Membrane Fouling Characteristics Research Articles

The effects of soluble organic matters on membrane fouling index

This study investigated the effects of soluble organic matters on membrane fouling characteristics, using silt density index (SDI) and modified fouling index (MFI) to evaluate the fouling potential. Experimental results demonstrated that humic acid had significant effects on membrane fouling indexes. When its concentration was in the range of 0.01–0.05 mg/L, the SDI 15 and MFI were 2.9–3.9 and 5.4–13.8 s/L 2, respectively. According to the linear equations of MFI measurements, the fouling potential was in the order of humic acid > nucleic acid ≒ protein > glucose. Moreover, the molecular weight of dextran played an important role in membrane fouling indexes. Furthermore, a mathematical analysis of filtration experiments based on saturation curve was developed in this study. The maximum accumulated filtrate ( V max ) and the constant of filtration ( k f ) could be obtained to improve the precision of membrane fouling prediction.

MEMBRANE FOULING IN CANE-SUGAR SYRUP FILTRATION

Crossflow microfiltration (MF) and ultrafiltration (UF) experiments of limed mixed juice to pH of 7.5 were carried out in plate and frame module employing polysulphone membranes. The experimental study was carried out to examine the effects of membrane molecular weight (MWCO) and the transmembrane pressure (TMP) on the membrane fouling characteristics and flux decline behavior. A comparative analysis using one MF membrane (GRM0.1PP MF) and one UF membrane (GR40PP UF) was done at different TMP (viz. 1.0, 1.5 and 2.0 bar). Three theoretical models (viz. cake filtration model, internal pore blocking model, and complete pore blocking model) were used to predict the membrane resistances and permeate flux. The models were used further to help in the analysis and to isolate a suitable membrane among the studied one with respect to flux and resistance. It was noticed that in all cases the membrane fouling was the highest at the first period of operation. Study of the mechanisms responsible for flux decline in MF and UF of limed mixed juice suggested that the membrane fouling is in good correlation with the cake filtration model and the pore narrowing (internal pore blocking) model for both the GRM0.1PP MF and GR40PP UF.

Characterising humic acid fouling of nanofiltration membranes using bisphenol A as a molecular indicator

Fouling of nanofiltration (NF) membranes by humic acids was investigated using bisphenol A (BPA) as an indicator chemical to differentiate between various mechanisms that may lead to a change in solute rejection. Three commercially available NF membranes were investigated and an accelerated fouling condition was achieved with a foulant mixture containing humic acids in an electrolyte matrix. The effects of membrane fouling on the rejection of BPA were interpreted with respect to the membrane pore sizes and the fouling characteristics. Results reported here indicate that calcium concentration in the feed solution could be a major factor governing the humic acid fouling process. Moreover, a critical concentration of calcium in the feed solution was observed, at which membrane fouling was most severe. Membrane fouling characteristics were observed by their influence on BPA rejection. Such influence could result in either an increase or decrease in rejection of BPA by the three different membranes depending on the rejection mechanisms involved. It is hypothesized that these mechanisms could occur simultaneously and that the effects of each might not be easily distinguished. However, it was observed that their relative contribution was largely dependent upon membrane pore size. Pore blocking, which resulted in a considerable improvement in rejection, was prominent for the more open pore size TFC-SR2 membrane. In contrast, the cake-enhanced concentration polarisation effect was more severe for the tighter NF-270 and NF-90 membranes. For hydrophobic solutes such as BPA, the formation of the fouling layer could also interfere with the solute–membrane interaction, and therefore, exert considerable influence on the separation process.

Behavior of extracellular polymers and bio-fouling during hydrogen fermentation with a membrane bioreactor

The characteristics of membrane fouling were investigated by examining the behaviors of extracellular polymer substances (EPSs) produced by hydrogen-producing bacteria during hydrogen fermentation from a submerged membrane bioreactor (MBR). The MBR consisted of a 1.4-L submerged membrane filtration tank and 3-L hydrogen fermenter. An intermittent suction operation was selected to maintain stable filtration performance. The operation of the suction pump was alternately shifted to ON for 7 min followed by OFF for 3 min, with bio-gas sparging at a flow rate of 5.0 L/m 2/h (LMH), and manually regulated. Most of the EPS during the continuous hydrogen fermentation using an MBR had accumulated in the reactor because they were retained by the membrane by adsorption onto the polymeric membrane surface. The amount of proteins in the EPS extracted was increased to 179 mg/L and that of carbohydrates was increased to 58 mg/L. Cu 2+, Mg 2+, Zn 2+ in the EPS were increased in the range of 1.6–3.3 mg/L. The high concentration of EPS that is produced has a higher chelation potential in the formation of ligand complexes with metals or cations than that in a conventional continuous stirred tank reactor (CSTR). The EPS directly affected the decrease in the permeate flux, which resulted in the clogging of the membrane.

Characteristics of membrane fouling in submerged membrane bioreactor under sub-critical flux operation

Recently, the membrane bioreactor (MBR) process has become one of the novel technologies to enhance the performance of biological treatment of wastewater. Membrane bioreactor process uses the membrane unit to replace a sediment tank, and this can greatly enhance treatment performance. However, membrane fouling in MBR restricts its widespread application because it leads to permeate flux decline, making more frequent membrane cleaning and replacement necessary, which then increases operating and maintenance costs. This study investigated the sludge characteristics in membrane fouling under sub-critical flux operation and also assessed the effect of shear stress on membrane fouling. Membrane fouling was slow under sub-critical flux operation. However, as filamentous microbes became dominant in the reactor, membrane fouling increased dramatically due to the increased viscosity and polysaccharides. A close link was found between membrane fouling and the amount of polysaccharides in soluble EPS. The predominant resistance was the cake resistance which could be minimized by increasing the shear stress. However, the resistance of colloids and solutes was not apparently reduced by increasing shear stress. Therefore, smaller particles such as macromolecules (e.g. polysaccharides) may play an important role in membrane fouling under sub-critical flux operation.

Effect of coagulant addition on membrane fouling and nutrient removal in a submerged membrane bioreactor

This study was conducted to investigate the characteristics of membrane fouling and removal of phosphorous in a membrane bioreactor process by addition of inorganic coagulants directly into aerobic tank of a pre-anoxic nutrient removal system. In order to determine the specific cake resistance of activated sludge floc for different coagulants used, jar test and dead-end filtration experiments were performed. The permeate flow rate increased as well as phosphate removed effectively, as the dosage of alum increased, both in the jar test and dead-end filtration experiments. Considering the phosphorous removal capacity, flux variations, pH range and specific resistance, it seems that the use of alum was effective in concentration range from 200 to 500 mg/L. Ferric chloride was efficient in causing decline in specific resistance, on the other hand, it caused decrease in pH than that of alum. The bench scale MBR performance test was conducted to evaluate the effect of coagulant addition into a MBR process. It was found that addition of alum in the aeration tank had a positive effect on phosphorus removal along with minimization of membrane fouling. The MBR performance elucidated that addition of 30 mg/L of alum was effective to remove 3 mg/L phosphorous in feed for MBR process with reduction in membrane fouling factor without deterioration in nitrogen removal efficiency. The particle size analysis further confirmed that coagulation with alum could minimize membrane fouling because the relative small particles in the MBR reactor, which are considered as a cause of membrane fouling, could became bigger particles due to the addition of coagulant.

Characteristics of Membrane Fouling in an Anaerobic-(Anoxic/Oxic)n-MBR Process

Abstract The characteristics of membrane fouling and cleaning, in a hybrid MBR process, was investigated. Under the condition of sub-critical flux operation, a characteristic three-stage trans-membrane pressure (TMP) profile is observed as time passes. The initially extended period of slow pressure rise, followed by a somewhat faster rise, is then supplanted by a sudden transition to rapid pressure rise. Membrane cleaning experiments and SEM examination make it apparent that the rapid TMP rise is mainly caused by the accumulation of a surface cake layer, which is a reversible fouling that can be removed by tap water washing. Fouling caused by a gel layer, which is an irreversible fouling, can be removed efficiently by chemical cleaning. NaClO can oxidize the gel layer, which is formed mainly of macro-molecular organic substances. The HCl can remove inorganic particles formed by Ca 2+ , Mg 2+ ions etc. The sequence of chemicals used in membrane cleaning has an influence on the cleaning result. The effect of the NaClO+HCl cleaning procedure is superior to that of the HCl+NaClO one. Particle size distribution measurements (PSD) reveal that fine particles are inclined to deposit or attach on the membrane surface, or in the membrane pores, and caused rapid fouling.

Effect of hydraulic retention time on membrane fouling and biomass characteristics in submerged membrane bioreactors

In this paper, three identical membrane bioreactors (MBRs) were operated in parallel in order to specify the influence mechanism of hydraulic retention time (HRT) on MBR. The results showed that the removal efficiency of chemical oxygen demand (COD) was stable though it decreased slightly as HRT decreased, but biomass activity and dissolved oxygen (DO) concentration in sludge suspension decreased as HRT decreased. The filamentous bacteria grew easily with decreasing HRT. The extracellular polymeric substances (EPS) concentration and sludge viscosity increased significantly as filamentous bacteria excessively grew. The over growth of filamentous bacteria, the increase of EPS and the decrease of shear stress led to the formation of large and irregular flocs. Furthermore, the mixed liquid suspended solids (MLSS) concentration and sludge viscosity increased significantly as HRT decreased. The results also indicated that sludge viscosity was the predominant factor that affecting hydrodynamic conditions of MBR systems.

The effects of natural organic matter (NOM) fractions on fouling characteristics and flux recovery of ultrafiltration membranes

Natural organic matter (NOM) which is a complex mix of particulate and soluble materials in surface water has been identified and reported by previous studies as responsible for membrane fouling. However the component of NOM which primarily causes the fouling problem is still not well understood, especially relating to the specific fraction that is mainly responsible for flux decline. Therefore, the main objective of this study is to gain a better understanding on the ultrafiltration membrane fouling characteristics due to fractionated NOMs. The 68 kDa MWCO polysulfone (PSF) and 50 kDa MWCO cellulose acetate (CA) membranes formed by a simple phase inversion technique were employed, and the experiments were carried out in a submerged membrane configuration. The Ulu Pontian River water was fractionated into several components which were hydrophobic (HPO), transphilic (TPI) and hydrophilic (HPI) fractions using DAX-8 and XAD-4 ion exchange resins. The experimental results showed that the HPI component of NOM exhibited the worst flux decline despite lesser DOC and UV 254 rejections compared to other fractions (HPO and TPI). On the other hand, the HPO fraction showed the highest DOC and UV 254 removals despite possessing lower organics concentration than HPI fraction. Therefore the degree of flux decline is not necessarily proportional with the DOC and UV 254 removals and in fact the degree of DOC and UV 254 rejections are relatively independent of the amount of organics in the source water. Thus, it is worth to note that rejection and fouling are not greatly dependent on the SUVA and DOC concentrations but mostly on the rejection mechanism resulted from the membrane-feed interactions. The hydraulic resistance in series assessment elucidated that the concentration polarization, adsorptive fouling and cake layer deposition were the dominant fouling mechanisms for HPO, HPI and TPI fractions, respectively. Furthermore the effect of cleaning solutions on flux recovery was found significant in nearly restoring the initial membrane permeability. Surprisingly, the HPI fraction was found to exhibit better flux recovery than HPO fraction despite its higher adsorption resistance ( R a ). Therefore, in future applications, the type of fouling resistance and the dominant fouling mechanism are useful tools which can be employed as quantitative measurements for flux recovery and cleaning techniques effectiveness.

Mitigated membrane fouling in a vertical submerged membrane bioreactor (VSMBR)

In a laboratory-scale study, characteristics of membrane fouling in an A/O (anoxic/oxic) series membrane bioreactor (MBR) and in a vertical submerged membrane bioreactor (VSMBR) treating synthetic wastewater were compared under the same operating conditions. Accordingly, fouling characteristics of a pilot-scale VSMBR treating municipal wastewater were studied under various operating conditions. Various physical, chemical, and biological factors were used to describe membrane resistances. As a result, it was concluded that high concentrations of extracellular polymeric substances (EPS), high viscosity and a high sludge volume index (SVI) corresponded to high membrane resistance indicating severe membrane fouling in both the laboratory-scale MBRs and the pilot-scale VSMBR. In addition, high fouling potential was observed in the pilot-scale VSMBR at 60-day sludge retention time (SRT). In this case, as hydraulic retention time (HRT) decreased from 10 to 4 h, EPS concentrations increased and the average particle size increased, leading to reduced settling of the sludge and increased membrane fouling. To mitigate fouling, two different methods using air bubble jets were adopted in the pilot-scale VSMBR. As a result, it was found that air backwashing was more efficient for fouling mitigation than was air scouring.

Effect of activated sludge properties and membrane operation conditions on fouling characteristics in membrane bioreactors

Biofouling control is considered to be a major challenge in operating membrane bioreactors (MBRs) for the treatment of wastewater. This study examined the impact of biological, chemical, and physical properties of activated sludge on membrane filtration performance in laboratory-scale MBRs. Sludges with different microbial communities were produced using pseudo-continuous stirred-tank reactors and pseudo-plug flow reactors treating a synthetic paper mill wastewater. Various filtration resistances were used to investigate membrane fouling characteristics, and molecular biology tools targeting 16S ribosomal DNA gene sequences were used to identify predominant bacterial populations in the sludges or attached to the fouled membranes. Filtration experiments using axenic cultures of Escherichia coli, Acinetobacter calcoaceticus, and Gordonia amarae were also performed to better understand the initiation and development of biofouling. The results showed that the tendency of membranes to biofoul depended upon membrane operating conditions as well as the properties of the activated sludge in the MBR systems. Specific bacterial populations, which were not dominant in the activated sludges, were selectively accumulated on the membrane surface leading to the development of irreversible biofouling.

Effect of internal recycle rate on the high-strength nitrogen wastewater treatment in the combined UBF/MBR system

An anaerobic/aerobic system combining an anaerobic upflow-sludge bed filter (UBF) and an aerobic membrane bioreactor (MBR) was operated to enhance organic and nitrogen removal efficiency. The internal recycle rate, which is one of the most important operation factors that affects overall removal efficiency, was varied from 100% to 300% of the influent flow. Under these conditions, the overall removal efficiencies of organic and nitrogen and characteristics of membrane fouling in the combined system treating the synthetic wastewater including high concentration of organics and nutrients were studied. As a result, nitrogen removal efficiency was increased to 67% when the internal recycle rate was 300% of influent flow rate. As the internal recycle ratio increased from 100% to 200%, protein content decreased by 17% and carbohydrate content increased by 12%. However, there was no remarkable difference in total extracellular polymeric substances (EPS) content. At the high recycle rate of 300%, the surface charge of sludge was decreased while hydrophobicity (specific ultraviolet absorbance, SUVA) was increased. The differences in SUVA and surface charge were 11% and 1%, respectively. It is concluded that SUVA and EPS composition were important parameters affecting membrane fouling in the combined system.

Effect of fractionated NOM on low-pressure membrane flux declines

The membrane fouling characteristics of natural organic matter (NOM) were assessed using single polypropylene hollow fibre membranes of pore size 0.2 μm. The membranes were liquid backwashed every 30 minutes and filtration runs of up to 48 hours (≤8 litres) were conducted. The NOM samples were fractionated into different chemical classes based on their adsorption properties. For the two waters investigated, the hydrophobic components were the major foulant for one water, and the hydrophilics were the major foulant for the other. Interaction between the strongly hydrophobic and weakly hydrophobic fractions was significant for one water, but the extent of interaction between these fractions was minor for the other. The long term membrane fouling characteristics could not always be deduced from short term fouling trials.

Fouling of UF membrane by humic substance: Effects of molecular weight and powder-activated carbon (PAC) pre-treatment

Fouling characteristics of membranes with various molecular weight cut-offs (MWCO) were investigated. The effects of the molecular weights (MW) of humic acids and pre-treatment with PAC on membrane fouling were studied. It was found that the hydraulic resistance caused by fouling materials calculated using cake resistance in series model is a better indicator than the percentage of flux decline to assess the fouling of membranes with various MWCO. The effects of MWCO of membranes and MW of humic acids on membrane fouling can be explained by the different types of fouling mechanisms.

Fouling characteristics and cleaning strategies in a coagulation-microfiltration combination process for water purification

The membrane fouling characteristics and cleaning strategies in a coagulation-microfiltration combination process for purification of micro-polluted raw water were investigated. The microcosmic observation on the exterior and inner surfaces of the fouled membrane by Scanning Electronic Microscope (SEM) combined. with Energy Dispersive Spectrometer (EDS) revealed that the fouling on the exterior surface was an integrated effect of microorganisms, organic and inorganic matters, while that on the inner surface was mainly due to the bio-fouling. The investigation on the recovery of the membrane permeability with various cleaning methods showed that alkaline cleaning effectively removed most of the microorganisms and organic foulants formed on both the exterior and inner surfaces of the membrane, and acidic cleaning more effectively removed the inorganic scales. Based on the analysis of the extracted solution during chemical cleaning, it was found that the majority of soluble organic foulants were of low molecular weight, and Ca was the major inorganic element in the foulants.

An advanced treatment of high-strength opium alkaloid processing industry wastewaters with membrane technology: pretreatment, fouling and retention characteristics of membranes

This paper presents the results of the laboratory and pilot-scale membrane experiments of opium alkaloid processing industry effluents. Different types of ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes were evaluated for membrane fouling, permeate flux and their suitability in separating COD, color and conductivity. Experiments demonstrated that membrane treatment is a very promising advanced treatment option for pollution control for opium alkaloid processing industry effluents. Almost complete color removal was achieved with NF and RO membranes. COD and conductivity removals were also greater than 95% and met the current local standards. Nevertheless, pretreatment was an important factor for the NF and RO membrane applications. Membrane fouling occurred with direct NF membrane applications without UF pretreatment. The total estimated cost of the UF and NF treatment system was calculated as $0.96/m 3, excluding the concentrate disposal cost.

Iron fouling in membrane gas transfer applications

Iron fouling characteristics of gas permeable composite and wetted microporous hollow fiber membranes were studied. Membrane modules, operated in the sealed end configuration, were pressurized with oxygen and enclosed within a tubular shell. A solution consisting of tap water and ferrous sulfate was metered though the shell side of the module. The added mass-transfer resistance due to iron fouling was determined from the required increase in oxygen feed pressure to the membranes in order to maintain constant flux across the membranes, and was evaluated as a function of operating time and process parameters. Process parameters investigated include Reynolds number, ferric and ferrous iron concentrations, iron oxidation rate, and oxygen flux. It was found that iron oxidation rate, ferric iron concentration, and oxygen flux had no impact on the rate of membrane fouling, whereas the rate and maximum extent of membrane fouling increased with Reynolds number and ferrous iron concentration. The induction period was also found to increase with Reynolds number. A maximum fouling resistance of 2779 s/cm was measured at 17 h for a Reynolds number of 9000 and ferrous iron concentration of 14.0 mg/l. This is 7.2 times greater than the overall mass-transfer resistance for the clean membrane. For all conditions in this study, constant flux across the membrane was maintained, and the impacts of fouling compensated for by operating at higher driving forces. For membrane gas transfer systems where the driving forces can be controlled, such as membrane oxygenation, driving forces can be used to compensate for increases in mass-transfer resistance. However, for applications where driving forces cannot be as easily manipulated, such as VOC stripping, iron fouling is likely to have significant impact on transfer performance.

Membrane performance with plug screw feeder pressate: operating conditions and membrane properties

Membrane performance can be significantly affected by operating conditions. The effects of transmembrane pressure and cross flow velocity are discussed for various membranes during ultrafiltration of a pulp mill effluent. The effluent contained suspended solids, colloidal particles such as resin and fatty acids and materials with a wide molecular weight distribution. This effluent had severe membrane fouling characteristics, the nature of which complicated interpretation of the results. A variety of commerically available membranes made with different polymers and pore sizes were evaluated. Permeation experiments were performed using thin-channel, flat sheet, test cells. Cross-flow velocities varied from 0.4 to 1.2 m/s and the transmembrane pressure from 345 to 1,035 kPa. Flux decline occurred by several mechanisms and the selection of the membrane material, pore size and the operating conditions determined the relative contribution of these mechanisms. A modified series resistance model using flux loss ratios qualitatively explained changes in membrane performance under different operating conditions.