after chlorination.•Increasing the chlorine dosage reduces fouling rates and increases HS permeability.•-DSlope325–375 can evaluate the NaOCl cleaning efficacy from forward flushing.Sodium hypochlorite (NaOCl) can effectively react with membrane foulants and thus maintain membrane performance in membrane-based water/wastewater treatment facilities. However, the evaluation of the cleaning efficacy of NaOCl agents remains a major and insufficiently understood issue due to the lack of information on the interactions between NaOCl and foulants. To achieve a better understanding of the cleaning efficacy of NaOCl, we investigated the potential of UV–vis spectral absorbance data to explore the cleaning process of NaOCl for fouled membranes by humic substances (HS). During chlorination, the spectral parameter -DSlope325–375 (the slope of the log-transformed absorbance spectra in the range of 325–375 nm) increased with reaction time at different pH (i.e., 6, 7 and 9) and Cl2:DOC ratios (i.e., 6 and 12). More interestingly, the increasing –DSlope325–375 can correlate well with the increasing hydrophilicity of HS compounds during chlorination, as well as their decreasing molecular weight. In addition, comparisons of the fouling behavior/permeability of pristine and NaOCl-treated HS samples further suggested that the chlorination at pH 7 and with higher amount of Cl2 led to a lower UFMI and higher –DSlope325–375, which was beneficial for the maintenance of membrane permeability. Notably, -DSlope325–375 measured from the forward flushing filtrate decreased at various rates with different chlorine dosages, corroborating its reliability to assess cleaning efficacy. The results of this study demonstrate that the spectral parameter -DSlope325–375 not only conveniently evaluates the cleaning efficacy of NaOCl agents in different pH environments but also helps reveal the NaOCl cleaning mechanism.Download high-res image (72KB)Download full-size image

•Changes in properties and structures of bio-macromolecules exposed to chemicals were studied.•Consistency index, size and zeta potential of treated bio-macromolecules varied differently.•Oxidants/alkali treatments destructed proteins, increased CO groups, and decreased lipids.•Filterability/permeability of bio-macromolecules was improved after exposed to chemicals.•This work sheds lights on chemical cleaning mechanisms in membrane technologies.Chemical cleaning is an essential process for the permeability recovery of fouled membranes, which is highly related to the interactions between chemicals and bio-macromolecules in fouling layers. In this study, three bio-macromolecules (i.e., effluent biopolymers (i.e., 0.45 μm–100 kDa) from a full-scale municipal wastewater treatment plant, bovine serum albumin (BSA) and dextran) were exposed to different chemicals (i.e., NaClO, H2O2, NaOH, and HCl) with varied concentrations to understand the changes in their properties and functional groups. The results showed that exposure to oxidants and alkali decreased the consistency index of all bio-macromolecules. With an increased oxidant dose, the molecular sizes of effluent biopolymers and dextran continuously reduced because of the oxidative cleavage of the long molecule chains. However, the molecular size of BSA sharply increased after being treated with oxidants and alkali, likely due to the cross-linkage of protein molecules. Three-dimensional fluorescence excitation-emission matrix (3D-EEM) spectra showed that the aromatic protein-like and humic substances in the effluent biopolymers were destructed readily during the treatments of oxidants and alkali. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyse further confirmed that exposures to NaClO, H2O2 and NaOH led to the destruction of protein structures (i.e., amide I, II and III), the increase of carbonyl and carboxyl groups, and the decrease of fatty acids/lipids, all of which could make the bio-macromolecules more hydrophilic. Most importantly, the bio-macromolecules exposed to chemicals had better filterability, and their permeability through membranes also significantly increased, which could be explained well by the above analysis. The chemical cleaning mechanisms of fouled membranes are understood in depth in this study, and all of the results shed light on the implementation of on-line chemical enhanced backwashing in membrane processes.Download high-res image (366KB)Download full-size image

Effluent organic matter (EfOM) from municipal wastewater treatment plants potentially has a detrimental effect on both aquatic organisms and humans. This study evaluated the removal and transformation of chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM) in a full-scale wastewater treatment plant under different seasons. The results showed that bio-treatment was found to be more efficient in removing bulk DOM (in term of dissolved organic carbon, DOC) than CDOM and FDOM, which was contrary to the disinfection process. CDOM and FDOM were selectively removed at various stages during the treatment. Typically, the low molecular weight fractions of CDOM and protein-like FDOM were more efficiently removed during bio-treatment process, whereas the humic-like FDOM exhibited comparable decreases in both bio-treatment and disinfection processes. Overall, the performance of the WWTP was weak in terms of CDOM and FDOM removal, resulting in enrichment of CDOM and FDOM in effluent. Moreover, the total removal of the bulk DOM (P < 0.05) and the protein-like FDOM (P < 0.05) displayed a significant seasonal variation, with higher removal efficiencies in summer, whereas removal of CDOM and the humic-like FDOM showed little differences between summer and winter. In all, the results provide useful information for understanding the fate and transformation of DOM, illustrating that sub-fractions of DOM could be selectively removed depending on treatment processes and seasonality.Download high-res image (206KB)Download full-size image

Metaproteomic analyses, including two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) separation and matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF)/TOF mass spectrometer (MS) detection, were used to trace and identify biocake proteins on membranes in a bench-scale submerged membrane bioreactor (MBR). 2D-PAGE images showed that proteins in the biocake (S3) at a low transmembrane pressure (TMP) level (i.e., before the TMP jump) had larger gray intensities in the pH 5.5–7.0 region regardless of the membrane flux, similar to soluble microbial product (SMP) proteins. However, the biocake (S2 and S4) at a high TMP level (i.e., after the TMP jump) had many more proteins in the pH range of 4.0–5.5, similar to extracellular polymeric substance (EPS) proteins. Such similarities between biocake proteins and SMP or EPS proteins can be useful for tracing the sources of proteins resulting in membrane fouling. In total, 183 differentially abundant protein spots were marked in the three biocakes (S2, S3, and S4). However, only 32 protein spots co-occurred in the 2D gels of the three biocakes, indicating that membrane fluxes and TMP evolution levels had significant effects on the abundance of biocake proteins. On the basis of the MS and MS/MS data, 23 of 71 protein spots were successfully identified. Of the 23 proteins, outer membrane proteins (Omp) were a major contributor (60.87%). These Omps were mainly from potential surface colonizers such as Aeromonas, Enterobacter, Pseudomonas, and Thauera. Generally, the metaproteomic analysis is a useful alternative to trace the sources and compositions of biocake proteins on the levels of molecules and bacteria species that can provide new insight into membrane fouling.

•Monovalent ions could give rise to changes in properties of organic matters.•The presence of monovalent cation ions led to increased size of BSA.•The presence of monovalent anion ions decreased fouling potential of BSA.•Enhanced electrostatic repulsion between BSA and the membranes could take place.In this study, we explored the response of membrane filtration behavior of bovine serum albumin (BSA) to various monovalent salts (i.e., NaCl, KCl, CsCl, NaBr, NaI) at 15 mM and 150 mM ionic strengths. The results demonstrated that the charge of BSA tended to be less negative with an increase in ionic strength. The presence of monovalent ions also resulted in the increased size of BSA. However, little change was observed in the viscosity of BSA with the involvement of monovalent ions (with the exception of 150 mM CsCl). The changes in the characteristics of BSA were primarily caused by their interaction with cations, and the greater specific interaction between Cs+ and BSA led to a significant increase in the size and viscosity of BSA. In spite of the substantial dependence of the characteristics of BSA (i.e., size, zeta potential) on cations, the fouling behaviors of BSA were primarily determined by anion identity. Specifically, the order of the ability of anion identity to reduce the flux decline rate seemed to be I− > Br− ∼ or > Cl−, which seemed to be ascribed to the changes in electrostatic repulsion between BSA and the membranes caused by anions.

Biogenic particles discharged by wastewater treatment plants play important roles in receiving water because of the large specific surface area and good mobility of the particles. In this study, the changes induced in biogenic particles by natural sunlight were investigated to understand the phototransformation of the particles in the receiving waters. The results showed that photoexposure resulted in significant decreases in the sizes of the biogenic particles but that photoexposure did not impact the zeta potentials. In addition, the photodissolution of biogenic particulate organic matter (POM) led to the generation of biogenic dissolved organic matter (DOM). Characterization using excitation–emission matrix (EEM) spectroscopy showed that photoexposure changed both the shapes and the intensities of the EEM spectra of the biogenic POM; the regions of the T1 and T2 peaks were susceptible to photoexposure. Modeling by parallel factor analysis (PARAFAC) decomposed the EEMs of the biogenic POM into four valid components, i.e., terrestrial or microbial humic-like substances, tryptophan-like proteins, tyrosine-like proteins and hydrophobic proteins. The humic-like substances in the biogenic POM from the effluents were subject to lower decreases (17.0 % and 11.6 %). Throughout the entire incubation time, the PARAFAC components in the biogenic DOM were dominated by elimination mechanisms rather than by production through photodissolution of the biogenic POM.

Pharmaceuticals and personal care products (PPCPs) are ubiquitous in wastewater streams, but little is known regarding their roles in membrane fouling during the operation of membrane bioreactors (MBRs). This study attempted to understand the role of ciprofloxacin exposure in control of filamentous bulking as well as membrane fouling mitigation. Compared with the control MBR, results show that the ciprofloxacin could selectively inhibit and kill filaments present in the mixed liquor, and, as a consequence, the sludge settleability improved significantly. The addition of ciprofloxacin had little influence on organic removals and nitrification, but had an adverse effect on denitrification. In addition, the sludge flocs exposure to ciprofloxacin still kept in compact structure; specifically, granular sludge was finally formed possibly due to the increase of polysaccharides in extracellular polymeric substances (EPS). Characterization by nuclear magnetic resonance (NMR) corroborated the enrichment of polysaccharides in the EPS exposure to ciprofloxacin. On the other hand, because of the release of EPS and the decay of filaments, soluble microbial products (SMP), particularly soluble polysaccharides, increased significantly in the MBR exposure to ciprofloxacin. Despite this, the results of this study show that the presence of trace contaminants in wastewater streams may play some positive roles in MBR fouling control.

•Findings in MBR fouling and its control reported in recent years were reviewed.•Fouling mechanisms of biopolymers in MBRs have been comprehensively revealed.•Biologically induced dispersal of biofilm is effective in fouling control.•Electrically assisted fouling mitigation has attracted much attention in MBRs.•The stability and performance of nano-based membranes in MBRs should be tested.The goal of the current article is to update new findings in membrane fouling and emerging fouling mitigation strategies reported in recent years (post 2010) as a follow-up to our previous review published in Water Research (2009). According to a systematic review of the literature, membrane bioreactors (MBRs) are still actively investigated in the field of wastewater treatment. Notably, membrane fouling remains the most challenging issue in MBR operation and attracts considerable attention in MBR studies. In this review, we summarized the updated information on foulants composition and characteristics in MBRs, which greatly improves our understanding of fouling mechanisms. Furthermore, the emerging fouling control strategies (e.g., mechanically assisted aeration scouring, in-situ chemical cleaning, enzymatic and bacterial degradation of foulants, electrically assisted fouling mitigation, and nanomaterial-based membranes) are comprehensively reviewed. As a result, it is found that the fundamental understanding of dynamic changes in membrane foulants during a long-term operation is essential for the development and implementation of fouling control methods. Recently developed strategies for membrane fouling control denoted that the improvement of membrane performance is not our ultimate and only goal, less energy consumption and more green/sustainable fouling control ways are more promising to be developed and thus applied in the future. Overall, such a literature review not only demonstrates current challenges and research needs for scientists working in the area of MBR technologies, but also can provide more useful recommendations for industrial communities based on the related application cases.Download high-res image (447KB)Download full-size image