Cause Of Fouling Research Articles

Microplastics fouling mitigation in forward osmosis membranes by the molecular assembly of sulfobetaine zwitterion

Forward osmosis (FO) membranes have potential for the efficient water and wastewater treatment applications. However, their development has faced significant challenges due to their fouling propensity. In this study, FO membranes modified with sulfobetaine zwitterions (i.e., [2-(Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide) were fabricated and used for the first time to address microplastic (MP) fouling issue. Water flux, reverse salt flux (RSF), fouling, and flux recovery were evaluated for the membranes loaded with different quantities of the zwitterions ranging from 0.25 % to 2 %. The developed membranes were tested over 49 h with feed solutions containing polyethylene MPs and bovine serum albumin (BSA) to evaluate their fouling resistance. The synergistic effects of the two foulants indicated that the MPs were the primary cause of fouling. The presence of BSA effectively reduced the blocking effect of MPs and therefore lowers overall fouling. Additionally, improved water flux, structural parameter (S), and RSF were reported for the modified membranes. The zwitterion's unique structure with hydrophilic groups (CO and OSO) resulted in high flux recovery rates of over 90 % for all modified membranes within only 30 min of physical cleaning upon fouling tests. The results demonstrate the high potential of the modification method for targeting the removal of MPs in TFC-based membranes.

Role of coagulation/flocculation as a pretreatment option to reduce colloidal/bio-colloidal fouling in tertiary filtration of textile wastewater: A review and future outlooks

Textile industry wastewater has become a growing concern in recent years due to it has been characterized by a high load of organic dyes, suspended and dissolved solids, alkaline pH, and low biodegradability. As a result, environmental authorities necessitate textile industries to treat effluents before discharge into the environment. Tertiary filters, particularly membrane filtrations, are the most preferable process to recover good-quality water at the tertiary treatment phase, which feeds from secondary effluents, in wastewater treatment processes. However, fouling is still a challenge due to a higher load of suspended solids, colloids, organic matter, and a high level of bio-colloids (mostly from secondary effluents) in the textile wastewater treatment process. Bio-colloids are any colloidal entities of organic matter including microorganisms and their exudates. Hence, a coagulation/flocculation unit process, as a pretreatment option, is critical both at the primary treatment stage and after secondary (biological) effluents to prevent fouling problems at the tertiary filters. We reviewed identifying major foulants causing tertiary filter damage and the available pretreatment option for the removal of these foulants. We focus on and suggest the coagulation/flocculation process as a good pretreatment alternative to prevent filter fouling as it provides a reliable process to treat high water turbidity that arises from a high load of solids and colloids. Amongst different types of foulants, we focus on and present the colloidal solids and bio-colloidal foulants that could be major causes of fouling. These foulants are less understood and expected to be dominant in the textile industry wastewater, and established pretreatment alternatives are not well developed for the bio-foulants fed from the secondary effluent. Thus, these foulants need to be critically identified in the textile wastewater treatment plants to integrate suitable pretreatment options to prevent fouling potentiality. We proposed a coagulation/flocculation unit process as a pretreatment option to reduce colloidal and bio-colloidal fouling before the tertiary treatment stage, next to the secondary effluent, is critical.

A Novel Approach for Visualizing Mixing Phenomena of Reactive Liquid-Liquid Flows in Milli- and Micro-Channels

Modular milli- and micro-structured systems represent a promising approach to exploit the potential of micro-process technology, including precise reaction control and scale-up. A major drawback of micro-structured devices is fouling and mixing mechanisms need to be investigated phenomenologically to better understand the processes that lead to fouling. Previous work was conducted to resolve 3D concentration fields by means of Laser-Induced Fluorescence (LIF) using a Confocal Laser Scanning Microscope (CLSM) (Frey et al., J Flow Chem, 2021, 11, 599–609). While the CLSM-LIF method yields detailed insight into concentration fields down to a few micrometers, it is limited to stationary flow structures only. Aubin et al. (Chemical Engineering Science, 2010, 65, 2065–2093) give a comprehensive review of methods to analyze mixing behavior. Most recent optical measurement methods rely on the detection of a single compound in mixtures. In case of reactive mixing, Tthe state of the art procedures to locally visualize micro mixing relies on tracking a reaction product which forms on molecular scale. In literature, only small micro-structures are manufactured from transparent materials, however larger milli-structures often lack optical accesses with sufficient quality. Selective laser-induced etching (SLE) is a new technique which enables the fabrication of larger milli-structures in transparent materials that are relevant for industry-scale applications. This work develops a method based on a concept of Kexel et al. (Chemie Ingenieur Technik, 2021, 93, 830–837) visualizing the selectivity of a competitive-consecutive gas-liquid reaction in a Taylor bubble flow. The main goal of this work is the analysis of the absorbance spectra of bromothymol blue (BTB) at different pH values in a miscible liquid-liquid system in a fused silica split-and-recombine mixer. The milli-structure of the mixer is manufactured by means of SLE. Backlight at different wavelengths is pulsed matching the recording frequency. In contrast to conventional UV/Vis setups, the absorbance is recorded locally within the mixer. The proposed method yields the 2D concentration distribution of multiple species with high spatial resolution. The spatially resolved reactant and product distribution unveils micro mixing and can yield important information about local root causes of fouling.

Effects of Ferrihydrite-Impregnated Powdered Activated Carbon on Phosphate Removal and Biofouling of Ultrafiltration Membrane

The presence of multiple contaminant species in surface waters makes surface water treatment difficult to accomplish through a single process. Herein, we evaluated the ability of an integrated adsorption/ultrafiltration (UF) membrane filtration system to simultaneously remove phosphates and dissolved organic matter (DOM). When bare powdered activated carbon (PAC) and PAC impregnated with amorphous ferrihydrite (FHPAC) adsorbents were compared, FHPAC showed a greater adsorption rate and capacity for phosphate. FHPAC had a phosphate adsorption capacity of 2.32 mg PO43−/g FHPAC, even when DOM was present as a competing adsorbate. In a lab-scale hybrid FHPAC-UF system (i.e. integrated adsorption by FHPAC with UF membrane filtration), irreversible membrane fouling was ca. three times lower than that in a PAC-UF system. When membrane fouling in the PAC-UF system was described with pore blockage models, we found that the main cause of fouling was bacterial deposition on the membrane surface. CLSM analysis determined that the chemical composition of foulants in the PAC-UF system included higher proportions of proteins, nucleic acids, and alpha-polysaccharides than that in the FHPAC-UF system. Overall, FHPAC’s ability to undergo ligand exchanges with DOM helped to reduce the nutrients and bacteria that cause biofouling to accumulate on the membrane surface.

Membrane fouling mitigation in different biofilm membrane bioreactors with pre-anoxic tanks for treating mariculture wastewater

This study compared the membrane fouling mitigation in two novel types of biofilm membrane bioreactor coupled with a pre-anoxic tank (BF-AO-MBR)—namely a fixed biofilm membrane bioreactor (FB-MBR) with fiber bundle bio-carriers and a moving-bed biofilm membrane bioreactor (MB-MBR) with suspended bio-carriers—relative to an anoxic/oxic MBR (AO-MBR), at salinities ranging from zero to 60 g/L. The results showed that the FB-MBR mitigated membrane fouling to a greater degree than the MB-MBR and AO-MBR. During operation, the FB-MBR exhibited the lowest fouling development, with three membrane filtration cycles, while the AO-MBR and MB-MBR had 22 and nine cycles, respectively. The key fouling factor in all reactors was cake layer resistance (RC), which contributed to 89.61, 62.20, and 83.17% of the total fouling resistance (RT) in AO-MBR, FB-MBR and MB-MBR, respectively. Additionally, in the FB-MBR, the pore blocking resistance (30.07%) was also an important cause of fouling. Fiber bundle bio-carriers and suspended bio-carriers reduced the RT by 37.68% and 21.24% (mainly the RC) compared to that of AO-MBR. Furthermore, FB-MBR and MB-MBR caused a decrease of suspended biomass (80.14 and 15.90%, respectively), and the latter exhibited a higher sludge particle size than AO-MBR, possibly resulting in the cake layer decline. The studied BF-AO-MBRs further alleviated the fouling propensity by reducing the amount of soluble microbial product (SMP) and extracellular polymeric substances (EPS) under all salinity levels, especially the FB-MBR. Among the protein components, the amounts of tryptophan protein-like substance and aromatic protein-like substance were significantly lower in the FB-MBR compared to the AO-MBR and MB-MBR. Additionally, at 60 g/L salinity, the structure of the microbial community in the FB-MBR had a lower abundance of Bacteroidetes and more biomacromolecule degraders, which may have contributed to the moderation of membrane fouling.

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.

Discrete phase-CFD simulations of asphaltenes particles deposition from crude oil in shell and tube heat exchangers

Fouling in crude preheat trains is a major cause of thermal inefficiency in petroleum refineries. The limited fundamental understanding of its causes and mechanisms led to ineffective fouling mitigation techniques. It is believed that asphaltenes precipitation and deposition is the major cause of fouling. The present work attempts to simulate the deposition of asphaltenes from crude oil in a multi-pass shell and tube heat exchanger through discrete-phase CFD simulations. The effects of bulk velocity, temperature gradients in the radial direction and particle sizes on asphaltenes deposition are investigated. In an effort to understand the effect of various forces on the rate of deposition of asphaltenes, forces such as gravity, drag, Saffman lift and thermophoretic are applied on the asphaltenes particles. The deposition velocities of the asphaltenes particles are determined based on solving the balance of these forces. The asphaltenes particles mass deposition rate is high in locations of higher particle deposition velocities. The CFD simulations indicate that the dominant forces to encourage particles deposition are gravity and drag. The deposition velocities and mass deposition rates are high for larger particle sizes. Low fluid velocities, higher temperature gradients and larger particle sizes favor higher particle deposition rates.

Membrane autopsy to provide solutions to operational problems of Jerba brackish water desalination plant

Fouling in membrane filtration processes is problematic. Causes are often specific depending on feed water constituents, the membrane and the chemical products used. Some fouling will badly affect membrane performance and cleaning will therefore be necessary. Cleaning can be supported by antiscalant treatments to reduce the risk of membrane fouling. However, some antiscalants may also lead to fouling. This paper presents the major problems of reverse osmosis plant in Jerba Island, namely the influence of antiscalant type on the water characteristics and membrane fouling. This was related to the increase of pressure drop, permeate conductivity and cleaning frequency. To identify the causes of membrane fouling, different investigations were carried out. Membrane autopsy, chemical analysis of feed water, follow-up of the operation parameters. The objective of membrane autopsy, by means of SEM/EDS, was to carry out a destructive analysis on a fouled membrane in order to identify the major causes of fouling. The results show that the film was composed mostly of organic matter, with significant presence of nitrogen, iron and silicates. These findings demonstrate how the antiscalants altered the water characteristics. Thus, the selection of the type of antiscalant should take into account the associated contribution to membrane fouling development.

Fouling behaviour of soluble microbial products and extracellular polymeric substances in a submerged anaerobic membrane bioreactor treating low-strength wastewater at room temperature

The properties of soluble microbial products (SMP) and extracellular polymeric substances (EPS) were determined and their role in membrane fouling behaviour was considered in an anaerobic membrane bioreactor (AnMBR) treating low-strength wastewater at room temperature. The SMP/EPS properties of interest were specific production, molecular weight distribution and adhesion force. The results showed the largest factor affecting the SMP/EPS properties and their diverse membrane fouling performances was the organic loading rate (OLR). An increase in the OLR resulted in an increase in the production of specific EPS and macromolecules in the SMP/EPS fractions, in effect exacerbating the flocculation ability of the mixed liquor in the AnMBR and thus facilitating the fast formation of cake layers. Furthermore, the EPS tended to be more viscoelastic and hydrophobic at a higher OLR and because the adhesion forces of the EPS-membrane and EPS-EPS were significantly enhanced as the OLR increased, cake fouling was significantly accelerated. The results indicated that the main cause of fouling was SMP-induced pore blockages, and that membrane resistance increased gradually until an OLR of 0.7 gCOD/L/d, but increased remarkably when the OLR was higher than 1.4 gCOD/L/d, caused by the EPS-induced fast-growth and compact cake layer on the membrane surface.

Evaluation of pretreatments for a blowdown stream to feed a filtration system with discarded reverse osmosis membranes

In reverse osmosis demineralization processes, spiral wound modules are used and after approximately 5years are discarded, since they lose their performance. These modules could be reused for other treatments that do not require a final permeate with so high quality, but still have limitations for feed water quality. The aim of this study is to propose a pretreatment configuration to a membrane filtration system that uses discarded membranes to treat the stream of a cooling tower for reuse as make-up water. Steps of coagulation/flocculation, sand filtration, sorption with GAC and combinations of these were proposed. Parameters as pH, electrical conductivity, turbidity, total hardness, COD, silica content and SDI were analyzed. The pretreated effluent was tested in a bench scale membrane filtration system. Permeate flux, salt retention and hydraulic permeability were evaluated to identify fouling. The best results were obtained coupling coagulation/flocculation with sand filtration. The RO discarded membranes presented high salt retention, about 97% and analysis of permeates indicates the possibility of reuse. MEV and EDS of SDI membranes showed a decrease of silica and total hardness scale, which are the main causes of fouling and the pretreated stream seems to present suitable characteristics to feed the RO filtration system.

Membrane biofouling mechanism in an aerobic granular reactor degrading 4-chlorophenol.

The membrane fouling of an aerobic granular reactor coupled with a submerged membrane in a sequencing batch reactor (SBR) was evaluated. The fouling analysis was performed by applying microscopy techniques to determine the morphology and structure of the fouling layer on a polyvinylidene fluoride membrane. It was found that the main cause of fouling was the polysaccharide adsorption on the membrane surface, followed by the growth of microorganisms to form a biofilm.

Evaluation of chemical cleaning efficiency of organic-fouled SWRO membrane by analyzing filtration resistance

Membrane fouling is an unavoidable phenomenon in the operation of seawater reverse osmosis and a major obstacle to economic and efficient operation. In particular, membrane fouling by organic matter negatively affects productivity, product quality, and process cost. Therefore, a chemical cleaning process is essential to prevent interruptions for an effective RO membrane filtration process. Firstly, this study focused on the proper chemical cleaning condition for commercial polyamide RO membranes purchased from two companies. The flux decline rate of SWC5+ membrane was higher than that of SW39HRLE400 membrane regardless of organic foulants because the initial zeta potential of SWC5+ membrane (-21.17 mV) was lower than that of SW30HRLE400 membrane (-30.11 mV) and the repulsive force between membrane surface and foulants was also lower. In addition, we attempted to evaluate cleaning efficiency according to the chemical cleaning conditions and investigate the cause of fouling by analyzing membrane resistance fraction after cleaning the organic-fouled membrane. As a result of cleaning in place (CIP), as the concentration and cleaning time increased when NaOH, trisodium phosphate 12, sodium tripolyphosphate, and ethylenediaminetetraacetic acid-4Na were used without sodium dodecylsulfate, cleaning efficiency increased significantly. Subsequently, in the condition of CIP using NaOH at pH 11 for 60 min, the cleaning efficiency of membrane fouled by humic acid was higher that of membrane fouled by sodium alginate (SA) due to the strong interaction between hydrophobic membrane and hydrophobic foulants in the case of both membranes. To find the fouling rate and fouling cause, total resistance (Rt) was fractionated by the initial membrane resistance (Rm), resistance removed by flushing (Rc), resistance removed by CIP (Ra), and irreversible resistance (Ri). Rc and Ra in both membranes showed a similar proportion in Rt. The cause of fouling of the membrane fouled by SA was mainly the absorption of membrane surface, and that of membrane fouled by HA was the absorption of membrane surface and pore blocking by cake layer.

HVAC: Maintaining HVAC equipment with air intake filters

Airborne debris, such as domestic waste, leaves, pollen, insects and even plastic carrier bags is the leading cause of fouling and clogging of cooling towers, heat exchangers, air cooled chillers and other HVAC systems. Richard Betts, managing director of UK-based RAB Specialist Engineers, provides Filtration+Separation with some practical advice on how to maintain HVAC equipment.

Improving heat exchanger supervision using neural networks and rule based techniques

The work is aiming to the supervision of heat exchangers fouling monitoring. The fouling known as deposition of undesirable material on the heat transfer surface degrades the performance of heat exchangers. The fouling of heat exchangers in process plants results in a significant cost impact in terms of production losses, energy efficiency, and maintenance costs. To overcome mentioned inconveniences a novel supervision strategy is proposed, reporting innovative techniques and main results of an application tool to diagnose the heat transfer efficiency of a heat exchanger of a pilot plant using neural network based models and parity space approaches associated to a rule based decision making strategy. The developed strategy is fragmented into several modules connected between them following a causal logic flowchart. The first module checks the consistence of the supervision system. The second module monitories the heat exchanger for fouling condition with the ability to diagnose the probable causes of fouling. A third module predicts the remaining operating time under acceptable conditions, associated to a decision making task to schedule the supervision flowchart.

Effects of wastewater effluent organic materials on fouling in ultrafiltration

The objective was to determine the effects of wastewater effluent organic materials (EfOM) on fouling of ultrafilters (100 kDa polyethersulfone (PES)). EfOM constituents were sequentially removed, first by removing particles down to the approximate ultrafilter pore size and then by removing dissolved EfOM based on functionality. Particles and colloids >20 nm accounted for 19% of total organic carbon (TOC), including 96% of EfOM >100 kDa. Removal of particles and colloids resulted in increased fouling, attributed to increased contact of dissolved EfOM with the membrane. Hydrophobic and hydrophilic (HPO/HPI) acids were 22% of total EfOM, and accounted for nearly all of the fouling. HPO/HPI base/neutrals were 59% of EfOM, but did not cause any significant fouling. Although HPO/HPI base/neutrals did not cause any fouling, they were the dominant EfOM constituent at the surface of fouled and then hydraulically cleaned membranes, as measured by attenuated reflectance infrared spectroscopy. Since the filtration runs were short, the effects of HPO/HPI base/neutrals on long-term fouling should be further investigated, but these results cast doubt on the presumption that organic materials that are identified during membrane autopsies are necessarily a primary cause of fouling. These results also indicate that wastewater EfOM should be treated to remove HPO/HPI acids prior to membrane filtration.

Effective factors in the treatment of kerosene–water emulsion by using UF membranes

The effects of different parameters including membrane type (regenerated cellulose and polysulphone), transmembrane pressure (TMP), the content of oil in the feed, the flow velocity of the feed and pH on the ultrafiltration of an emulsion of kerosene in water were studied. It was found that the important factors affecting ultrafiltration were, in order, membrane type, pressure and oil concentration. The greatest flux at the optimum conditions here of 3 bar, an oil content of 3% (v/v) and with membrane type C30F was predicted as 108 L/(m 2 h) that was within the range of the confidence limit of the measured value of 106 L/(m 2 h). The normalised FTIR results of the virgin cellulosic membranes C30F and C100F showed more abundant OH groups. The bigger number of OH groups implies a greater hydrophilicity. The larger observed flux in the C30F is related to a higher number of pores as well (surface porosity) compared with the C100F membrane. In the “polarised regime” from 3 bar upwards, flux was independent of pressure for all membranes and was assumed to be determined by the back diffusion transport. Despite the fact that both the PS100H and C100F membranes had the same cut-off (100 kg/mol), the hydrophilic C100F showed a superior permeate flux. The strongest drop of flux with time due to oil fouling was observed for the C100F although it was hydrophilic. In the case of the PS100H, both FTIR and SEM showed that cake layer formation was not the cause of fouling. Meanwhile the SEM and FTIR results of fouled C100F provided evidence of adsorptive and gel formation fouling.

Optimization of PACl dose to reduce RO cleaning in an IMS

In order to ensure stable treated water quality and to reduce chemical costs in any treatment plant it is necessary to study and optimize the coagulant dosing control (CDC). This research focused on the affects of coagulant in the integrated membrane (UF & RO) system employed for industrial water production. The dose of coagulant (PACl) might be associated (partly) with the frequency of cleaning in the RO units, and a value of 20 abs/m in the UF permeate is the control of the dosage process. This could suggest that organic fouling is directly and indirectly (inducing biofouling as well) the cause of fouling of the RO membranes. Nevertheless, high doses of PACl could produce scaling of aluminium and, in this work minimizing PACl to prevent operational problems in RO membranes was focused.The approach involved the study of the treatment processes, determination of the optimum dose of coagulant, evaluation of the removal efficiency of UV and DOC by coagulation. The available data, which comprised monthly/weekly measurements for a period of six years of operation, was studied and analyzed and an attempt was made to draw some conclusions for the plant regarding the coagulant dosage and the link with UV absorbance as control.The study of the coagulant dosing control revealed that the use of simple and robust online sensors like UV measurement allows an automatic dosing control although this parameter is not found to be sufficient to fully characterize nor predict fouling during membrane operation and there is no link between UV after the UF and the cleaning frequency of the RO. Parameters, as the added value allowed to verify the doses efficiencies in UV and DOC removal. Coagulant dose depends on the level of UV absorbance in the UF permeate and it should never be higher than 20 abs/m to ensure a RO cleaning frequency as long as possible. However, it was found that the target value of 20 abs/m produced substantial over dose of coagulant — 90% of the time – when UV removal is considered only. In the same way, for coagulant doses in excess of 5 mg/L, the additional removal of UV is less than 2.5% per mg coagulant/L, which suggests that the current dose is on the high side.Furthermore, a coagulant dose in excess of 5 mg/L did not produce significant additional UV removal, and thus a reduction from 14 mg/L (2005 dose) to ca. 7 mg/L could be considered.

Fouling of a hollow fibre submerged membrane during longterm filtration of activated sludge

Membrane fouling in membrane bioreactors (MBR) is caused by cake formation on the surface, mainly attributed to suspended solids, and also by surface adsorption connected with pore blocking, attributed to soluble components of activated sludge. In this study, long-term experiments were carried out in sub-critical conditions with frequent backwashing, and the nature of fouling was investigated. The irreversible blocking by adsorption has been identified as the major cause of fouling, as it had gradually decreased membrane permeability from 417 to 55 L m −2 h −1 bar −1 in 123 days of continuous experiment without chemical cleaning. Cake resistance was small due to a low flux and high aeration intensity and it remained constant for most of the experiment. A sudden acceleration of fouling has been observed after 119 days of operation and attributed to irreversible fouling that caused local fluxes to exceed critical flux and lead the filtration into critical conditions, where suspended solids start to deposit. Faster fouling, in comparison to the filtration of normal sludge, has been observed when starving biomass had been filtered. That was attributed to floc breakage.

Discussion of “Mechanistic Model for CaSO4 Fouling on Nanofiltration Membrane” by Che-Jen Lin, Saqib Shirazi, and Pritesh Rao

The mass transfer phenomenon that leads to concentration polarization and crystallization of salt on membrane surfaces during filtration of CaSO4 solutions is a dynamic process as presented in Fig. 5 of the original paper . The flux decline observed is due to the increases in both the density and thickness of the concentrated layers of CaSO4 i.e., concentration polarization and salt cake on the membrane surface Li et al. 2005 . Eqs. 1 and 2 in the paper are valid for steady state conditions, as the authors have noted. However, the authors have used these equations to estimate a dynamic parameter i.e., concentrations of CaSO4 on membrane surface at specific times . The estimated concentrations of CaSO4 at the membrane surface Table 1 of the original paper show a decrease in CaSO4 concentration over time as the membrane fouling progresses from point A to point D. With the progression of fouling over time, one would expect the CaSO4 concentration at the membrane surface to increase, as illustrated in Fig. 6 of the original paper , since this is the primary cause of fouling, which causes the flux decline presented in Figs. 3 and 7 of the original paper . Other researchers have used the steady state solution of the mass transfer equation for estimation of salt concentration on membrane surfaces at the time of nucleation or to determine if the precipitation is expected to occur in more complex systems van de Lisdonk et al. 2001; Le Gouellec and Elimelech 2002 . These limited applications of the steady state solution may be appropriate in such cases as the flux appears to be stable during the time of nucleation. For simplification purposes, assuming that the flux decline is entirely due to increase in concentration of CaSO4 at the membrane surface, then, the decrease in flux can be related to the increase in concentration of CaSO4 at the membrane

The effect of calcium on the membrane biofouling in the membrane bioreactor (MBR)

A continuous bench-scale submerged membrane bioreactor (BSMBR) was operated in two modes; low (LC) and optimum calcium (OC) concentrations, to investigate the effect of calcium on membrane biofouling. Both the cake layer and pore blocking resistances were reduced in the operation under OC conditions. In order to find the causes of fouling, molecular weight fractionations of the extracellular polymeric substances (EPS) and soluble microbial products (SMP) were conducted, and the relative hydrophobicity of the sludge, EPS and SMP were also measured. The reduction in the cake layer resistance was assumed to be due to the decrease of filamentous bacteria, and better flocculation caused by the calcium bridges and the increased hydrophobicity of EPS in the operation under OC conditions. It was suggested that the pore blocking resistance was reduced due to the low SMP rejection of the membrane and the fewer hydrophobic reactions of the SMP in the operation under OC conditions. Fourier-transform infrared analysis of the fouled membrane surface supported that proteins and carbohydrates were the main foulants during the BSMBR operation.