A new method for the evaluation of the reversible and irreversible fouling propensity of MBR mixed liquor

In this paper, a new fouling measurement method is presented as a pragmatic approach to determine a mixed liquor's fouling propensity. The MBR-VFM (VITO Fouling Measurement) uses a specific measurement protocol consisting of alternating filtration and physical cleaning steps, which enables the calculation of both the reversible and the irreversible fouling resistances. The MBR-VFM principle, set-up and measurement protocol are described as well as the evaluation of the fouling measurement method. Finally, the MBR-VFM was validated by comparing the fouling propensity measured on-line by the MBR-VFM in a lab-scale MBR with the fouling of the MBR membranes themselves. Our experiments indicated that the MBR-VFM can accurately measure fouling and that it can even be detected earlier than can be seen from the on-line filtration data of the lab-scale system itself. Furthermore, the differences measured in reversible and irreversible fouling seemed to be related to the observed impact of physical and chemical cleaning respectively. Therefore, the application of the MBR-VFM as an on-line sensor in an advanced control system, enabling the deployment of the measured fouling data for the control of membrane cleaning, seems feasible and will be tested in the near future.     

Synergetic cleaning procedure for a ceramic membrane fouled by beer microfiltration

Apart from considerations for hygienic operation, membrane cleaning is essential to maintain consistent permeability and selectivity of membrane systems for clarifying beer and beverages where balanced fractionation of particles/macromolecules is necessary. Experiments involved formulating and optimising chemical cleaning methods for a ceramic microfiltration membrane, which had been severely fouled during clarification of a commercial beer. The cleaning processes employed NaOH, HNO₃, H₂O₂, and Ultrasil 11 as the chemical cleaning agents. The cleaning ability and cleaning kinetics of the processes were evaluated in parallel with the study of the fouling mechanism, formation and strength so as to elucidate the synergetic relationship between fouling and cleaning. A three-step cleaning mechanism was postulated. This led to the development of a fast and effective combined simultaneous caustic cleaning and oxidation method (CSCCO), which was able to restore 87% of the original membrane's water permeability within 8 min. Analysis suggested the concept of a cleaning energy barrier E_c and a cleaning rate constant k_c0. This study confirmed the existence of a synergetic relationship between the prior fouling and optimum formulation of cleaner and optimal cleaning condition. The study varied beer filtration conditions. Transmembrane pressure (TMP) and crossflow velocity during fouling appeared to have a minimal effect on the membrane's subsequent cleanability, especially when the powerful CSCCO process was employed. The number of previous fouling/cleaning cycles was influential. A complete removal of the residual fouling, formed on the virgin membrane's surface proved beyond the means of the harsh chemical cleaning used under any conditions. The degree of residual fouling eventually reached a plateau and a level of 87% of the original water flux could be restored repeatedly.     

Modelling of submerged membrane bioreactor: Conceptual study about link between activated slugde biokinetics,aeration and fouling process

A mathematical model was developed to simulate filtration process and aeration influence on submerged membrane bioreactor (SMBR) in aerobic conditions. The biological kinetics and the dynamic effect of the sludge attachment and detachment from the membrane, in relation to the filtration and a strong intermittent aeration, were included in the model. The model was established considering soluble microbial products (SMP) formation-degradation. The fouling components responsible of pore clogging, sludge cake growth, and temporal sludge film coverage were considered during calculation of the total membrane fouling resistance. The influence of SMP, transmembrane pressure, and mixed liquor suspended solids on specific filtration resistance of the sludge cake was also included. With this model, the membrane fouling under different SMBR operational conditions can be simulated. The influence of a larger number of very important process variables on fouling development can be well quantified. The model was developed for evaluating the influence on fouling control of an intermittent aeration of bubbles synchronized or not with the filtration cycles, taking into account the effects of shear intensity on sludge cake removal.     

Effects of nanoparticles on the ultrafiltration of surface water

The effects of nanoparticles on the fouling behavior of UF membranes were investigated by filtering river water containing natural organic matter (NOM). Self-dispersible carbon black (70–200 nm) was employed to model nanoparticles in natural water. The presence of nanoparticles transformed the mode of initial fouling from internal pore adsorption of NOM to intermediate pore blocking, which caused a significant flux reduction. The use of powdered activated carbon to adsorb organic micromolecules reduced internal pore fouling, but this effect on initial fouling mode did not much mitigate the overall flux decline. As filtration proceeded, cake filtration became the dominant fouling mode. The resistance-in-series model revealed that boundary-layer resistance contributed significantly to increased filtration resistance in the filtration of river water. The nanoparticles nullified boundary-layer resistance plausibly by removing organic macromolecules from river water, but aggravated cake resistance, which required chemical cleaning. Addition of calcium significantly increased the aggregate size of nanoparticles from 0.18–0.35 μm to 3.4 μm, and thus reduced pore blocking and total cake resistance.     

Blocking of capillaries as fouling mechanism for dead-end ultrafiltration

In this paper plugging of capillaries in the potting is investigated. A lot of research has been done on fouling of the membrane surface (pore blocking, cake filtration) but research on other types of membrane fouling like plugging of capillaries is not so common. Experiments were performed with a lab-scale test installation under constant flux conditions with synthetic feed water containing ferric hydroxide flocks as a fouling component. The experiments showed that during operation capillaries became blocked by fouling plugs. The presence of blockages, especially in the potting at the concentrate side of the capillaries, could not be detected by measuring the clean water resistance. However such blockages did result in an increased forward flush pressure. A combination of the clean water resistance and the forward flush pressure is suitable for determining the fouling of a membrane and the effectiveness of a cleaning procedure. The part of the capillaries in the potting is not backwashed and therefore the hydraulic as well as the chemical cleaning is not efficient at this place.     

UF of pulp and paper effluent: membrane fouling-prevention and cleaning

Effluent arriving from the Mondi Kraft paper mill at Piet Retief, South Africa, was filtered through tubular poly(ether sulphone) (PES) ultrafiltration membranes under constant pressure cross-flow conditions. The effluent that was fed into the membranes and permeate produced during filtration were characterised by UV–VIS light-spectroscopy. Substances that absorbed onto membranes during filtration caused changes to the permeability characteristics of the membranes. Changes in membrane performance were monitored by pure-water and product flux (pf) measurements.A colourimetric staining technique was developed to determine the nature of foulants adsorbed onto the membranes. Membrane cleaning solutions were subsequently selected using information obtained from the characterisation studies. In addition, the anti-fouling potential of non-covalently attached coating materials was investigated as a possible membrane pretreatment technique. Results showed that foulants present in the effluent are of phenolic and hydrophobic nature. Increasing the hydrophilic characteristics of membranes prior to filtration could reduce the amount of organic foulants that adsorbed onto the membranes. Membrane pretreatment not only reduced fouling, but also improved the effectiveness of cleaning methods. Membranes were effectively cleaned by a combination of mechanical and chemical cleaning techniques.     

Irreversible membrane fouling in microfiltration membranes filtering coagulated surface water

Chemical coagulation has been widely used as a method to mitigate membrane fouling in MF/UF membranes used for drinking water treatment. Optimization of coagulation as pre-treatment of membrane processes has not been achieved yet: the optimum condition of coagulation for conventional treatment systems is not necessarily applicable to membrane-based treatment systems. This study investigated (physically) irreversible membrane fouling in an MF membrane used with pre-coagulation by aluminum salt. In a series of bench-scale filtration tests, feed water containing commercially available humic acid or organic matter isolated from surface water was coagulated with polyaluminum chloride (PACl) under various conditions and subsequently filtered with an MF membrane with the nominal pore size of 0.1 μm. It was found that coagulation conditions had great impacts on the degree of physically irreversible fouling. Acidic conditions improved the quality of treated water but generally caused greater physically irreversible fouling than did neutral or alkaline conditions. Also, dosage of coagulant was found to be influential on the degree of membrane fouling: high dosage of coagulant frequently caused more severe irreversible fouling. Sizes of flocs seemed to become small under acidic conditions in this study, which was indicated by high concentrations of aluminum in the permeate under acidic conditions. It is thought that small flocs produced under acidic conditions could migrate into micropores of the membrane and caused physically irreversible fouling by plugging or adsorption. These findings obtained in the bench-scale tests were verified in a long-term pilot-scale test.     

Enzymatic cleaning of ultrafiltration membranes fouled by protein mixture solutions

Compared to other typical cleaning agents, application of enzyme in cleaning of membranes fouled with protein solution promised the high cleaning efficiencies with lower environmental impact. This paper is focused on the mechanisms of protein removal by enzyme cleaning agent from the membrane surface by analysis hydraulic resistance, total protein removal using Lowry method, and membrane surface analysis using MALDI-MS and gel electrophoresis to estimate the foulant composition. Using single and binary protein solutions of bovine serum albumin and beta-lactoglobulin as the feed solution for filtration process, the experimental results indicate that optimum cleaning time and cleaning agent concentration is due to the competition between foulant removal and deposition of enzymes on the membrane during the cleaning process. The removal rate of different protein species in the fouling layer is varied, indicating that cleaning strategies can be tailor-made for fouling layer with different protein compositions.     

Periodic air/water cleaning for control of biofouling in spiral wound membrane elements

The main problem during the operation of nanofiltration or reverse osmosis membrane plants is fouling of feed spacers in membrane elements due to biofouling and particulate fouling. In order to control biofouling and particulate fouling in membrane elements, both daily air/water cleaning (AWC) and daily copper sulphate dosing (CSD) were investigated and compared to a reference without daily cleaning. A pilot study was carried out for 110 days with three parallel spiral wound membrane elements; AWC, CSD and the reference which were fed by tap water enriched with a biodegradable compound (100 μg acetate-C/L). The CSD element, which combined daily copper sulphate dosing and sporadically air/water cleaning, performed best with an increase in pressure drop of 18% and a biomass concentration of 8000 pg ATP/cm² within 110 days. This was followed by the AWC element with a pressure increase of 37% and biomass concentration of 20,000 pg ATP/cm² within 110 days. The reference element showed a pressure increase of 120% within 21 days. The presented approach is considered very successful in controlling particulate fouling and biofouling, especially when air/water cleaning is combined with copper sulphate dosing.     

Application of heat-activated peroxydisulfate process for the chemical cleaning of fouled ultrafiltration membranes

Na Cl O has been widely used to restore membrane flux in practical membrane cleaning processes, which would induce the formation of toxic halogenated byproducts. In this study, we proposed a novel heatactivated peroxydisulfate(heat/PDS) process to clean the membrane fouling derived from humic acid(HA). The results show that the combination of heat and PDS can achieve almost 100% recovery of permeate flux after soaking the HA-fouled membrane in 1 mmol/L PDS solution at 50 °C for 2 h, which is att...     

Controlled backwashing in a membrane sequencing batch reactor used for toxic wastewater treatment

A new control algorithm for performing filtration in a membrane sequencing batch reactor (MSBR) to prevent fouling is presented. Based on continuous measurements of the transmembrane pressure (TMP) and the permeate flux, the algorithm decides when to initiate backwashing. The algorithm was tested on a laboratory scale bioreactor treating synthetic wastewater containing 4-chlorophenol (4CP) as model toxic compound and filtration was carried out using a submerged tubular membrane module and a diaphragm pump. Several controller configurations were tested for different MSBR cycles. The results showed that the proposed algorithm was robust against the highly varying mixed liquor characteristics and was able to keep the TMP below critical values and maintain the flux at a maximum for most of the filtration time. Therefore, despite possible frequent backwashes, the total filtration time was minimized.     

Influence of Extracellular Polymeric Substances on Membrane Fouling and Cleaning in a Submerged Membrane Bioreactor

There is little information available on the correlation between the concentration of extracellular polymeric substances (EPS) and membrane fouling as well as cleaning efficiency. In this study, two lab-scale flat submerged membrane bioreactors (SMBRs) at sludge retention times (SRTs) of 25 and 250 days were operated at a constant permeate flux (12.5 l m⁻² h⁻¹). Samples of activated sludge were tested to quantify the concentration of extractable EPS using cation exchange resin. Batch filtration tests were also performed to determine the specific cake resistances and the flux recoveries. The extractable EPS and protein concentrations were relatively low at the prolonged SRT, leading to cake layers easily removable by the physical manual cleaning or the de-ionized water backwashing and the chemical cleaning with sodium hypochlorite methods. The extent of flux recoveries (both in SMBRs and batch filtration tests) and macroscopic as well as microscopic images indicated that the chemical cleaning could enhance the effectiveness of cleaning. The membrane fouling and cleaning mechanisms were also discussed.__________From Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 392–397.Original English Text Copyright © 2005 by Chackrit Nuengjamnong, Ji Hyang Kweon, Jinwoo Cho, Kyu-Hong Ahn, Chongrak Polprasert.This article was submitted by the authors in English.     

Chemical Cleaning of Microfiltration Membranes Fouled by Whey

Millipore hydrophobic polyvinylidene fluoride (PVDF) microfiltration membranes were used for whey processing. Fouled membranes were cleaned with acid (HCl), alkaline (NaOH) and surfactant (Triton‐X100). The latter resulted in maximum flux recovery and resistance removal. Hydrochloric acid had a moderate effect and sodium hydroxide was the weakest cleaning agent. This is due to the cleaning strength of emulsifiers compared to acid or alkali. However acids are more efficient than alkaline solutions for removal of mineral compounds which remain on the membrane surface. Cleaning efficiency depends on the concentration of cleaning agent being higher for higher surfactant concentration. For acids and alkali, the efficiency increases with increasing the concentration of the reagent reaches a maximum (optimum concentration) and then decreases. This can be explained by changes in permeability of the deposit layer with the concentration of the cleaning agent. Another explanation is the breakage of proteins by acid or alkali which produces more fouling materials and causes less cleaning efficiency. Operating conditions affect the cleaning process. At higher stirring speeds (turbulent flow) or longer cleaning time better removal of deposits and higher cleaning efficiency were observed. The sequential cleaning process may or may not improve the cleaning efficiency. When acidic cleaning was followed by washing with a surfactant an improvement was achieved. This can be attributed to the incomplete removal of deposits by acid. However further cleaning with acid can not improve the cleaning efficiency. During whey processing fouling occurs by deposition of foulants of mostly proteins and macromolecules on the membrane surface or in the membrane matrix. Large substances (compared to the membrane pores) settle on the membrane surface and the small species penetrate and are adsorbed in the membrane pores. Cleaning dissolves and removes the adsorbed foulants from the membrane.     

Long-term study of an intermittent air sparged MBR for synthetic wastewater treatment

Membrane bioreactors (MBR) combine biological processes with membrane filtration. Advantages of MBR in municipal wastewater treatment include high effluent quality and reduced space requirements. Steady operation of membrane plants requires careful management of membrane fouling. Even though it might be impossible to prevent, fouling can be limited by techniques such as gas sparging. The injection of gas bubbles increases the shear stress and removes fouling material from the membrane surface. Most cited literature on air sparging refers to short-term experiments, often times in bench scale. The aim of this study was therefore long-term investigations in pilot plant scale of a 70 L reactor fed with glucose-based synthetic wastewater. The main focus was on enhancing permeate flux by air sparging. The results showed that using air sparging significantly increased the permeate flux was doubled even over several weeks. The findings were interpreted using the dimensionless fouling and shear stress number. The fouling resistance was found to decrease significantly with air injection ratios between 0.4 and 0.5. When air sparging was applied after a period without air sparging, the shear stress number doubled. This increase in shear led to a reduction of the fouling number by approximately 30%. During several weeks air sparging only a slow fouling number increase was. In contrast to that after air sparging was ceased, an exponential increase of the fouling number was observed.     

Chemical cleaning of oil contaminated polyethylene hollow fiber microfiltration membranes

The primary aim of this paper was to develop a more effective and economical procedure for cleaning polyethylene hollow fiber microfiltration membranes that have been used for removing oil from contaminated seawater. Alkaline cleaning showed higher recovery of operating cycle time but lower permeate flux recovery than acid cleaning. The combination of both alkaline and acid cleaning agents gave the best operating cycle time and flux recoveries (e.g. 96% and 94%, respectively). As the cleaning agent soaking time was reduced, the actual operating cycle time was reduced. However, the ratio of operating time/chemical cleaning time increased as the soaking time was reduced. The soaking time was recommended to be as short as possible (8–10 h) in the design of small capacity plants and 30 h or higher in case of large capacity plants. SEM analysis showed that in case of alkaline cleaning, most of the pores remained covered with a foulant layer, resulting in low flux recovery. The SEM results of acid cleaned membranes showed more complete removal of the foulant layer from the pores resulting in better flux recovery. Surface analysis of membranes cleaned with combined acid/base agents showed the best results. A membrane surface similar to the original one was obtained. The long-term objective is to increase the understanding of membrane fouling phenomena, preventive means and membrane cleaning processes as it applies to the clean-up and desalination of oil contaminated seawater.     

Microfiltration of post-fermentation broth with backflushing membrane cleaning

Separation of microorganism cells from broth is a very important stage in the recovery of fermentation products. The microfiltration of fermented glycerol solutions was studied. During this process, the filter cake building up on the membrane surface caused an increase of filtration resistances, resulting in the decrease of the permeate flux. In this work, short time reverse flow of permeate was used to remove the fouling layer after each cycle of the filtration. The applied periodical membrane cleaning led to minimization of the observed fouling effects.     

Ultrasonic time-domain reflectometry as a non-destructive instrumental visualization technique to monitor inorganic fouling and cleaning on reverse osmosis membranes

Fouling is readily acknowledged as one of the most critical problems limiting the wider application of membranes in liquid separation processes. A better understanding of fouling layer formation and its monitoring is needed in order to improve on existing cleaning techniques. The overall objective of this research was to develop a non-destructive, real-time, in situ visualization technique or device for fouling layer monitoring. Ultrasonic time-domain reflectometry (UTDR) was employed as a visualization technique to provide real-time characterization of the fouling layer. The fouling experiment was carried out with 2 g/l calcium carbonate solution. Results confirmed that there is a correspondence between the flux decline behavior and the UTDR response from membranes in reverse osmosis (RO) modules. The ultrasonic technique could effectively detect fouling layer initiation and growth on the membrane in real-time at different axial velocities. In addition to the measurement of fouling, the ultrasonic technique was also successfully employed for monitoring membrane cleaning. The UTDR technique, due to its extremely powerful capabilities and its use in monitoring devices, can be of great significance in the membrane industry.     

An enzymatic approach to the cleaning of ultrafiltration membranes fouled in abattoir effluent

Membrane fouling severely curtails the economical and practical implementation for the purification of biologically related process streams such as abattoir effluent (Jacobs, WRC Report no. K5/362, 1991, Pretoria, South Africa [1]). Mechanical and chemical removal of foulants usually lead to membrane damage and additional pollution. Enzymes, specific for the degradation of proteins and lipids, were tested as key components of biological cleaning regimes for membranes fouled in abattoir effluent. Fouling of polysulphone membranes was assessed as previously described by Maartens et al. (J. Membrane Sci., 119 (1996) 1 [2]) and optimal enzyme concentrations and incubation times were determined for the different preparations. The ability of each cleaning agent to remove adsorbed protein and lipid material, as well as their ability to restore the water-contact angle and the pure-water flux of the fouled membranes, were determined and compared. These variables were also used to compare the cleaning efficiency of enzymatic cleaning agents with conventional chemical agents under optimal conditions. The enzymes and enzyme detergent mixtures were effective cleaning agents and the pure-water flux of statically fouled membranes could be restored by treatment with these agents.     

Effects of several different flux enhancing chemicals on filterability and fouling reduction of membrane bioreactor (MBR) mixed liquors

The main objective of this work was to determine the effectiveness of various chemicals on filterability and fouling reduction in MBR mixed liquors. Different lab-scale experiments were conducted with a total of 7 different additives (3 cationic polymers (MPL30, MPE50, KD452), a biopolymer (Chit), a starch (Sta), and 2 metal salts (FeCl₃, PACl)). Initially, batch shaker tests were performed for each additive to determine the optimum dosages in terms of soluble microbial products (SMP) removal. Then, short-term filtration trials and critical flux tests were performed. All tested additives were able to remove SMP, but at different extent; 33, 45, 51, 36, 38, 54, and 56% for MPL30, MPE50, KD452, FeCl₃, PACl, Chit, and Sta, respectively. The cationic polymer KD452 exhibited the best performance in terms of the extent of SMP removal and the required dosage. All tested cationic polymers, starch and chitosan significantly reduced fouling rates and increased permeability values. At their optimum dosages, the cationic polymers MPE50, MPL30 and KD452 provided 96, 80 and 74% reductions in fouling rates, respectively. The enhancements in critical flux achieved by MPL30, MPE50, KD452, FeCl₃, PACl, Chit, and Sta were 38, 46, 38, 14, 14, 0, and 22% in comparison with raw mixed liquor. Cationic polymers increased critical flux values to levels above 50 L m⁻² h⁻¹. SMP removal from MBR mixed liquors and further improvement in filtration performance and fouling control did not always correlate. Overall, based on the lab-scale tests conducted, cationic polymeric additives were found to be favorable over the other additives due to their steady and successful performance in fouling control. The performance of cationic polymers was independent of small variations in dosing, while for other additives over- or under-dosing showed detrimental effects on filterability.