MBR cleaning articles − ten research papers

Below is a selection of abstracts from ten papers published in peer-reviewed journals relating to MBR membrane cleaning, with the selection of papers provided by a range of research groups from the past 3−4 years.

The selection includes a review of mechanical cleaning by scouring agents, a study of ceramic membrane cleaning, and the use of an alternative oxidant. The latter relates to the concern over halogenated byproducts generated from conventional hypochlorite cleaning, which is the focus of another paper on this list.

There are some quite unusual approaches to cleaning featured in the list. There’s a study of a highly novel approach to cleaning, based on coupling electrochemical oxidation with a non-woven fabric (NWF) filter, another based on ultrasonic cleaning and one using nitrous oxide to mitigate biofilm formation. There’s also a paper focusing on iron fouling mitigation, and another comparing various physicochemical cleaning methods. Finally, the most recent paper listed concerns the use of neural networks to optimise the scheduling of cleaning. All in all, it’s a pretty eclectic mix.

Ten MBR membrane cleaning research abstracts (reverse chronological order)

'The membrane fouling is an important factor of restricting wide application of MBR (Membrane Bio-Reactor), which causes the fall of membrane flux and reduces the membrane cleaning period. So the Bandelet neural network is proposed through combining Bandelet transform and neural network, which predicts membrane flux and its recovery rate for making proper membrane cleaning decision. Firstly, the main affecting factors of membrane fouling are discussed. Secondly, the architecture of Bandelet neural network is designed with Bandelet function and its scale function as activation functions of hidden and output layers respectively. Thirdly, the improved Bat algorithm is established, which is applied to improve the optimization effect of parameters of Bandelet neural network.

'Finally, the simulation analysis is carried out, the improved bat algorithm has higher performance than the traditional bat algorithm through analyzing the single objective optimization problem from 2018 CEC competition, the optimal number of nodes in hidden layer is confirmed based on comparison analysis and statistical tests. The proposed BNN-IBA has obvious superiority in prediction accuracy and speed according to prediction simulation results of membrane fouling of MBR, which has better prediction results than other state-of-art prediction models optimized by the novel optimal algorithms. In addition, the proper membrane cleaning period and method are confirmed according to the prediction results of membrane flux and its recovery rate.' © 2020 Elsevier B.V.

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'Online chemical cleaning of membrane bioreactor (MBR) has been widely applied as a promising alternative to alleviate membrane fouling, while sustaining the permeability of MBR. However, the conventional chlorine-based chemical cleaning of MBR has been receiving increasing concerns on product water quality, environmental sustainability and human health due to the generation of harmful halogenated by-products. Therefore, in this study, peroxymonosulfate (PMS), a chlorine-free and thermodynamically strong oxidant, was explored as an environmentally friendly agent for online chemical cleaning of MBRs. Results show that the cleaning efficiency of fouled membrane by PMS was comparable with that by NaClO at the same dosages. Moreover, Fe2+-activated PMS could significantly improve the cleaning efficiency of the membrane, leading to a 75% reduction in chemical dosage against NaClO.

'It was also found that Fe2+-activated PMS was more effective than PMS in removing organic foulants from the cake layer and supressing cell survivability, ultimately leading to significantly reduced adhesion force of foulants to membrane surfaces. More importantly, as a chlorine-free chemical cleaning agent, Fe2+-activated PMS had a lower tendency of forming halogenated by-products during chemical cleaning. Consequently, this study indicates that Fe2+-activated PMS offers a promising alternative to commonly used NaClO for online chemical cleaning of MBR, not only for more effective cleaning at lower dosages but also the lack of toxic by-products.' © 2020 Elsevier B.V.

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'This study reports that electrochemical oxidation (EO) coupled with non-woven fabric (NWF) filter synergistically mitigated fouling in a submerged membrane bioreactor (MBR) under hydraulic retention time (HRT) of 1 h. Our NWF-EO-MBR was equipped with NWF prefilter cage surrounding microfiltration (MF) membrane with IrO2 anodes in vicinity. Continuous operation showed a retarded increase of transmembrane pressure at intermittent current density (5 mA cm−2) to prolong operation duration by 40% under constant flux (13.75 L m−2 h−1). Primary separation of bio-flocs by NWF not only reduced physically removable fouling on MF, but also allowed augmented concentration of electrolytic free chlorine (FC) near 4.0 mgCl2 L−1 inside the NWF cage. The FC degraded dissolved foulants to reduce physically irremovable hydraulic resistance of MF.

'A limited nitrification under the short HRT was assisted by electrochemical breakpoint chlorination, resulting 72% total nitrogen removal efficiency on average. Terminal restriction fragment length polymorphism fingerprinting indicated that the microbial community in mixed liquor was less influenced by FC than that in bio-cake on MF. The energy consumption of EO was approximated to 28 Wh per m3 of permeate.' © 2019 Elsevier B.V.

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'Membrane fouling is the major disadvantage of MBRs, which leads to decreased membrane performance and increased operating expenses. In this study, fouling was monitored in a pilot-scale submerged MBR system fed with municipal wastewater and operated under intermittently aerated conditions. Transmembrane pressure (TMP) was measured online on the membrane module during the whole operating period; permeability and resistance were estimated on a daily basis as well. The most common fouling prevention processes were systematically assessed and optimized.

'To control TMP increase owing to biosolids accumulation on membrane surface, successive backflushing cycles, backwash volume increase, air-cross flow velocity increase and in/ex situ mechanical cleaning were applied. Hydraulic cleaning resulted in TMP improvement and flux recovery of 40% and 32%, respectively. Ex situ and in situ mechanical cleaning led to TMP improvement of 25% and 39%, corresponding to flux recovery of 63% and 189%, respectively. Increased aeration intensity improved TMP and increased permeate flux by 63% and 56%, respectively. In the case of fouling that was caused by pore blocking and cake layer formation, chemical cleaning was implemented on the membrane module. Extensive chemical cleaning with NaOCl solution led to permeate flux increase of 90%, corresponding to TMP improvement of 44%.' © 2019 Desalination Publications. All rights reserved.

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'Reverse osmosis concentrates (ROC) produced from water reclamation plants can threaten the environment if it is not appropriately treated before discharge. A membrane bioreactor (MBR) process to treat ROC was used in this project. In an MBR, fouling is an essential and inevitable phenomenon which leads to higher operational and capital costs. A comparative study on chemical cleaning, such as sodium hypochlorite (NaOCl) and nitric oxide (NO), was experimentally evaluated together with the influence of filtration flux. Exposure to a low concentration of NO reduced biofilms in an MBR system. NO treatment delayed the formation of new biofilm biomass on the membrane. NO also showed good performance in reducing membrane fouling and had no adverse effect on activated sludge and the environment. In MBR, the bacterial community was dominated by Proteobacteria (61%), with Alpha and Beta-proteobacteria representing approximately 54% of the community. After NO treatment, the relative abundance of the Proteobacteria decreased to 44%, and this was also reflected in a reduction in Alpha and Beta-proteobacteria, to 30% and 5% respectively. Thus, NO treatment resulted in the decrease of the relative biofilms associated with reduced MBR performance.' © 2019 Elsevier B.V.

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'An in situ continuous off-line chemical cleaning system for membrane bioreactors was developed, and a related engineering test was carried out. The cleaning method achieved the same effect as traditional off-line chemical cleaning of the MBR (membrane bioreactor) but reduced the cleaning time by 60%. There was a linear relationship between sodium hypochlorite/citric acid and pH. The proposed sodium hypochlorite soaking time was 15–24 h, and the citric acid soaking time was 8–20 h. An intermittent aeration process was used to blow sludge particles off of the surface of membrane to improve the cleaning efficiency when the aeration intensity was 25 Nm3/m2·h. This system and method did not affect the effluent production and had the same effect as traditional off-line chemical cleaning methods while saving space and reducing capital. This system was successfully used in a full-scale municipal wastewater treatment plant and is an important complement to traditional chemical cleaning of MBRs.' © 2018 Elsevier Ltd

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'This study characterized the dissolved organic matter (DOM) and byproducts generated after the exposure of activated sludge to ozone and NaClO in ceramic MBR. It was found that NaClO triggered more significant release of DOM than ozone. Proteins with the molecular weight greater than 20 kDa and humic acid like-substances were the principal components of DOM generated by NaClO, while ozone was found to effectively degrade larger biopolymers to low molecular weight substances. The results showed that more than 80% of DOM generated by NaClO and ozone could pass through the 0.2-μm ceramic membrane. Furthermore, total organic chlorine (TOCl) was determined to be the principal species of halogenated byproducts in both cases, while the generation of TOCl by NaClO was much more significant than that by ozone. Only a small fraction of TOCl was removed by the 0.2-μm ceramic membrane. More importantly, the toxic bioassays further revealed that the supernatant of sludge suspension and permeate in the MBR with NaClO cleaning exhibited higher developmental toxicity to the polychaete embryos than those by ozone. The results clearly showed that on-line chemical cleaning with ozone should be a more eco-friendly and safer approach for sustaining long-term membrane permeability in ceramic MBR.' © 2018 Elsevier Ltd.

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'Four pathways for cleaning of iron oxide-fouled membranes, namely i) proton (acid)-assisted, ii) ligand-promoted (citric acid), iii) reductive (ascorbic acid), and iv) ligand-promoted reductive (ascorbic acid-citric acid-mediated) were examined and their effectiveness were compared in this study. The cleaning effectiveness under oxic conditions followed the order: proton-assisted < ligand-promoted < reductive < ligand-promoted reductive. Iron oxide dissolution rate initially increased with increase of citrate concentration in the ascorbic acid-citric acid solutions but declined at higher citrate concentrations indicating that an intermediate citrate concentration was required for optimal cleaning.

'The mechanism of ligand-promoted reductive dissolution mediated by ascorbate and citrate was investigated through studies of dissolution behaviour under both oxic and anoxic conditions with citrate shown to have a mitigating effect on the consumption of oxygen, apparently by reducing the iron catalyzed oxidation of ascorbate. Kinetic modelling showed that the dynamics of dissolution could be reasonably well simulated with the inclusion of a surface > Fe(III)-citrate-Fe(II) ternary complex which facilitates the detachment of surface Fe(II). Use of dual reagents (ascorbic acid and citric acid) under oxic conditions is recommended for the cleaning of iron-fouled membranes in view of the extreme cleaning effectiveness though it is critical that cleaning conditions be carefully optimised.' © 2017 Elsevier B.V.

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'Ultrasonic irradiation is one of the most promising membrane cleaning techniques for membrane bioreactors (MBRs) because of several advantages such as high flux-recovery capacity and in situ application without interrupting the filtration process. However, significant contradictions may be found and, consequently, this method has not yet been widely developed. In this paper, four MBRs equipped with hollow-fibre polyvinylidene fluoride ultrafiltration membranes were operated continuously. The cleaning method applied consisted of sonication at low power (15 W) with different frequencies (20, 25, 30, and 40 kHz) for each module and aerated backwashing.

'The different MBRs were analysed comparatively between them and with a conventional MBR in order to check the effects of the irradiated waves on membrane integrity, effluent quality and process performance. Effluent turbidity and chemical oxygen demand, total and volatile suspended solid concentration and activated sludge viscosity were affected by biomass fragmentation or membrane cake removal, mainly at lower frequencies. The best transmembrane pressure control was achieved at the frequency of 20 kHz without a significant effect on membrane integrity. The results showed that under these operational conditions, no negative effects on effluent quality or membrane integrity were found, suggesting that this method was suitable for this type of membrane.' © IWA Publishing 2017.

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'Membrane bioreactor (MBR) is a reliable and promising technology for wastewater treatment and reclamation applications. In spite of more than a decade of significant advances in developing fouling mitigation methods, different physical and cleaning protocols are still necessary to be developed to limit the membrane fouling. The use of scouring agents in MBR applications has been paid attention as a new approach as an energy-efficient way to control membrane fouling. Recently, mechanical cleaning by scouring agents is becoming an intense research area considering high efficiency of fouling reduction while requiring low energy consumption. In this review, fundamental and comprehensive assessments of the mechanical cleaning concepts and their applications with porous and nonporous scouring agents for MBR system are critically reviewed. The existing challenges and future research prospects on the mechanical cleaning technology associated with scouring agents for the MBR applications are also discussed.' © 2016 Elsevier B.V.

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I am thinking if your process can reduce a COD of 600 down to 200 and below?


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