Effects of feed water temperature on inorganic fouling of brackish water RO membranes

A bench scale RO process simulator was operated in a batch concentration mode to determine the effects of product water recovery and feed water temperature on flux, rejection, and inorganic fouling by gypsum scale formation for simulated brackish water. As feed water temperature increased, salt rejection and concentration polarization decreased (reducing scale formation potential at a given recovery). However, gypsum crystal nucleation and growth rates increased with temperature. Specifically, at 15 and 25°C gypsum scale formation resulted in slow, steady flux decline at recoveries as low as 10–20%. At these temperatures, many small crystals formed over the entire membrane surface. In contrast, at 35°C flux decline was due to the increasing feed solution osmotic pressure — up to a recovery of about 70%. At this recovery, we observed a sudden, rapid loss of flux and a concomitant spike in feed water turbidity. Relatively few (in number), large crystals formed on the membrane towards the brine outlet of the RO simulator, but the entire membrane surface was covered with “needle-like” crystal fragments. The crystal fragments broke off from growing gypsum rosettes and re-deposited uniformly across the membrane forming a “cake layer” that caused the massive flux decline. These results suggest that high temperature operation of brackish water RO processes could enable higher recovery and lower energy consumption, but operating near the limiting recovery (at elevated temperature) creates an increased risk of a catastrophic fouling event.     

A Comparative study of organic fouling in hollow fiber and spiral wound membranes

The occurrence of flux decline in brackish water reverse osmosis (RO) plants due to dissolved organics is a topic of ongoing research. This type of organic fouling has also been found in seawater RO plants. A study was undertaken to compare organic fouling in hollow fiber and spiral wound membranes using a seawater feed that possessed a high concentration of huraic acid. This study was undertaken at an RO plant on Grand Cayman Island, British West Indies. The feed water came from a sea well and possessed a concentration of humic acid that varied between 35 and 60 mg/l.The hollow fiber membrane was operated at a recovery of 25% while the recovery with the spiral wound membrane varied between 5 and 25%. The performance data which included permeate flow, salt rejection, pressure drops across the membrane and analysis of the membranes for organic fouling were undertaken. This study compared the performance data and organic fouling between the hollow fiber and spiral wound meembranes.     

Treatment of Leather Plant Effluent by Membrane Separation Processes

Abstract

A scheme is proposed for the treatment of the leather plant effluent using membrane based separation processes. The effluent coming out from the various upstream units of the leather plant (except chrome tanning) are combined and a two step pressure driven membrane processes involving nanofiltration (NF) and reverse osmosis (RO) are adopted after a pretreatment consisting of gravity settling, coagulation, and cloth filtration. The entire membrane separation scheme is validated by conducting experiments under a continuous cross flow mode. A detailed parametric study for cross flow experiments is investigated to observe the effects of the operating conditions, i.e., the transmembrane pressure drop and the cross flow velocity on the permeate flux and quality for both NF and RO. Using a combination of osmotic pressure and solution diffusion model for both NF and RO, the effective osmotic pressure coefficient, solute diffusivity, and the solute permeability through the membrane are obtained by optimizing the experimental permeate flux and concentration (in terms of total dissolved solids) values for this complex industrial effluent. The BOD and COD values of the finally treated effluent are well within the permissible limits (in India).     

Application of reverse osmosis to automotive electrocoat paint wastewater recycling

Field and bench-scale reverse osmosis experiments on wastewater derived from automotive electrocoat paint operations are reported. Field experiments were performed continuously over a six-month interval using a reverse osmosis unit equipped with cellulose acetate membranes. An operating pressure of 3100 kPa (450 psi) and a temperature of 24–27°C was maintained. These RO experiments were unusual for two reasons. First, certain solutes in the feedwater were allowed to permeate through the membrane along with water. This was done to permit solute recycling and reuse. Since these solutes (ethyl, butyl and hexyl glycol ethers) were present in a 3% total concentration in the feedstream, their permeation through the membrane eliminated a flux reduction mechanism. This would arise from the increased upstream osmotic pressure if these solutes were concentrated. Second, the experiments were performed under conditions where colloidal lead was present. This caused limited membrane fouling which was controlled via weekly cleaning with dilute lactic acid. Some supporting bench-scale RO experiments were carried out to probe factors influencing this RO application.     

Control of scale formation in reverse osmosis by membrane rotation

Scale formation of soluble salts is one of the major factors limiting the application of reverse osmosis (RO) membranes. In this study, rotating RO, which takes advantage of Taylor-Couette flow instabilities to reduce concentration polarization and membrane fouling, was investigated as a novel method to control CaSO₄ scale formation. The permeate flux for rotating RO at ω = 180 rpm remains constant up to a volume concentration factor (VCF) of 4.2, while the permeate flux declines steadily with increasing VCF for no rotation. This is probably because vortices in rotating RO induce bulk crystallization and prevent scale particle deposition on the membrane surface. The anti-scaling effect in rotating RO increases with increasing rotational speed and depends to some extent on transmembrane pressure.     

Development of the MFI-UF in constant flux filtration

The MFI-UF, based on cake filtration, was developed to measure and predict the particulate fouling potential of feedwater to membrane filtration installations. The MFI-UF is determined in constant pressure filtration with the flux deceasing during the test. However, many membrane systems, e.g., reverse osmosis (RO), operate at constant flux with pressure increasing when fouling occurs. As both pressure and flux contribute to cake compression, determining the MFI-UF in constant flux with correction to the flux of an RO system is expected to more closely simulate particulate fouling Therefore, this research investigated the development of the MFI-UF test in constant flux filtration applying low (tap water) and high fouling (diluted canal water) feedwater. Preliminary experiments were promising; the fouling index (I) (and hence the MFI-UF) of all feedwater could be determined within 2 h under constant flux filtration. Cake filtration was demonstrated as (1) a minimum in the fouling indexvs time plot and (2) by linearity of the fouling index with feedwater particulate concentration. The fouling index increased with increasing applied flux due to cake compression. Further investigation at higher and lower applied flux is required to identify a reference test flux and to develop a method to correct the fouling index to the reference test flux and/or the flux of a membrane filtration system The fouling index can then be applied in a model to predict fouling.     

Osmotic power production from seawater brine by hollow fiber membrane modules: Net power output and optimum operating conditions

This study analyzes the net energy output and optimum operating conditions for osmotic power generation from seawater brine based on the currently available hollow fiber membranes in the module scale. Factors that are influential on membrane performances, such as external concentration polarization, internal concentration polarization, salt reverse diffusion, and dilution have been taken into account. Net power density is defined and applied to characterize the efficiency of the PRO system, in terms of power production minus pumping energy, pretreatment cost and energy consumption by pressure drop in the membranes. When using 1 M NaCl as the draw solution and 10 mM NaCl as the feed, it is found that up to 7 W m^?2 net power density can be harvested by the PRO system depending on the water sources. Coupling with the existing RO plant is highly beneficial in terms of readily available high pressure source, high salinity and less or negligible pretreatment costs for the draw solution. Sources with higher salt concentrations are preferred. The optimum hydraulic pressure, module length, flow rate to membrane area ratio and feed to draw flow rate ratio have also been analyzed to maximize the net power output. In addition, implications on hollow fiber development are discussed. Fibers with high water permeability, lower structural parameter, good mechanical stability, better fouling resistance, and outer‐selective configurations are recommended for further studies. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1216–1225, 2016     

A Study on Concentration Polarization in Ultrafiltration

Abstract

A finite-difference solution of coupled transport equations for momentum and solute continuity is presented to model the concentration polarization in a tubular ultrafiltration (UF) system. The model includes the effects of solute osmotic pressure and solute rejection at the membrane surface, axial pressure drop and resistance of the gel layer. This provides a fundamental understanding of the dynamics of various operating parameters on concentration polarization and transmembrane flux. Simulation results are presented for a wide range of operating variables to show their effects on local variation of solute concentration and transmembrane flux. The numerical results were also compared with previously published experimental data, which shows that a concentration polarization model based on constant membrane permeability (usually obtained from pure water flux data) grossly overestimates the flux behavior. If the effect of gel polarization is included, the model can predict the actual permeate flux very closely. Thus, in modeling ultrafiltration, one needs to be careful in using the appropriate membrane permeability terms. The commonly used intrinsic membrane permeability which is usually a constant, may not describe the true flux behavior in ultrafiltration. Actually the nature of the feed, solute-surface interaction and gel layer formation control the effective permeability, which varies axially along the membrane length.     

Influence of filtration conditions on the performance of nanofiltration and reverse osmosis membranes in dairy wastewater treatment

Filtration performance and fouling of nanofiltration (NF) and reverse osmosis (RO) membranes in the treatment of dairy industry wastewater were investigated. Two series of experiments were performed. The first one involved a NF membrane (TFC-S) for treating the chemical-biological treatment plant effluents. The second one used a RO membrane (TFC-HR) for treating the original effluents from the dairy industry. The permeate flux was higher at higher transmembrane pressures and higher feed flowrates. The curves of permeate flux exhibited a slower increase while the feed flowrate decreased and the pressure increased. Membrane fouling resulted in permeate flux decline with increasing the feed COD concentration. Furthermore, the flux decline due to the COD increase was found higher at higher pressures for both NF and RO membranes.     

Processing of composite industrial effluent by reverse osmosis

Reverse osmosis (RO) is a well‐established process for water desalination and effluent treatment and it is anticipated that its application could be extended to complex mixtures of industrial effluents. Pilot‐scale experiments using a spiral‐wound thin‐film composite (TFC) polyamide membrane were carried out to investigate the potential of RO for processing a composite effluent, which was a mixture of various wastewaters from bulk drug and pharmaceutical factories. Separation performance was evaluated at various feed pressures (0–70 bar) and feed concentrations (2–30 gdm^?3), and was found to improve with increasing pressure. High rejection of dissolved solids (～98%), COD, BOD and almost complete removal of color were achieved with reasonable flux rates and water recovery. The effect of concentration polarization and fouling on flux and rejection rates as functions of time was evaluated. An approximate cost estimate for an aerobic process vis‐à‐vis a RO membrane process for treatment of the composite effluent is presented. Copyright © 2003 Society of Chemical Industry     

Boron removal from seawater using high rejection SWRO membranes — impact of pH, feed concentration, pressure, and cross-flow velocity

The main objective of this work was to investigate boron removal from seawater using two commercial high rejection SWRO membranes. The impact of solution pH, feed concentration, pressure, and cross-flow velocity on boron rejection and permeate flux was determined. The membranes used were the Toray™ UTC-80-AB and Filmtec™ SW30HR. A lab-scale cross-flow flat-sheet configuration test unit was used for all RO experiments. Seawater sample was collected from the Mediterranean Sea, Alanya-Kızılot shores, south Turkey. For all experiments, mass balances were between 91% and 107%, suggesting relatively low loss of boron on membrane surfaces during 14 h of operation. Operation modes did not have any impact on boron rejection, indicating that boron rejection were independent of feedwater boron concentrations up to 6.6 mg/L. For both membranes, much higher boron rejection were obtained at pH of 10.5 (>98%) than those at original seawater pH of 8.2 (about 85–90%). Permeate boron concentrations less than 0.1 mg/L were easily achieved at pH 10.5 by both membranes. The dissociated boron species are dominant at this pH, thus both electrostatic repulsion and size exclusion mechanisms are responsible for the higher boron rejection. The rejection of salts in seawater did not correlate with boron rejection at constant conditions. For each membrane type, permeate fluxes at constant pressure were generally lower at pH of 10.5, which may be partially explained by membrane fouling and enhanced scale formation by Mg and Ca compounds from concentration polarization effect at higher pH values. While somewhat higher boron rejection was found for one membrane type as the pressure was increased from 600 to 800 psi, increasing pressure did not affect boron rejection for the other membrane. Feed flowrate thus the cross-flow velocity (0.5–1.0 m/s) did not exert any significant impact on boron rejection at constant conditions.     

Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes

The influence of colloidal fouling and feed water recovery (or concentration factor, CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated. Fouling experiments were carried out using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate the CF and recovery as commonly observed in full-scale RO/NF systems. For feed waters containing only salt (NaCl), permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes. On the other hand, a sharp decrease in permeate flux and significant decline in salt rejection with increasing CF were observed under conditions where colloidal fouling takes place. For both RO and NF membranes, the marked permeate flux decline was attributed to the so-called “cake-enhanced osmotic pressure”. The decline in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes. In all cases, the decline in salt rejection was higher under conditions of more severe colloidal fouling, namely at higher ionic strength and initial permeate flux.     

膜形态、膜污染和浓差极化对流动电位的影响

引　言膜与水溶液接触后 ,由于自身所具有的固定荷电基团的静电吸附或膜表面ｖａｎｄｅｒＷａａｌｓ力对溶液中某些反离子的不等量吸附作用 ,使被吸附的离子紧贴在固体表面 ,形成一个固定的吸附层 ,叫Ｓｔｅｒｎ层 ,同时从Ｓｔｅｒｎ层到液体内部又形成了扩散层 ,带电膜的表面与电解质溶液相对移动时在Ｓｔｅｒｎ层和扩散层之间形成一个剪切面 ,剪切面与溶液体相的电位差即 ζ电位 ,其大小与膜表面吸附离子的性质与数量有关 ,反映出膜表面荷电性能 .多年来 ,许多关于膜荷电性能的研究工作表明膜表面的荷电会影响膜的通量和对溶液中带电微粒…     

Rotating reverse osmosis for water recovery in space: influence of operational parameters on RO performance

Rotating reverse osmosis (RO), which is based on Taylor-Couette flow, offers a means to minimize flux decline due to concentration polarization and membrane fouling. However, the operating conditions play a significant role in determining the effectiveness of the system. In this study, the effect of operating conditions on system performance was explored using a theoretical model. Flux, rejection, recovery, and theoretical power consumption were calculated for a wide variety of operating parameters including transmembrane pressure, rotational speed, and concentrate flow rate. Flux and rejection increase with increasing transmembrane pressure and rotational speed. Operating in the vortical flow regime enhances the filtration performance. Higher concentrate flow increases flux, but decreases recovery. The power consumption for rotating RO is similar to that for conventional RO except at very high rotational speeds.     

Interactions controlling biopolymer fouling of reverse osmosis membranes

Laboratory experiments and model calculations were performed to elucidate the fundamental interactions that control organic fouling in reverse osmosis (RO) processes. Bovine serum albumin and alginic acid were selected as model aquatic organic macromolecules (organic foulants). An extended Derjaguin-Landau-Verwey-Overbeek (DLVO) characterization analysis was used to elucidate mechanisms of organic matter fouling on a commercial, polyamide composite RO membrane. Surface tension parameters derived from contact angle analyses are used to demonstrate that the apparent thermodynamic stability of macromolecules determines and adhesive free energy between membranes and macromolecules explained the observed differences in flux decline. Further, foulant–membrane and foulant–foulant interfacial forces helped explain why hydrophilic macromolecules formed polarization layers causing minimal flux decline, while hydrophobic macromolecules formed gel (or cake) layers that led to severe flux decline.     

Short-term fouling propensity and flux behavior in an osmotic membrane bioreactor for wastewater treatment

The short-term fouling behavior of forward osmosis (FO) membrane in an osmotic membrane bioreactor (OMBR) was investigated, using NaCl or MgCl₂ as the draw solutions. The effect of membrane orientation, mixed liquor suspended solids (MLSS) concentration and draw solution (DS) osmotic pressure on water flux and membrane fouling behaviors was examined, along with the effects of simulated elevated salinity on sludge properties and on membrane fouling. Water flux and membrane fouling were not significantly affected by both MLSS concentration (4.91–12.60 g/L) and osmotic pressure (3.0–15.0 MPa), but were severely affected by elevated salinity, due to changes in activated sludge properties, in particular the increase in extracellular polymeric substances (EPS) and sludge hydrophobicity. MgCl₂ as the DS showed more significant influence on activated sludge properties and membrane fouling than NaCl but gave rise to lower salt accumulation. Analyses of the membrane foulants showed that small sludge floc/particles and EPS (in particular, proteins) were enriched in the fouling layer. UPLC–MS/MS analyses of the proteins showed that hydrophobic proteins were the main cause of membrane fouling.     

ULTRAFILTRATION FLUX AND REJECTION CHARACTERISTICS OF BLACK LIQUOR AND POLYETHYLENE GLYCOL

Comparative experimental studies were carried oul on ultrafiltration with black liquor, and polyethylene glycol as a standard molecule, using an asymmetric membrane in a stirred batch cell. Effects of pressure, concentration, and stirrer speed were studied on flux and salute rejection characteristics for both the solutes.

The experimental results were analysed using the osmotic pressure limitation model to account for concentration polarization. It was found that, apart from the rise in osmotic pressure with pressure and concentration, there was a uniform rise in the polarization layer resistance (R_p) with membrane surface concentration (c_m). The results were correlated by the equation:

where ∝ and βt are constants. The membrane was characterized by determining the solution permeability (P_m) and reflection coefficient (a). The value of a was found to be close to unity—a representative value for high rejection membranes.     

Analysis of Laminar flow precipitation fouling on reverse osmosis membranes

Precipitation fouling on reverse osmosis membranes was analyzed by the boundary layer integral method for laminar flow in a parallel plate passage. Flux decline and concentration polarization levels are governed by a complex coupling between precipitation kinetics, mass transfer, and the deposit structurePrecipitation at the membrane relieves the concentration polarization of the precipitating salt. Concentration polarization is also relieved by the flux decline accompanying the increased hydraulic resistance of the growing deposit. As precipitation rate increases with growing concentration salt convection into the boundary layer is balanced by the rate of precipitation. In the usual case where an inert salt is also present, the concentration polarization of the precipitating salt may pass through a maximum. Permeate flux is reduced by the increasing osmotic pressure of the inert salt, thereby reducing the concentration polarization of the precipitating salt.The analysis stresses the importance of distinguishing between those effects mainly attributable to precipitation kinetics and those which are primarily a function of the deposit structure. Flux decline data alone are inadequate to elucidate kinetic aspects of precipitation fouling phenomena.     

The effects of performance and cleaning cycles of new tubular ceramic microfiltration membrane fouled with a model yeast suspension

The efficiency of crossflow microfiltration processes is limited by membrane fouling and concentration polarization leading to permeate flux decline during operation. The experiments that were carried out in the laboratory were conducted to determine and investigate the performance, behaviour and the fouling susceptibility of new ceramic tubular microfiltration membranes during the crossflow filtration of yeast suspensions. The tubular membranes of nominal pore size 0.5 microns were fouled over a varied range of concentration, temperatures, pH, crossflow velocities and system pressures. The typical filtration conditions were at a temperature of 25°C, a system pressure of 1.5 bar and a concentration of 0.03 g/L yeast suspension. These parameters varied during subsequent investigations. After each experiment, the membrane and the rig were cleaned using a three stage cleaning process and was reused in order to replicate industrial filtration conditions. The effects of repeated fouling and cleaning cycles upon membrane flux over time and cleaning efficiency are investigated and their influence over time is also documented. For every experiment, the flux data was recorded over a 50 min period and the membrane was changed after the PWF declined considerably due to excessive fouling over time. Chemical cleaning consisted of a sequential application of a 1% caustic solution through the rig followed by a 2% hypochlorite solution and a 2% nitric solution, all at 50°C. The permeate flux was shown to decrease with filtration time during the development of the fouling layer. Once the fouling layer was developed and established, there appeared to be a leveling of permeate flux. The experimental results are presented in the report and the flux values at different conditions are presented.     

Comparison of Different Pitch Lengths on Static Promoters for Flux Enhancement in Tubular Ceramic Membrane

Abstract

The use of turbulence promoters in membrane based processes have been investigated and are increasingly been used in industrial applications to minimize fouling and enhance the membrane flux. The efficiency of crossflow microfiltration is limited by membrane fouling and concentration polarization leading to flux decline during operation. A detailed study was carried out in the microfiltration of yeast suspensions using an in‐house rig and three different static turbulence promoters of varying pitch lengths. The design of the promoters incorporates a helical thread around the length of the insert, which induces turbulent flow through the membrane. This promotes good mixing of the feed fluid and minimizes concentration polarization effects. The testing of tubular membranes with the static inserts has been carried out and the results are included in the report. The pitch lengths used were 7 mm, 10 mm, and 14 mm and the parameters investigated included temperature, CFV, concentration of feed suspension and pressure. The flux decline data was recorded over a 50 minute filtration cycle and the cleaning protocol was employed after every cycle to restore the permeability of the membrane. A comparison of the membrane performance and efficiency of the three swirls inserts of varying pitch lengths together with a comparison of the degree of total, reversible, and irreversible fouling data amongst others are reported and discussed. The results obtained during the investigations of flux enhancement via static turbulence promoters into the tubular membranes are presented and are selected to differentiate the efficiency of the inserts and the degree of fouling associated with them.