The cake layer formation in the early stage of filtration in MBR: Mechanism and model

Microscale fluid and particle dynamics in filtration processes in water treatment: A review

The complex filtration processes in water treatment, granular filtration and membrane filtration, often suffer from filter fouling, and the fundamental understanding of microscale fluid and particle dynamics is a key to improving filtration efficiency and stability. In this review, we identify and review several key topics in filtration processes: drag force, fluid velocity profile, intrinsic permeability and hydraulic tortuosity in microscale fluid dynamics, and particle straining, absorption, and accumulation in microscale particle dynamics. The paper also reviews several key experimental and computational techniques for investigating filtration processes at microscale considering their applicability and capability. Then, major findings in previous studies on these key topics are comprehensively reviewed in terms of microscale fluid and particle dynamics. Last, future research is discussed in terms of techniques, scopes and links. The review provides a comprehensive overview of microscale fluid and particle dynamics in filtration processes for water treatment and particle technology communities.

Novel strategy for membrane biofouling control in MBR with nano-MnO2 modified PVDF membrane by in-situ ozonation

In this study, ozonation catalyst nano-MnO₂ blended polyvinylidene fluoride (PVDF) membrane was fabricated via phase inversion method and applied to membrane bioreactors (MBR), and then coupled with in-situ ozonation to study the anti-biofouling performance and reveal its mechanism. Results showed that, compared with pristine PVDF membrane (MBR_M₀), 0.75 wt% and 1.00 wt% nano-MnO₂ modified PVDF membrane (MBR_M_0.75 and MBR_M_1.00) could mitigate the membrane biofouling rate. Meanwhile MBR_M_1.00 coupled with in-situ ozonation could increase the membrane cleaning cycle to 1.5 and 2.7 times, compared with MBR_M₀ and MBR_M_0.75 without in-situ ozonation. The possible mechanisms included that the nano-MnO₂ modification coupled with in-situ ozonation directly removed the biofouling on the membrane surface, improved the hydrophilicity of the membrane surface and enhanced the chemical oxidation and biodegradation of membrane biofouling contaminants in the sludge mixture. The results of this work provide a new strategy for the control of membrane biofouling in MBR to treat industrial wastewater.

Self-generated electric field to suppress sludge production and fouling development in a membrane bioreactor for wastewater treatment

Compared with conventional sludge reduction techniques, electric field assisted membrane bioreactor (MBR) is a cost-effective technology with low power consumption. In this study, spontaneous electric field without extra power supply was introduced into the MBR for wastewater treatment to complete the in situ sludge reduction and membrane fouling mitigation. A novel spontaneous electric field membrane bioreactor (SEF-MBR) equipped with Cu-nanowires (Cu-NWs) conductive microfiltration membrane as cathode was established by using baffles to form anaerobic and aerobic tanks. SEF-MBR 1 with external resistance of 500 Ω maintained a highest electric field intensity of 1.25 mV/cm. Compared with Control-MBR, the reduction of mixed liquor suspended solids (MLSS) growth rate, extracellular polymeric substances (EPS) growth rate, total cell number and water content of SEF-MBR 1 reached 50.0%, 43.0%, 37.1% and 6.4%, respectively. After 43 days operation, SEF-MBR 1 obtained the minimum MLSS concentration and sludge volume, which were 29.9% and 83.8% lower than that of it in Control-MBR. The total biovolume of the contaminants (i.e., EPS and cells) on the membrane surface of SEF-MBR 1 was 68.8% lower than that of Control-MBR. SEF-MBR 1 exhibited a better performance with a lower membrane fouling rate (0.58 kPa/d) than Control-MBR (1.09 kPa/d). Economic analysis showed that a total of 148.1 kWh/m³ of electric energy was saved in the SEF-MBR 1. This technology reduced the sludge production in the sewage biological treatment process, which realized the sludge reduction had a positive impact on the membrane fouling mitigation.

The growth process of the cake layer and membrane fouling alleviation mechanism in a MBR assisted with the self-generated electric field

The electric field assisted membrane bioreactor (MBR) is an effective technique to alleviate membrane fouling. In this study, the spontaneous electric field was introduced into the MBR to observe the growth process of cake layer on the membrane surface. The external resistance for spontaneous electric field MBR (S-50) and S-500 were 50 Ω and 500 Ω respectively. During the experiments, S-50 maintained the highest electric field intensity of 11.83 mV/cm. The reduction of extracellular polymeric substances (EPS) content in activated sludge, transmembrane pressure (TMP) growth rate reached 52.8% and 51.7% respectively. After 28 days operation, S-50 obtained the minimum contaminant specific biovolume (23.316 μm³/μm²), which was 68.2% lower than that of it in Control-MBR. The metal oxide or metal hydroxide were distributed in the cake layer. EPS played a significant role in the formation and growth of the cake layer. Based on the results obtained in this study, the growth of the biofouling layer on the membrane surface could be divided into three stages. EPS first deposited on the membrane surface, and then microorganisms embedded in the cake layer to form clusters. After that, EPS and total cells further increased and led to a faster biovolume growth rate. Subsequently, the biovolume growth rate decreased in the cake layer. The spontaneous electric field delayed the deposition of EPS on the membrane surface. The produced H₂O₂ and •OH were beneficial to the degradation of organics, causing the smaller contaminant biovolume on the membrane surface. This work aims to provide a theoretical basis for the practical application of the electric field to control membrane fouling.

Characterization of the Initial Fouling Layer on the Membrane Surface in a Membrane Bioreactor: Effects of Permeation Drag

In this study, the properties of the initial fouling layer on the membrane surface of a bioreactor were investigated under different operating modes (with or without permeate flux) to improve the understanding of the effect of permeation drag on the formation of the initial fouling layer. It was found that protein was the major component in the two types of initial fouling layers, and that the permeation drag enhanced the tryptophan protein-like substances. The attraction of the initial foulants to the polyvinylidene fluoride (PVDF) membrane was ascribed to the high zeta potential and electron donor component (γ-) of the membrane. Thermodynamic analyses showed that the permeation drag-induced fouling layer possessed high hydrophobicity and low γ-. Due to permeation drag, a portion of the foulants overcame an energy barrier before they contacted the membrane surface, which itself possessed a higher fouling propensity. A declining trend of the cohesive strength among the foulants was found with the increasing development of both fouling layers.

2D kaolin ultrafiltration membrane with ultrahigh flux for water purification

Membrane separation technology is an important option for the treatment of contaminated surface waters but the relatively high cost of materials and membrane fabrication represent a significant obstacle to the wider use of membrane processes. In this study, we describe the development and testing of a new kind of membrane made from two-dimensional (2D) kaolin nanosheets. The fabrication involved a layer-by-layer stacking of the nanosheets with a cationic polyacrylamide cross-linking agent, assembled on a cellulose acetate supporting layer. The kaolin membrane exhibited an ultrahigh flux (∼4000 L.m-².h^-1.bar^-1) which was almost ten times greater than that of a commercial polyether sulfone (PES) ultrafiltration (UF) membrane. The membrane was tested using a range of influent water types, including samples of a lake water, river water and three natural organic matter solutions. The results showed that the kaolin membrane was stable and behaved as an UF membrane, in terms of its pore size distribution (peak distribution at 20-35 nm) and comparable treatment performance to the PES UF membrane. The kaolin membrane showed a substantially reduced rate of fouling, compared to PES membrane, despite a much greater flux, which was partly attributed to its highly hydrophilic nature. The advantages of lower cost, much higher flux and lower fouling propensity make the 2D-kaolin membrane a potentially important development in UF membrane technology for drinking water treatment, and possibly other applications.

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.

Pilot study on the effects of operating parameters on membrane fouling during ultrafiltration of alkali/surfactant/polymer flooding wastewater: optimization and modeling

Alkali/surfactant/polymer (ASP) flooding wastewater is commonly produced in enhanced oil extraction processes and needs to be properly treated prior to reuse due to the potential threat of formation damage. Ultrafiltration (UF) is an effective technique for treating ASP flooding wastewater to meet the requirements for reinjection water. Membrane fouling is the major challenge to UF application. In this study, the operating parameters were modified to research their effects on membrane fouling in a UF pilot study in Daqing, China. The effects of trans-membrane pressure (TMP), cross-flow velocity (CFV), concentration factor (CF) and temperature on membrane flux were systematically investigated, and optimal operating conditions were established by an orthogonal experiment. A temperature of 22 °C, TMP of 2.12 bar, CFV of 3.00 m s⁻¹ and CF of 5 were the most feasible operating conditions for the membrane types and raw water quality parameters in the study. The quality of the permeate met the water quality standards for injection to oilfield low-permeability layers. The results could provide a reference and guidance for practical operations. To learn more about the influences of the operating parameters, a model including external and internal pollution factors was developed based on the Hagen–Poiseuille equation and classical membrane fouling theory. The operating parameters had a more significant effect on external pollution than on internal pollution. The fouling on the membrane surface was much affected by TMP and CFV.

Ultrafiltration (UF) is an effective technique for treating ASP flooding wastewater to meet the requirements for reinjection water.