Evaluation of fouling formation and evolution on hollow fibre membrane: effects of ageing and chemical exposure on biofoulant

Core carbon fixation pathways associated with cake layer development in an anoxic-oxic biofilm-membrane bioreactor treating textile wastewater

Microbial carbon fixation pathways have not yet been adequately understood for their role in membrane case layer formation processes. Carbon fixation bacteria can play critical roles in either causing or enhancing cake layer formation in some autotrophic-prone anoxic conditions, such as sulfur-cycling conditions. Understanding the microbes capable of carbon fixation can potentially guide the design of membrane biofouling mitigation strategies in scientific ways. Thus, we used meta-omics methods to query carbon fixation pathways in the cake layers of a full-scale anoxic-oxic biofilm-MBR system treating textile wastewater in this study. Based on the wastewater constituents and other properties, such as anoxic conditions, sulfide-reducing and sulfur-oxidizing bacteria could co-exist in the membrane unit. In addition, low-light radiation conditions could also happen to the membrane unit. However, we could not quantify the light intensity or total energy input accurately because the whole experimental setup was a full-scale system. Potentially complete carbon fixation pathways in the cake layer included the Calvin-Benson-Bassham cycle, Wood-Ljungdahl pathway, and the 3-hydroxypropionate bicycle. We discovered that using aeration could effectively inhibit carbon fixation, which resulted in mitigating membrane cake layer development. However, the aeration resulted in the 3-hydroxypropionate bicycle pathway, presumably used by aerobic sulfur-oxidizing prokaryotes, to become a more abundant carbon fixation pathway in the cake layer under aerobic conditions.

A combination of membrane relaxation and shear stress significantly improve the flux of gravity-driven membrane system

Gravity-driven membrane (GDM) filtration system is a promising process for decentralized drinking water treatment. During the operation, membrane relaxation and shear stress could be simply achieved by intermittent filtration and water disturbance (created by occasionally shaking membrane model or stirring water in membrane tank), respectively. To better understand the impact of membrane relaxation and shear stress on the biofouling layer and stable flux in GDM system, action of daily 60-min intermission, daily flushing (cross-flow velocity = 10 cm s^-1, 1 min), and the combination of the two (flushed right after the 60-min intermission) were compared. The results showed that membrane relaxation and shear stress lonely was ineffective in improving the stable flux, while their combination enhanced the stable flux by 70%. A more open and spatially heterogeneous biofouling layer with a low extracellular polymeric substance (EPS) content and a high microbial activity was formed under the combination of membrane relaxation and shear stress. In-situ optical coherence tomography (OCT) observation revealed that, during intermission, the absence of pushing force by water flow induced a reversible expansion of biofouling layer, and the biofouling layer restored to its initial state soon after resuming filtration. Shear stress caused abrasion and erosion on the biofouling surface, but it exerted little effect on the interior of biofouling layer. Under the combination, however, both the surface and interior of biofouling layer were disturbed because of 1) the water vortexes caused by rough biofouling layer surface, and 2) the porous structure after 60-min intermission. This disturbance, in turn, helped the biofouling layer maintain its roughness and porosity, thereby improving the stable flux of GDM system.

Quantitative analysis of cake characteristics based on SEM imaging during coagulation-ultrafiltration process

Cake formed by flocs is a crucial factor to affect membrane fouling during coagulation-ultrafiltration process. To investigate the role of floc properties on cake, cake characteristics under various coagulant dosage conditions were calculated by scanning electron microscope (SEM) imaging. Results found that one SEM image with × 5000 magnification could accurately estimate cake porosity with relative error lower than 5.00% for all conditions, whereas more SEM images with × 10,000 magnification or × 20,000 magnification should be applied to calculate cake porosity precisely. This could be explained by different pore information of SEM images with various magnifications. Compared to single SEM image with × 10,000 magnification and × 20,000 magnification, single SEM image with × 5000 magnification contained the most comprehensive pore information and slightly overestimated pore area for pore smaller than 0.4 μm² due to lower resolution. To verify feasibility by SEM image evaluating cake characteristics, cake porosity calculated by SEM image and Carman-Kozeny equation were analyzed. The results showed that cake porosity estimated by these two methods were nearly the same, proving the feasibility of this method. Moreover, with the increase of coagulant dosage, cake porosity presented similar variation with floc average size, indicating that floc average size was likely to dominate cake porosity in this study. For pore characteristics, pore average characteristic length and pore average area were in accordance with floc fractal dimension, whereas pore fractal dimension and pore amount were consistent with floc average size. This gives specific information about the relation between floc properties and cake characteristics.

Solid-phase fluorescence excitation emission matrix for in-situ monitoring of membrane fouling during microfiltration using a polyvinylidene fluoride hollow fiber membrane

Controlling membrane fouling is challenging and information regarding the causes of fouling is critical for this. While liquid-phase fluorescence spectroscopy excitation emission matrix (LPF-EEM) has previously been applied to identify the characteristics of membrane foulants, we applied EEM measurements to solid samples to identify foulants accumulated on the membrane. This solid-phase fluorescence EEM (SPF-EEM) enables sensitive and nondestructive identification of different organic solids. LPF-EEMs and SPF-EEMs were used on natural organic matter (NOM) isolated from secondary-treated wastewater, which revealed differences in peak positions and in spectral shapes. SPF-EEMs and LPF-EEMs of hydrophobic (HPO), transphilic (TPI) and hydrophilic (HPI) fractions showed that peaks of HPO fraction disappeared while those of TPI and HPI fractions shifted to a longer excitation and emission position through solidification. Then, the surface of the membrane fiber was continuously monitored using SPF-EEM during filtration. Three peaks appeared as expected during membrane fouling progression, indicating that in-situ monitoring of foulants was successful. Comparison of the EEM peaks between foulants and isolated NOM fractions shows the presence of both liquid-phase proteinaceous substances and gels formed from HPI and TPI fractions. Changes in peak intensity confirmed that the former proteinaceous substances were responsible for both reversible and irreversible fouling, while the latter gels mainly contributed to the irreversible fouling. We demonstrated the functionality of the SPF-EEM as an in-situ fouling monitoring tool.

Anaerobic treatment of N,N-dimethylformamide-containing high-strength wastewater by submerged anaerobic membrane bioreactor with a co-cultured inoculum

The anaerobic treatment of wastewater containing approximately 2000 mg L^-1N,N-dimethylformamide (DMF) was conducted by a lab-scale submerged anaerobic membrane bioreactor (SAnMBR). The inoculum consisted of aerobic DMF-hydrolyzing activated sludge (DAS) and anaerobic digested sludge (ADS). A rapid start-up was achieved with thorough DMF methanogenic degradation on the first day. The results of a 250-day long-term experiment demonstrated that under a low organic loading rate (OLR) of 3.14-4.16 g COD L^-1 d^-1, SAnMBR maintained excellent DMF removal efficiency along with high methane conversion. However, the elevation of OLR significantly limited DMF hydrolysis. When OLR exceeded 6.54 g COD L^-1 d^-1, both removal efficiency and methane production dramatically dropped. The DMF-hydrolyzing bacteria originating from the DAS gradually decayed under the anaerobic condition, resulting in the weak hydrolysis of DMF. The shortening of hydraulic retention time (HRT) is not recommended for the SAnMBR because severe membrane fouling occurred when HRT was shortened to 8 h. To handle high OLRs, an appropriate solution is to maintain a low F/M ratio by increasing both the influent DMF concentration and sludge concentration. The high CH₄ content in the biogas, exceeding 85%, was shown to be the reason for the suitability of anaerobic treatment to DMF. Some improvements which would help to maintain the effective hydrolysis are proposed: a side-stream system to replenish DAS to the SAnMBR is helpful; slight dosage of nitrate could also help to enrich the DMF-hydrolyzing bacteria; and the co-digestion of DMF and other organics might be convenient to establish a stable DMF-degrading consortium.

A Brief Review on the Resistance-in-Series Model in Membrane Bioreactors (MBRs)

The cake layer deposited on the membrane modules of membrane bioreactors (MBRs), especially under a submerged configuration, represents a relevant and fundamental mechanism deeply influencing the development of membrane fouling. It negatively affects the total resistance to filtration, while exerting a positive effect as a "pre-filter" promoting the "dynamic membrane" that protects the physical membrane from internal fouling. These two opposite phenomena should be properly managed, where the submerged membranes are usually subjected to a periodical cake layer removal through ordinary (permeate backwashing and air scouring) and/or irregular cleaning actions (manual physical cleaning). In this context, the physical removal of the cake layer is needed to maintain the design filtration characteristics. Nevertheless, the proper evaluation of the effect of physical cleaning operations is still contradictory and under discussion, referring in particular to the correct evaluation of fouling mechanisms. The aim of the present work was to summarize the different aspects that influence the fouling investigations, based on simple models for the evaluation of the resistance to filtration due to the cake layer, through physical cleaning operations.

Characterization of the Evolution of Crystallization Fouling in Membranes

Liquid-to-air membrane energy exchangers (LAMEEs) are promising in heating, ventilating, and air-conditioning applications because they are able to use semipermeable membranes to transfer heat and moisture between air and liquid desiccant streams. However, the development of crystallization fouling in membranes may pose a great risk to the long-term performance of LAMEEs. The main aim of this paper is to characterize the evolution of crystallization fouling in membranes through the use of both noninvasive and invasive methods. Noninvasive methods are used to study the development of fouling in the LAMEE by monitoring the changes in moisture flux through the membrane and overall moisture-transfer resistance of the LAMEE. On the other hand, invasive methods are implemented to characterize fouled membranes by using optical microscopy and scanning electron microscopy (SEM) to depict the morphology of crystal deposits and energy-dispersive X-ray spectroscopy (EDX) to identify the composition of the deposits. Experiments are performed by using air to dehydrate MgCl₂(aq) at two operating conditions of low and high fouling rates. The results show that the moisture flux decreases and the moisture-transfer resistance increases more considerably during the test at the high fouling rate than in the test at the low fouling rate. SEM micrographs show that cake crystal deposits cover the membrane surface in the test at the high fouling rate, whereas only few crystal particles are observed on the membrane in the test at the low fouling rate. Furthermore, the crystal deposits undergo more structural changes in the tests at the high fouling rate than in the tests at the low fouling rate, possibly because of the higher moisture transfer rate through the membrane in the tests at the high fouling rate. Finally, the SEM-EDX analysis confirms that the crystal deposits primarily consist of Mg, Cl, and O elements.

Assessing membrane biofouling and its gel layer of anoxic/oxic membrane bioreactor for megacity municipal wastewater treatment during plum rain season in Yangtze River Delta, China

This study assessed membrane biofouling and its gel layer of anoxic/oxic membrane bioreactor (A/O-MBR) for megacity municipal wastewater treatment during plum rain season, which was continuous rainy weather, in Yangtze River Delta, China. A laboratory-scale A/O-MBR was operated to treat the municipal wastewater from Quyang wastewater treatment plant, which located at the typical megacity of Shanghai in Yangtze River Delta, from April to July accompanying with plum rain season. As reactor performance showed, COD_Cr, NH₄⁺-N, TN, TP of the influent gradually decreased during plum rain season, and inhibited pollutant removal due to organic carbon shortage. However, dissolve inorganic carbon and inorganic components in mixed liquid had an obvious increase under rainy weather. Membrane filtration results indicated that plum rain season enhanced pore blocking behavior, further leading to the serious membrane biofouling but inhibiting gel layer formation. Additionally, gel layer analysis predicted that plum rain season led to plenty of inorganic components and precipitate flew into A/O-MBR reactor. Inorganic components with elements of Ca, Mg Ba, Fe, Al and Si seriously blocked membrane pores. Those components also accumulated into gel layer in the form of SiO₂, CaCO₃, CaSiO₃, MgNH₄PO₄, BaCO₃, AlPO_4, etc. Consequently, plum rain season enhanced pore blocking behavior and led to severe membrane biofouling but with the inhibition of gel layer formation.

Behaviour of biopolymeric substances in the activated sludge of an MBR system working with high hydraulic retention time

This study was undertaken to analyse the activated sludge of a membrane bioreactor (MBR), the behaviour of extracellular polymeric substances (EPS) and soluble microbial products (SMP) as well as their biopolymers composition, in the activated sludge of a membrane bioreactor (MBR) and their influence on membrane fouling were analysed. For the experiment an experimental fullscale MBR working with real urban wastewater at high hydraulic retention time with a variable sludge-retention time (SRT) was used. The MBR system worked in denitrification/nitrification conformation at a constant flow rate (Q = 0.45 m³/h) with a recirculation flow rate of 4Q. The concentrations of SMP in the activated sludge were lower than the concentrations of EPS over the entire study, with humic substances being the main components of the two biopolymers. SMP and, more specifically, SMP carbohydrates, were the most influential biopolymers in membrane fouling, while for EPS and their components, no relation was found with fouling. The SRT and temperature were the operational variables that most influenced the SMP and EPS concentration, causing the increase of SRT and temperature a lower concentration in both biopolymers, although the effect was not the same for all the components, particularly for the EPS carbohydrates, which increased with longer SRTs. Both operational variables were also the ones most influential on the concentration of organic matter of the effluent, due to their effect on the SMP. The volatile suspended solid/total suspended solid (VSS/TSS) ratio in the activated sludge can be applied as a good indicator of the risk of membrane fouling by biopolymers in MBR systems.

Electro-ultrafiltration to remove sodium dodecyl sulfate in proteins extracted for proteomics

A creative electro-ultrafiltration method was developed to remove sodium dodecyl sulfate (SDS) from proteins extracted for proteomics analysis.

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.

Durable antifouling polyvinylidene fluoride membrane via surface zwitterionicalization mediated by an amphiphilic copolymer

The antifouling properties of PVDF membrane were remarkably enhanced by facile incorporation of an amphiphilic triblock copolymer PDMAEMA-b-PDMS-b-PDMAEMA and subsequent surface zwitterionicalization.