Cake-enhanced Osmotic Pressure Research Articles

Evaluation of the Anti-Fouling Effects of Micro-Nano Bubbles on the Performance of Reverse Osmosis Membrane

Colloidal fouling on a brackish water reverse osmosis (BWRO) membrane was simulated by a lab-scale plate-and-frame module in the presence and absence of air micro-nano bubbles (AMNBs). Synthetic feed water samples with the same physical and chemical properties, but various concentrations of colloidal silica (50, 100, 200, and 300 mg/L), were used for experiments. The results illustrate that colloidal fouling caused a severe decrease in permeate flux (24%–56%) and salt rejection (1.25% to 4.18%) in the absence of AMNBs. This reduction in membrane performance was attributed to the high hydraulic resistance and the cake-enhanced osmotic pressure (CEOP) of a fouled gel layer that were intensified with increase in the colloidal concentration. On the other hand, presence of AMNBs decreased the deposition rate of colloidal particles significantly and increased the porosity of fouling layer. Thus, an improvement in the membrane permeate flux (21%–40%) and salt rejection (1.2%–2.6%) were seen in the different concentrations of colloidal particles. The SEM images also confirmed the formation of loose fouling layers which were easily removed from the membrane surface by a clean-in-place (CIP) process. This research introduces AMNBs technology as an effective in-line method to control the adverse effects of colloidal fouling in reverse osmosis (RO) systems.

Impact of salt accumulation in the bioreactor on the performance of nanofiltration membrane bioreactor (NF-MBR)+Reverse osmosis (RO) process for water reclamation

The impacts of salt accumulation, through adjusting the solid retention time (SRT), in the bioreactor on the bioprocess as well as membrane performance of a high retention nanofiltration membrane bioreactor (NF-MBR) and subsequent reverse osmosis (RO) process for water reclamation are addressed in this study. The build-up of salts (i.e., Ca, Mg, PO4) is a function of SRT, hydraulic retention time (HRT) and membrane rejection. Despite the accumulation of salts, both NF-MBRs at SRT of 30 and 60 days, achieved (i) similar biodegradation efficiency; (ii) excellent organic removal (> 97%); and (iii) excellent ammonia removal (> 98%). Extending the SRT could improve the microbial bio-flocculation capability, but did not influence the microbial activity, viability, and community structure. However, more severe membrane fouling was observed in the NF-MBR with elevated salt levels, which was attributed to the greater formation of calcium phosphate scale and Ca-polysaccharides complex (i.e., irreversible fouling layer) as well as the cake-enhanced-osmotic-pressure (CEOP) effect. Although both NF-MBRs produced comparable quality of permeate, a higher RO membrane fouling rate was observed when the permeate of NF-MBR with SRT at 60 days was fed to the RO system, implying organic compositions in NF-MBR permeate may influence RO performance.

Permeate flux increase by colloidal fouling control in a vibration enhanced reverse osmosis membrane desalination system

Experiments and Computational Fluid Dynamics (CFD) simulations were performed to investigate the colloidal fouling control of a vibration enhanced reverse osmosis (VERO) membrane system for up to 12 h of operation time. A porous cake mass transfer model for the reverse osmosis (RO) colloidal fouling analysis was derived by combining the cake filtration model, cake enhanced osmotic pressure (CEOP) effect, the critical flux theory, and the solute convection-diffusion model. The process of colloidal particle deposition and fouling formation on the membrane surface were visualized using this model. The permeate flux variation and cake layer distribution along the membrane were depicted. Systematic studies including different initial permeate fluxes, Reynolds numbers, particle concentrations, and vibration frequencies were carried out to investigate the system performance under different operation conditions. The results suggest that colloidal fouling induced permeate flux decline could be improved by the high-frequency vibration in the VERO module. Both simulations and experiments demonstrated that, with a fixed vibration amplitude, membrane module with higher vibration frequencies will have less NaCl accumulation and higher permeate flux under the influence of colloidal fouling.

The feasibility of nanofiltration membrane bioreactor (NF-MBR)+reverse osmosis (RO) process for water reclamation: Comparison with ultrafiltration membrane bioreactor (UF-MBR)+RO process

This study examines the feasibility of a novel nanofiltration membrane bioreactor (NF-MBR) followed by reverse osmosis (RO) process for water reclamation at 90% recovery and using an ultrafiltration MBR (UF-MBR)+RO as baseline for comparison. Both MBRs adopted the same external hollow fiber membrane configurations and operating conditions. The collected permeates of the MBRs were subsequently fed to the respective RO systems. The results showed that the NF-MBR (operated at a constant flux of 10 L/m2h) achieved superior MBR permeate quality due to enhanced biodegradation and high rejection capacity of the NF membrane, leading to lower RO fouling rates (∼3.3 times) as compared to the UF-MBR. Further analysis indicated that the cake layer fouling that caused the cake-enhanced osmotic pressure (CEOP) effect contributed predominantly to the transmembrane pressure (TMP) increase in the NF-MBR, while irreversible pore fouling was the major reason for UF membrane fouling. Furthermore, it was found that the biopolymers (i.e., organics with MW > 10 kDa) were the main components present in the foulants of the NF/UF membranes and RO membranes. The analysis indicated that the NF-MBR + RO system at recovery of 90% has comparable energy consumption as the UF-MBR + RO system at recovery of 75%. Our findings proved the feasibility of the NF-MBR + RO for water reclamation at a high recovery rate.

Highly condensed polyvinyl chloride latex production by forward osmosis: Performance and characteristics

Forward osmosis (FO) has attracted attention in industry because of its low energy consumption and low fouling tendency. In this paper, the condensation of polyvinyl chloride (PVC) latex with FO as a pretreatment step prior to the final drying process was investigated. The effect of agitation speed, FO membrane type, and the NaCl concentration of the draw solution (DS) were evaluated. For the membrane performance, the water flux, NaCl flux, and total organic carbon (TOC) flux were evaluated. When the feed solution was sufficiently agitated, a PVC concentration of 75wt% was achieved using model seawater as the DS. FO performance was experimentally compared with reverse osmosis (RO) performance using similar apparatus and the same water flux conditions. The TOC rejection in FO was higher than that in RO. Latex condensation using FO was characterized by a decrease in outlet water flux by cake-enhanced osmotic pressure, a decrease in inlet water flux by the capillary force resistance in the cake layer, and an increase in TOC rejection by the retarded forward diffusion of solutes. The control of cake-layer formation was important for the condensation of PVC latex by FO.

Impact of feed ionic concentration on colloidal and organic fouling of osmotically driven membrane process

The impact of feed ionic concentration in the presence of organic and colloidal foulant on osmotically driven membrane processes (ODMPs) was demonstrated. The normalized flux of ODMPs was significantly decresed in both DI and 10 mM NaCl feed solution when the feed solution contained colloidal particles as foulants due to the deposition of colloidal particles on the membrane surface. The deposited colloidal particles strongly promoted cake-enhanced osmotic pressure near the surface of the membrane. In contrast, the normalized flux of ODMPs was slightly increased when humic acids (HA) were added as the organic foulant into the feed solution. The attachment of HA molecules changed the membrane surface conducting it more hydrophilic, and induced a stronger diffusion of water molecules into the membrane active layer. As a result, both the increased feed ionic strength and deposited colloidal fouling did not significantly alter the reversal salt selectivity of the ODMPs, while the reversal salt selectivity was significantly incresed after the addition of HA molecules to the feed solution because the HA molecules changed the properties of the membrane surface.

Study of flux decline and solute diffusion on an osmotically driven membrane process potentially applied to municipal wastewater reclamation

One major challenge in this study was to investigate the performance of an osmotically driven membrane process, such as forward osmosis (FO) in a case of using raw wastewater that was obtained from a municipal wastewater treatment plant, with a focus on the flux decline and solute diffusion. First, to determine the effect of suspended solids (SS) in wastewater, wastewater was used containing 20SSmg/L and filtered by a 0.45μm filter to remove the SS. The results showed that a noticeable flux decline was observed in the case of the existing SS, but flux was slightly decreased without the SS. Furthermore, a larger decline in reverse salt flux (Js) was also obtained with the SS, thus it can be implied that cake enhanced osmotic pressure (CEOP) phenomenon occurred. In other words, the SS could accelerate membrane fouling, resulting in a flux decline and hindered reverse salt diffusion. There was also a comparison of the FO performance using wastewater and a MBR permeate. As hypothesized, it has been found that wastewater resulted in a higher flux and reverse salt flux (Js) decline through the consecutive fouling experiments (four times), but a MBR permeate also brought about substantial flux decline, which was contrary to what was conjecture. These findings indicate that an effective method to control fouling, such as pretreatment and cleaning, may be required even if the treated water resulting from wastewater treatment is used as feed water. As the fouling was getting severe, UV254 rejection was gradually decreased. That phenomenon might possibly be attributed to the increased humic substances on the membrane surface, changes in the membrane characteristics, and Js decline.

Combined colloidal and organic fouling of FO membranes: The influence of foulant–foulant interactions and ionic strength

In this work we determined the exact foulant–foulant interactions during combined membrane fouling in forward osmosis (FO). FO fouling tests were performed using sodium alginate and colloidal silica (ST-ZL, 139nm) as model foulants. The fouling potential of each foulant and their mixture was investigated using feed solutions containing fixed concentrations of Na+ and Ca2+ (total ionic strength of 0.5M). Changes on the foulant surface charges upon mixing with cationic species were monitored using zeta potential analysis. The inter-foulant interactions were determined by conducting sequential fouling experiments (the membrane was fouled by each foulants in alternating sequences) as well as Quartz Crystal Microbalance with Dissipating monitoring (QCM-D). Fouling tests with the mixture of alginate and silica colloids resulted in a flux decline trend very similar to that of alginate alone, an observation attributed to the primary effect of alginate during the gel layer formation. QCM-D analysis revealed that alginate macromolecules adsorb onto the surface of silica colloids and thus influencing the subsequent colloid–colloid interactions resulting in a cake layer with similar hydraulic resistance to that formed by alginate alone. There was no evidence of synergistic effects when alginate and silica colloids co-existed mixed in same feed matrix. It was also found that inter-foulant interactions were governed by non-electrostatic forces because at seawater level ionic strength there was excessive reduction of the Debyle length and electric double layer (EDL) such that normal electrostatic forces were greatly suppressed and short range non-electrostatic forces became dominant. The Extended-Derjaguin-Landau-Verwey-Overbeek (XDLVO) approach was used to understand foulant membrane interactions during fouling and it could not compliment the flux decline trends observed from fouling with combined foulant feed stream. The newly proposed approach of determining combined fouling, whereby the membrane was fouled with the two different single foulants in alternating sequences revealed that gel layer hydraulic resistance, hindered colloid back diffusion and cake enhanced osmotic pressure (CEOP) were responsible for flux loss during combined alginate–colloid fouling.

Prediction of reverse osmosis fouling using the feed fouling monitor and salt tracer response technique

In this study an on-line feed fouling monitor (FFM) combined with a salt-tracer-response technique (STRT) was used to predict reverse osmosis (RO) fouling under constant flux filtration. The FFM was used to capture foulant loads using a small ‘collection’ ultrafiltration (UF) membrane at the same crossflow hydrodynamics as in the RO experiments. A UF membrane was used in the FFM to decrease the monitoring time and improve the accuracy because it is more responsive to the fouling resistance than an RO membrane. Since the deposits captured by the FFM are potential RO foulants, the resulting information can be used to predict the transmembrane pressure (TMP) rise for the RO membrane. The STRT was used to measure the development of concentration polarization that is important in estimating the cake-enhanced osmotic pressure (CEOP) contribution. A model was developed that includes both the cake resistance and the CEOP effect due to cake formation and was used to predict RO fouling trends.Model foulants were humic acid and colloidal silica. The major focus was on organic fouling by humic acid (20mg/l) in 2000mg/l sodium chloride (NaCl) as the ionic background for the RO and FFM fouling experiments. The RO and FFM fouling experiments were conducted at different constant fluxes using the same feed solutions and at the same crossflow velocity (0.1m/s).The results indicated that higher fluxes cause an increased fouling rate for both RO and the FFM for both types of solute. The CEOP effect, measured by the salt-tracer-response in the RO experiments, was also strongly enhanced by the flux.The model was validated by plotting the predicted RO transmembrane pressure (TMP) as a function of time for different fluxes based on the resistivity from the FFM and the CP obtained from the STRT. For both organic foulants and colloidal silica the results show that the combination of the FFM and salt-tracer-response STRT is a promising method to provide a good estimate of the RO fouling trends. It also underscores the contribution of CEOP to the increase in TMP during RO fouling of saline feeds.

Nano-colloidal fouling mechanisms in seawater reverse osmosis process evaluated by cake resistance simulator-modified fouling index nanofiltration

Fouling potentials caused by particulate matters are generally evaluated by fouling index, such as SDI and MFI, widely used in RO membrane practices. However, these fouling indices failed to predict the effect of nano-colloidal sizes on flux decline, implying that colloidal fouling is too complex to be analyzed by simple fouling index. Thus, in this study, nano-colloidal fouling mechanism in seawater reverse osmosis (SWRO) desalination was fundamentally investigated by employing new approach. Specifically, the specific cake resistance of colloidal foulants was first determined by a novel method, cake resistance simulator-modified fouling index nanofiltration (MFI-NFCRS), which was conducted under operating pressure and solute environment similar to those of real SWRO desalination. Then colloidal deposition and resulting cake-enhanced osmotic pressure (CEOP) were quantitatively assessed by fitting RO experimental data to the calculations from the CEOP model. The results clearly demonstrated that the flux decline caused colloidal deposition in the SWRO process depended greatly on the CEOP. The newly developed methodology including MFI-NFCRS is expected to contribute significantly to better understand nano-colloidal fouling mechanisms and to accurately predict their fouling potentials in the SWRO desalination.

CFD analysis of tracer response technique under cake-enhanced osmotic pressure

A cake-layer mass transfer model applicable for RO, that incorporates the cake-enhanced osmotic pressure (CEOP) effect of a particulate fouling layer, is presented. This model includes the effect of a variable dissolved solute concentration on the specific cake resistance and porosity of the cake layer. The model is based on one-dimensional diffusion of the dissolved solute through the cake layer, and uses the solute concentration at the cake surface and the cake mass per unit area to calculate the solute concentration at the membrane surface and the trans-membrane osmotic pressure. The cake-layer mass transfer model is incorporated into a commercial Computational Fluid Dynamics (CFD) software package. Simulations are validated against experimental data, and the model predictions are within ±7% for permeate fluxes and within ±14% for measured concentration polarisation.The model is used to interpret and assess tracer response test results for estimating concentration polarisation and fouling resistance. Model predictions confirm the assumption for the tracer experiment that the average concentration polarisation along the membrane wall does not change significantly after a step change in the feed concentration of the tracer solute. However, it was found that the tracer experiment over-estimates the concentration polarisation index and under-estimates the fouling resistance, particularly under fouled conditions. The sources of error are discussed and a multiple tracer response test is proposed to minimise estimation error.

Development of a new technique to predict reverse osmosis fouling

In this paper RO colloidal fouling was predicted using an on-line Feed Fouling Monitor (FFM) combined with a monitor based on ultrasonic time domain reflectometry (UTDR) under constant flux filtration. The FFM incorporated the relevant cross-flow hydrodynamics and detected the development of foulant load and resistance from a continuous sample passing over a small UF membrane. A UF membrane was used in the FFM, rather than a RO membrane, because it was more sensitive to fouling resistance and this decreased monitoring time and increased accuracy. The UTDR was coupled to a RO membrane cell to monitor the rate of cake thickness increase, which is the required information for the estimation of cake-enhanced osmotic pressure (CEOP). A model was developed to predict fouling in RO using data from the two monitors and combined both the cake resistance and the CEOP effect due to cake formation.RO and FFM fouling experiments were performed at different constant fluxes using the same feed solutions (200mg/l SiO2, 2000mg/l NaCl) and the same cross-flow velocity (0.1m/s). The results showed that higher fluxes caused an increased fouling rate for both RO and the FFM. The foulant layer thickness measured by UTDR in the RO experiments increased faster at higher applied fluxes and the estimated CEOP effect was also strongly influenced by the flux.The results show that the model, combined with the FFM and UTDR measurements, can provide a good estimation of the RO fouling profile over a range of applied fluxes. This study also underscores the importance of CEOP in contributing to the increase in transmembrane pressure due to colloidal fouling in RO of saline feeds.

Modeling of colloidal fouling in forward osmosis membrane: Effects of reverse draw solution permeation

A numerical model for predicting the flux decline due to colloidal fouling was developed for a forward osmosis (FO) membrane system. We derived the kinetic equation of the cake layer growth based on a first-order reaction and control volume approach. Based on the model simulation, it was found that the deposited particles on a membrane surface are proportional to the feed concentration and the permeate flux. Moreover, the simulation result reveals that the cake-enhanced osmotic pressure (CEOP) is a key factor diminishing the permeate flux for large colloidal foulants. For small colloidal foulants, the hydraulic resistance of the cake layer is dominant in flux decline at the beginning of the fouling and CEOP increasingly become significant as fouling progresses. The effects of the reverse draw solute permeation on the flux decline were also simulated. Interestingly, the increased reverse draw solute permeation obtained by increasing the solute permeability showed little effect on the flux decline. Contrarily, variation of the diffusivity significantly influenced the flux decline. Consequently, the numerical model developed in this paper suggests that the selection of draw solute for an FO membrane process should be carefully regarded, along with the fouling mechanism.

The influence of feed pH on the performance of a reverse osmosis membrane during alginate fouling

The performance of a brackish water reverse osmosis membrane in the presence of a model polysaccharide foulant, sodium alginate, is investigated. The concentration of foulant, the feed pH and the presence of calcium are all shown to impact upon this performance. The flux decline is greatest at low feed pH, high calcium concentrations and high foulant concentrations. Sodium and chloride rejection falls as foulant concentration increases, due to cake-enhanced osmotic pressure. Effects are again greatest at low feed pH and high calcium concentrations. In part, this reflects a shift of the isoelectric point of the membrane from 4.3 to below 3.0 as alginate is added, reflecting the lower pKa of the alginate foulant. Conversely, calcium rejection improves in the presence of even small quantities of alginate foulant at all pH values. These increases in rejection are believed to relate to a direct interaction between the dissolved ions and the alginate which act to sequester the salt.

Colloidal fouling in forward osmosis: Role of reverse salt diffusion

Colloidal fouling behavior in forward osmosis (FO) was investigated, focusing on the role of reverse salt diffusion. Two suspensions of silica nanoparticles, with average particle diameters of 24 and 139 nm, were used as model colloidal foulants. To verify the effect of reverse salt diffusion on the colloidal fouling behavior, NaCl and LaCl 3 were employed as draw solutions because they exhibit different reverse diffusion rates. Our results suggest that in colloidal fouling of FO, salts diffuse from the draw to the feed solution and accumulate within the colloidal fouling layer that forms on the membrane surface. The accumulated salts result in a marked acceleration of cake-enhanced osmotic pressure (CEOP), which reduces the net osmotic driving force for permeate water flux. Fouling was not observed with the small, 24-nm particles because of the lack of substantial cake formation, but was notable for the 139-nm particles and for a feed containing a mixture of the 24 and 139 nm particles. Our findings further indicate that colloidal fouling is enhanced under solution conditions (ionic strength and pH) within the colloidal cake layer that promote aggregation or destabilization of the silica particles. Colloidal fouling reversibility was also examined by varying the cross-flow velocity during the FO fouling runs. We showed that in the absence of colloidal particle destabilization/aggregation, the permeate flux during colloidal fouling in FO recovered almost completely when the cross-flow velocity was increased from 8.5 to 25.6 cm/s. Our results suggest that reverse salt diffusion in FO is a key mechanism that controls colloidal fouling behavior as well as fouling reversibility. Therefore, minimization of reverse salt diffusion through the selection of proper draw solutes and optimization of FO membrane selectivity are important for minimizing colloidal fouling as well as enhancing FO operation efficiency.

Investigations of the coupled effect of cake-enhanced osmotic pressure and colloidal fouling in RO using crossflow sampler-modified fouling index ultrafiltration

The aim of this study was to investigate the potential of using the resistivity and deposition rate data from Crossflow Sampler-Modified Fouling Index Ultrafiltration (CFS-MFI UF) measurements to determine the coupled effects of colloidal fouling and cake enhanced osmotic pressure (CEOP) effect. Cake filtration derived from CFS-MFI UF was combined with a CEOP model to predict the crossflow RO fouling profile under constant flux filtration. The prediction based on resistivity, I′ from CFS-MFI UF measurement alone was found to underestimate the RO fouling for high salinity solutions. However, when incorporating the mass information from the CFS-MFI UF test to account for the CEOP effect, the prediction showed good agreement with the TMP profile of the RO system. The results indicated that the CFS-MFI UF test which includes the CEOP effect is a very promising technique to provide an estimation of the RO colloidal fouling profile. When the changes of cake thickness and porosity throughout the filtration were considered, the predicted TMP profile based on the model clearly indicated a two-stage of fouling profile which agreed well with the experimental data. Additional studies on the effects of cake thickness and porosity on CEOP highlighted the important influence of cake structure on CEOP.

Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO)

Fouling behaviors during forward osmosis (FO) and reverse osmosis (RO) are compared. Alginate, humic acid, and bovine serum albumin (BSA) are used as model organic foulants, and two suspensions of silica colloids of different sizes are chosen as model particulate foulants. To allow meaningful comparison of fouling behavior, identical hydrodynamic operating conditions (i.e., initial permeate flux and cross-flow velocity) and feed water chemistries (i.e., pH, ionic strength, and calcium concentration) are employed during FO and RO fouling runs. The observed flux-decline behavior in FO changed dramatically with the type of organic foulant, size of colloidal foulant, and the type of the draw solution employed to generate the osmotic driving force. Based on these experimental data and the systematic comparisons of fouling behaviors of FO and RO, we provide new insights into the mechanisms governing FO fouling. In FO, reverse diffusion of salt from the draw solution to the feed side exacerbates the cake-enhanced osmotic pressure within the fouling layer. The elevated osmotic pressure near the membrane surface on the feed side leads to a substantial drop in the net osmotic driving force and, thus, significant decline of permeate flux. Our results further suggest that the structure (i.e., thickness and compactness) of the fouling layers of FO and RO is quite different. By varying the cross-flow velocity during the organic fouling runs, we were able to examine the fouling reversibility in FO and RO. The permeate flux during organic fouling in FO recovered almost completely with increasing cross-flow velocity, while no noticeable change was observed for the RO system. Our results suggest that organic fouling in FO could be controlled effectively by optimizing the hydrodynamics in the feed stream without employing chemical cleaning.

Monitoring of CaSO4 deposition behaviors on biofilm during nanofiltration via ultrasonic time-domain reflectometry (UTDR)

The ultrasonic time-domain reflectometry (UTDR) as a non-destructive real-time method was employed to monitor the CaSO(4) deposition behaviors on biofilm during nanofiltration (NF). Two parallel experiments were performed to compare the different behaviors of CaSO(4) deposition with and without biofilm on the membrane. Results showed that the flux decline during combined fouling was slower than that in case of CaSO(4) fouling alone. The Ca(2 + ) rejection obtained with biofilm was higher than that without. A larger acoustic differential signal obtained by UTDR in the combined fouling revealed a denser and thicker layer formed on the membrane surface. Furthermore, the amount of CaSO(4) deposition on the biofouled membrane was more than that on non-biofouled membrane as a result of microorganisms as crystal nucleus to induce CaSO(4) crystallization and deposition. SEM images indicate that the CaSO(4) crystals deposited in order on the non-biofouled membrane, whereas on the biofouled membrane they were embedded in the biofilm. The denser and thicker fouling layer formed with biofilm was impermeable, resulting in a high Ca(2 + ) rejection. The complexation of Ca with polysaccharide in biofilm would eliminate the cake-enhanced osmotic pressure effect leading to a slow flux decline. To sum up, the independent measurements corroborate the ultrasonic measurements.

Effect of colloids on salt transport in crossflow nanofiltration

The role of colloid deposition on the performance of a salt-rejecting NF membrane was evaluated by modeling salt transport using a two-layer transport model, which quantified the relative contributions of advection and diffusion in the cake and the membrane layers, and the effects of flux on the membrane sieving coefficient. The model was able to accurately describe how the measured permeate concentration, rejection, osmotic pressure, and flux decline varied with time. The two-layer model confirmed that the Peclet number in the cake layer was about an order of magnitude higher than that in the membrane layer, leading to significant concentration polarization at the membrane surface, as shown by others. However, the cake layer also increased overall resistance, which resulted in flux decline during constant pressure operation. Flux decline caused an increase in the actual sieving coefficient, leading to higher solute flux, lower observed rejection, and thus lower the bulk concentration. These coupled phenomena tended to mitigate the increase in concentration polarization caused by the cake. Therefore, as predicted by the model and verified by experiment, the osmotic pressure does not increase monotonically as the cake grows, and in fact can decrease when the cake layer is thick and the flux decline is significant. In our experimental system, the pressure drop across the cake layer, which was proportional to the cake thickness, was significant under the conditions studied. The effects of cake-enhanced osmotic pressure analyzed here are lower than those observed in previous studies, possibly because the transport model employed explicitly accounts for the effect of flux decline due to cake growth on the membrane sieving coefficient, and possibly because we used a somewhat different methodology to estimate cake porosity.

Implications of critical flux and cake enhanced osmotic pressure (CEOP) on colloidal fouling in reverse osmosis: Modeling approach

This article describes a predictive model to account for the performance of a reverse osmosis (RO) system under the influence of colloidal fouling (using silica particles as model colloids) in terms of transmembrane pressure increase (δTMP) in constant flux operation or flux drop (δJ v) in constant pressure operation. The predictive model considers the implications of both the critical flux (J crit) and cake enhanced osmotic pressure (CEOP) phenomenon and is based on the membrane retention, concentration polarization, mass transfer, resistance in series and osmotic pressure model, cake resistance, and critical flux concept.