Mark Wilf

Concentration polarization in ultrafiltration and reverse osmosis: a critical review

Abstract A primary reason for flux decline during the initial period of a membrane separation process is concentration polarization of solute at the membrane surface This can occur in conjunction with irreversible fouling of the membrane as well as reversible gel layer formation Experimental and mathematical studies have been performed by various groups to gain a better understanding of concentration polarization phenomena in ultrafiltration and reverse osmosis This article critically reviews published studies on concentration polarization in both systems It presents progress made in determination of, for example, critical or limiting flux, and recommends specific models such as surface renewal, and experimental methods such as laser-based refractometry, for quantification of the problem.

Optimization of seawater RO systems design

The paper describes the configuration and operating parameters of current large seawater desalination systems. Major advances of RO seawater desalination technology that lead to a remarkable decrease of desalted water costs are evaluated. Process improvements that enable compliance with more stringent requirements of permeate water quality are discussed. Results of field tests conducted to demonstrate a new process approach are described. Some examples of process optimization resulting in lower power consumption and more efficient system operation are presented.

Effect of feed temperature on permeate flux and mass transfer coefficient in spiral-wound reverse osmosis systems☆

The objective of the present study was to analyze and model concentration polarization in spiral-wound seawater membrane elements. In particular, the influence of feed temperature, salinity and flow rate on permeate flow and salinity was evaluated. Membrane lifetime and permeate fluxes are primarily affected by the phenomena of concentration polarization (accumulation of solute) and fouling (i.e., microbial adhesion, gel layer formation and solute adhesion) at the membrane surface. Results show that the polymer membrane is very sensitive to changes in the feed temperature. There was up to a 60% increase in the permeate flux when the feed temperature was increased from 20 to 40°C. This occurred both in the presence and absence of solute. Surprisingly, the permeate flux appears to go through a minimum at an intermediate temperature. There was up to a 100% difference in the permeate flux between feed temperatures of 30 and 40°C. The differences were statistically significant (p<0.05). A doubling of the feed flow rate increased the permeate flux by up to 10%, but only at a high solute concentration. Membrane parameters were estimated using an analytical osmotic pressure model for high salinity applications. A combined Spiegler-Kedem/film theory model described the experimental results. The modeling studies showed that the membrane transport parameters were influenced by the feed salt concentration and temperature.

Application of low fouling RO membrane elements for reclamation of municipal wastewater

Abstract Membrane fouling encountered in reclamation of municipal wastewater represents serious design and operational concern. This is because the municipal effluent, after secondary treatment, contains high concentrations of suspended particles, colloids and high level of biological activity. Application of membrane technology for treatment of municipal wastewater requires very extensive pretreatment prior to the RO process. The conventional multi-step treatment approach, based on disinfection, flocullation, clarification and media filtration, still produces RO feed water with very high fouling potential. Extensive field results from pilot and commercial RO system operation indicate high fouling rates, regardless of the nature of membrane material: cellulose acetate or composite polyamide. Membrane cleaning has to be applied very frequently in order to maintain the design product capacity. Recently a new pretreatment technology is being used in RO processing of municipal effluent. It consists of backwashable microfiltration and ultrafiltration membrane elements in a capillary configuration. This new membrane pretreatment technology is capable to treat secondary effluent and maintain stable performance of filtrate flow and operating pressure. The capillary technology produces RO feed water of a very high quality. The capillary filtrate has a much lower concentration of colloidal and suspended particles than can be produced in a conventional pretreatment process. In reclamation plant that uses membrane pretreatment the fouling rate of the RO membranes operating on capillary effluent has been reduced significantly. The fouling rate has been reduced even more by introduction of new generation of low fouling composite membranes (LFC1). In low fouling membranes the surface of the salt rejection layer has been modified to make it more hydrophilic and reduce its affinity to dissolved organics. Field results of operation of the low fouling membranes in municipal wastewater reclamation systems indicate that the fouling rate is very low, comparable with that observed in RO operation with clean well water. The low fouling rate is attributed to a lower rate of adsorption of dissolved organics on the LFC1 hydrophilic membrane surface. Apparently, in the low fouling membranes, the bonding between the adsorbed organic layer and the membrane surface is relatively weak. The paper will describe properties of low fouling membrane technology and present results of its application with conventional and capillary pretreatment. Performance in municipal wastewater reclamation applications will be compared with that of conventional membrane technology. Results of operation of capillary UF membrane pretreatment on municipal secondary effluent and optimization of operating parameters will be described as well.

Improved performance and cost reduction of RO seawater systems using UF pretreatment

RO seawater systems that operate on a surface feed water originating from an open intake source require an extensive pretreatment process in order to control membrane fouling. Considerations of long-term performance stability lead to a design concept of operation at a low permeate flux rate and low permeate recovery. In recent years the nominal performance of composite seawater membrane elements has improved significantly, and new effective water microfiltration technologies have been introduced commercially. These developments can be utilized to improve the quality of surface seawater feed to the level comparable to, or better than the water quality from the well water sources. These new developments enable a more advanced RO system design which should result in increased reliability and lower water cost.

Performance limitation of the full-scale reverse osmosis process

The mechanisms controlling the performance of a full-scale reverse osmosis (RO) process (typically a pressure vessel holding six 1 m long modules in series) under various operating conditions are carefully examined in this study. We demonstrate that thermodynamic equilibrium imposes a strong restriction on the performance of a full-scale RO process under certain circumstances. This thermodynamic restriction arises from the significant increase in osmotic pressure downstream of an RO membrane channel (owing to the phenomenon of salt accumulation within the RO channel as a result of permeate production). The behavior of the full-scale RO process under thermodynamic restriction is much different from that of the process when it is controlled by mass transfer. The conditions for an RO process to shift from mass transfer-controlled regime to thermodynamically restricted regime are delineated and discussed.

Field evaluation of capillary UF technology as a pretreatment for large seawater RO systems

RO seawater systems operating on a surface feed water, originating from an open intake source, require an extensive pretreatment process in order to control membrane fouling. Considerations of long-term membrane performance stability have lead to an initial design concept of operation of seawater RO systems at a low permeate flux rate and low permeate recovery. In recent years the nominal performance of composite seawater membrane elements has improved significantly, and in parallel new backwashable microfiltration and ultrafiltration capillary technologies have been introduced commercially. This new membrane technology can be utilized to treat seawater from a surface sources. Use of membrane capillary technology as a pretreatment step can improve quality of the surface feed water to a level comparable or better than the water quality from the well water sources. A better feed water quality enables more effective optimization of operating parameters in the RO systems. Both permeate flux and system recovery rate can be increased considerably without creating membrane fouling conditions. Operation of seawater systems at higher permeate flux and recovery rate results in improved economics of RO seawater desalting. Field evaluation of hybrid membrane systems consisting of UF membrane pretreatment unit and a RO seawater unit was conducted subsequently at two test sites. The first test site was at the Red Sea (Eilat site) and the second test site was on the Mediterranean (Ashdod site). The RO membranes were commercial seawater elements in spiral wound configuration. The UF equipment utilized capillary backwashable elements operated in dead end flow mode. For comparison, a second pilot system consisting of conventional pretreatment and an RO unit was operated in parallel at the above sites. The conventional pretreatment unit included in line flocculation followed by media filtration. The tests were conducted over a period of two years. Raw water quality reflected seasonal changes of composition and weather conditions. The performances of UF and RO equipment were evaluated over a wide range of operating parameters such as recovery and flux rate. Field results were used to project and compare the economics of the seawater RO desalting process using conventional and membrane pretreatment. This paper will describe the experimental procedure and results of parallel operation of an integrated membrane unit and a conventional RO sweater system. The economic analysis of both designs based on local site conditions, will be provided as well.

Design consequences of recent improvements in membrane performance

The introduction of low pressure, high rejection ESPA membranes enables the achievement of one of the goals of commercial RO technology: the ability to operate a given RO system at the minimum value of feed pressure, which is the pressure equal to the osmotic pressure of the concentrate plus the pressure losses along the system. However, at the operating conditions of high feed water temperature, high feed water salinity or high permeate recovery rate, conditions could be created of excessive permeate flux rate from the lead elements and negligible NDP at the end of the system. This is due to a very high specific permeate flux value of the ESPA membranes. Such operating conditions result in higher permeate salinity and the possibility of an increasing fouling rate. Corrective measures could include the use of an interstage pump or a hybrid membrane system design. The hybrid design consists of the use of membranes with lower specific permeate flux in the lead position (first stage), followed by the high flux membranes. The hybrid design does not utilize the full extent of energy savings possible with the ESPA membranes but provides more uniform flux distribution and improved permeate quality. Due to the low values of NDP required by the ESPA membranes, a high permeate flux system design becomes more economically attractive. There is a possibility of improving the efficiency of a RO system operation by changing the configuration of array. These new system configurations, however, require design optimization to assure long-term stable performance.

Emergence of thermodynamic restriction and its implications for full-scale reverse osmosis processes

The production rate of permeate in a reverse osmosis (RO) process controlled by mass transfer is proportional to the net driving pressure and the total membrane surface area. This linear relationship may not be the only mechanism controlling the performance of a full-scale membrane process (typically a pressure vessel holding six 1-m-long modules in series) which utilizes highly permeable membranes. The mechanisms that control the performance of an RO process under various conditions were carefully examined in this study. It was demonstrated that thermodynamic equilibrium can impose a strong restriction on the performance of a full-scale RO process under certain circumstances. This thermodynamic restriction arises from the significant increase in osmotic pressure downstream of an RO membrane channel due to the accumulation of rejected salt within the RO channel as a result of permeate water production. Concentration polarization is shown to have a weaker influence on the full-scale RO process performance than the thermodynamic restriction. The behavior of the process under thermodynamic restriction is quite different from the corresponding behavior that is controlled by mass transfer. The transition pressure for an RO process to shift from a mass transfer controlled regime to a thermodynamically restricted regime was determined by the basic parameters of the full-scale RO process.

Restoration of commercial reverse osmosis membranes under field conditions

Abstract A membrane restoration program was conducted at the site of a large (over 3 MGD) reverse osmosis brackish water desalting plant. The procedure applied consisted of cleaning the membranes with an alkaline solution at low pressure followed by dosing with a coloidal solution of high molecular weight polymer at high pressure. The initial restoration tests were performed on single 8″ elements, which were taken out from the commercial desalting unit due to the excessive salt passage. After obtaining positive results with a few single elements the restoration procedure was applied on the elements as installed in the desalting units. One of the units, equipped with 8″ and 10″ hollow fiber elements, had an average salt passage of 14%. Following the restoration it was reduced to a level of 6% to 10% and was maintained at that level for several thousand operating hours by applying continuous online treatment. The reduction of salt passage was accompanied by a decrease of about 10% in productivity. A similar treatment was applied to a number of old 8″ polyamide, spiral wound elements, which had a salt passage in the range of 14% to 24%, and were therefore disconnected from the unit. As a result of the treatment, an average decrease in salt passage of 40%, accompanied by a tolerable (≈10%) decrease of productivity, was obtained. The treated spiral wound elements were reinstalled in the desalting unit and operated for over a thousand hours with no significant increase of salt passage.