Reverse Osmosis Membrane Desalination Research Articles

Minimizing energy consumption in reverse osmosis membrane desalination using optimization-based control

Abstract This work focuses on the design and implementation of an optimization-based control system on an experimental reverse osmosis (RO) membrane water desalination process in order to facilitate system operation at energy optimal conditions. A nonlinear model for the RO process is derived using first principles and the model parameters are computed from experimental data. This model is combined with appropriate equations for reverse osmosis system energy analysis to form the basis for the design of a nonlinear optimization-based control system. The proposed control system is implemented on UCLA’s experimental RO desalination system and its energy optimization capabilities are evaluated.

Recent developments in reverse osmosis desalination membranes

Reverse osmosis (RO) desalination is one of the main technologies for producing fresh water from seawater and other saline water sources. The membrane properties greatly affect the water productivity and energy costs in the reverse osmosis desalinatin processes. Recent years have seen significant research efforts devoted to developing high-performance RO membranes. This article reviews recent activities in the development of RO membranes with improved flux and salt rejection, chlorine tolerance, fouling resistance and thermal stability. In particular, this review mainly focuses on the modification of current polymeric membrane materials, and synthesis and separation performance of new polymer membranes, inorganic membranes and mixed matrix membranes.

Desalination membranes from pH-controlled and thermally-crosslinked layer-by-layer assembled multilayers

We introduce a novel and facile approach to improve the desalination performance of pressure-driven layer-by-layer (LbL) assembled membranes. Electrostatic LbL multilayers composed of weak polyelectrolytes (PEs), e.g., cationic poly(allylamine hydrochloride) (PAH) and anionic poly(acrylic acid) (PAA), were prepared on commercial polysulfone substrates. In order to measure the ion rejection and permeate flux of these membranes, the ionic concentration of the feed solution and operating pressure were fixed at 2000 ppm NaCl and 20 bar, respectively. It was observed that the crosslinked (PAH pH 7.5/PAA pH 3.5)n=10,20 multilayers, which were assembled at the pH conditions allowing a low charge density of the respective PEs, show relatively high ion rejection compared to other multilayers. This result suggests that the optimal structures for desalination membranes should contain a large amount of freely charged groups with densely-packed structures via crosslinking. Finally, the recycling process was employed to further improve the desalination performance. In this case the (PAH pH 7.5/PAA pH 3.5)n=10,20 multilayers exhibited the ion rejection up to 99.8%. This pH-controlled and thermal crosslinking method suggests a new route for the design of well-defined desalination reverse osmosis membranes based on LbL multilayers.

Leveraging Resist Chemistry Research for Water Purification Membrane Technology

This paper gives examples of the connection of new research on water purificiation membranes to earlier research on resist chemistry. Two new membrane materials are reported here. The first is a reverse osmosis desalination membrane material based on a new hexafluoro alcohol-modified aromatic polyamide. This polymer is made in a reactive casting operation using interfacial polycondensation. The membrane has intriguing performance with pH-dependent properties. The second membrane presented here is designed to have fouling resistance. We found through the course of this work that the material had intrinsic nanofiltration properties and data is presented that supports this finding. Research experiences in high performance functional materials can accelerate new efforts in other areas of technology, in this case sustainability.

Oxidation-reduction potential in saline water reverse osmosis membrane desalination and its potential use for system control

Chlorination and dechlorination are required in most saline water (brackish and seawater) reverse osmosis membrane desalination processes, regardless of whether a membrane or conventional chemical conditioning is employed as a pre-treatment. Wastage of chemicals occurs when the dosing process is not automated. Titrations of chlorinated water (using sodium hypochlorite) with sodium meta-bisulfite were undertaken using water of different salinities, which were obtained by varying volume ratios of seawater to deionized water (4:0, 3:1, 2:2, 1:3 and 0:4). Changes in reduction and oxidation potential (ORP) were studied with variations in chlorine doses (0 to 5 mg L-1NaOCl) and salinities. As expected, the ORP values were greater at higher than at lower chlorine doses for any given water. The greater the ratio of seawater to deionized water, the lower was the ORP value, with a difference of greater than 100 mV between the deionised water and the seawater at a high chlorine dosage. The results suggest that the ORP level would be lower for seawater than for brackish water in control of chlorine dose. The ORP decreased with sodium meta-bisulfite addition at high chlorine dosages ≤2 mg l -1) while the opposite was observed at low chlorine dosages (≥1 mg L-1). Free chlorine disappeared instantly following sodium meta-bisulfite addition. As more than 80% of the added chlorine was diminished in the seawater, 2 mg L-1 sodium meta-biosulfite was sufficient to reduce free residual chlorine to less than 0.1 mg L-1 as required for most of the cellulose acetate RO membrane in seawater desalination. To be safe and cost-efficient, a chemical dose controller can be developed using the ORP titration information.

Effect of Thermodynamic Restriction on Energy Cost Optimization of RO Membrane Water Desalination

Advances in highly permeable reverse osmosis (RO) membranes have enabled desalting operations, in which it is practically feasible for the applied pressure to approach the osmotic pressure of the exit brine stream. However, energy cost remains a major contributor to the total cost of water produced by RO membrane desalination. Reduction of the overall cost of water production represents a major challenge and, in the present work, various elements of water production cost are evaluated from the viewpoint of optimization, with respect to various costs (energy, membrane area and permeability, brine management, and pressure drop), as well as the important thermodynamic cross-flow constraint, utilization of energy recovery devices, and operational feed and permeate flow rate constraints. More specifically, in the present study, an approach to the optimization of product water recovery at pressures that approach the osmotic pressure of the exit brine stream is presented via several simple RO process models that...

Diagnostic characterization of gypsum scale formation and control in RO membrane desalination of brackish water

The utility of a laboratory diagnostic approach was investigated for a systematic evaluation of the impact of operating conditions, membrane type, and antiscalant effectiveness on membrane mineral salt scale formation. The study focused on low-pressure reverse osmosis (RO) membrane desalting of brackish water with high potential for scaling by calcium sulfate dihydrate (gypsum). Gypsum scale formation, in the surface crystallization regime, was studied in a plate-and-frame RO system via flux decline measurements and membrane surface imaging. Membrane scale coverage increased with increasing level of surface gypsum supersaturation. However, membrane scaling propensity, as quantified by both the surface scale coverage and flux decline, indicated that there were measurable differences among the four commercial membranes studied, particularly in response to antiscalant treatment. Axial development of scale coverage indicated higher scale coverage as the membrane channel exit was approached, consistent with increased level of gypsum supersaturation along the membrane. As a result, antiscalant effectiveness, at a given dosage, was also location dependent. The present study illustrates a practical approach of combining flux decline and surface scale coverage measurements as a means of quantifying the impact of scale formation and its dependence on operating conditions and membrane type. Clearly, further studies are needed to elucidate the impact of membrane surface roughness and charge as well as module geometry on surface mineral salt scale formation and its mitigation.

Cellulose membranes for reverse osmosis part II. Improving RO membranes prepared from non-woody cellulose

Experiments were carried out to minimize the stages of preparing reverse osmosis (RO) desalination membranes at economical cost and to improve the transport properties of RO membranes prepared from non-wood fibrous materials (cotton linters and bagasse pulp) to approach those prepared from imported viscose pulp and purchased cellulose acetate (see Part I). Further study was carried out on examining the effects of long-term testing and subsequent washing of membranes by a special mixture. Finally the quality of the product water reverse osmosis was examined by comparing its water analysis with standard specification for drinking and domestic water.

Comparison of three pilot studies using Microza ® membranes for Mediterranean seawater pre-treatment

Until recently, in desalination plants seawater was pre-treated with a combination of conventional techniques such as air flotation, clarification, sand filtration and cartridge filtration prior to spiral wound reverse osmosis desalination membranes. Microfiltration or ultrafiltration membranes are now considered as an alternative solution for seawater pre-treatment. This paper describes pilot test results obtained at three different locations treating Mediterranean seawater using 0.1 μm hollow fiber membranes in Pall's Microza modules. Depending of the location, the pilots operated with or without pretreatment using ferric chloride and with or without specific flux maintenance (SFM) protocols performing daily highly chlorinated backwash. The study shows that system optimization yields stable, reproducible permeate flow and permeate SDI of 1.8 or less, with mean SDI of 1.43 when seawater was coagulated or SFM was performed. At these conditions, reverse osmosis system recovery is enhanced and total integrated system performance is optimized.

Low-pressure RO membrane desalination of agricultural drainage water

Agricultural drainage water is a complex mixture of dissolved and suspended chemical species and may contain a wide variety of microorganisms. The application of membrane systems for desalination of agricultural drainage (AD) water requires careful consideration of feed water quality, suitable membrane selection and operating conditions. In order to evaluate the potential applicability of low-pressure reverse osmosis (RO) to the treatment of AD water, a diagnostic approach to membrane selection and process evaluation was undertaken in support of a pilot field study in the California San Joaquin Valley. Five candidate membranes were evaluated in a diagnostic laboratory membrane system which provided an initial selection based on salt rejection and product water flux performance for model salt solutions of univalent and divalent cations. Biofouling potential of the selected membranes was also evaluated using two standards strains of bacteria. Preliminary pilot plant performance, based on the selected membranes, was encouraging and has provided the basis for long-term pilot plant testing at higher recoveries to assess the impact of fluctuating AD water feed composition.

The use of microfiltration membranes for seawater pre-treatment prior to reverse osmosis membranes

Since the early 1980s, microfiltration and ultrafiltration have been widely used for drinking water applications. More recently, membranes have started to be used in membrane bioreactors or on secondary wastewater effluents for reuse application. Until recently, in desalination treatment plants seawater was pre-treated with a combination of conventional techniques such as air flotation, clarification, sand filtration and cartridge filtration prior to spiral wound reverse osmosis (RO) desalination membranes. Microfiltration or ultrafiltration membranes are now considered as an alternative solution for seawater pre-treatment. This paper describes pilot test results obtained on Mediterranean seawater using a 0.1 μm hollow fiber Microza ® module. A pilot rig was operated at different fluxes ranging from 80 to 140 L/h.m 2 with and without ferric chloride addition. Run cycles between chemical cleanings varied between 10 days and 30 days depending on the operating conditions and seawater quality. During the tests, seawater silt density index (SDI) ranged between 6 to non-measurable while the microfiltered water SDI remained below 2 under the optimized operating conditions. With low and constant microfiltered water SDI values, it is possible to design a full-scale plant with fewer RO membranes and to expect longer Ro run cycles.

Development of reverse osmosis desalination membranes composition and configuration: Future prospects

The most common reverse osmosis (RO) membranes which attained the stage of economic application in desalination plants are made of cellulose acetate (CA) or polyamide (PA), in either hollow fiber (HF) or spiral wound (SW) configurations. The first application problem of CA membrane is its sensitivity to hydrolysis being both a polysaccharide and polyester. The resulting deacetylation of the surface film explains the observed loss of selectivity and reverse osmosis failure. In the hot countries of the Middle East this problem acquires particular dimensions in view of the accelerated rate of hydrolysis together with the usually higher frequency of biofouling and chemical cleaning. Failure of CA membrane is also due to the inadequate mechanical and thermal stability of CA polymer which result in progressive decline of RO performance due to membrane compaction and shrinkage. Results of comparative testing and experience in application of PA membranes in plants originally designed to use CA ones of capacities ranging between 25,000 to 66,000 m 3/d are reviewed. With only minor system modifications, this replacement led to generally more steady performance at a higher salt rejection and for a longer life time. Furthermore, a remarkable energy saving is achieved by the operation of RO plants at less than the half of the original operational pressure of CA membranes. On the other hand, the reported replacement of CA membranes by the PA ones is not as straightforward as would be indicated by the oversimplified system design projection programs of membrane producers. Aspects of the mentioned replacement are discussed in detail. Despite their higher surface charge and surface roughness, PA composite membranes did not show excessive biofouling susceptibility. Biofouling was investigated by destructive RO element autopsy, cell test of fouled sheet membranes, surface analysis by scanning electron microscopy and energy dispersive X-ray. The RO element configuration determines the hydrodynamic conditions inside the RO element. Due to this effect, the HF configuration, whether with PA or CA membranes, requires more critical pretreatment and has a lower response to cleaning. Recently introduced modifications to elements of the plate-and-frame configuration enabled successful treatment of high organics, wastewaters. Further systematic studies are required to optimize the performance of the now-available RO membrane polymers as well as to develop new ones. Membranes suitable to operate at high temperatures will enable higher process energy efficiency together with control of biofouling. Membranes of promoted hydrophobicity will need only low operational pressures and will have minimized fouling susceptibility by organics for application in industrial wastewater treatment. Chlorine-tolerant membranes will enable easier and more efficient treatment of water contaminated with microorganisms or sewage water.

Experimental RO facility to study the heating effect of raw water on the varying main parameters

Utilizing power plants in seawater desalination is one of the recent trends to cover the gap between the increasing water demand and the shortage in water resources in the new millenium. The fast development in the reverse osmosis (RO) desalination technique and membrane manufacturing technology renders it one of the promising desalination techniques to be coupled with power plants. Therefore, it is important to study all aspects related to thermal matching between the condenser cooling water outlet and RO system feedwater. In this concern an experimental RO facility was designed for Nuclear Power Plants Authority in Egypt (NPPA) to study the effect of the following parameters on the permeate quality, production, membrane life and aging and system's economy: feedwater temperature; feedwater pressure; recovery ratio. Three types of four-inch diameter spiral wound membranes were selected to carry out the experiments (Fluid Systems, FilmTec and Hydranautics). Based on statistical analysis, five of each membrane type was considered as an optimum sample size. The system design included: 1) Feedwater heating and temperature control facility, consisting of fresh water heater and fresh water to feedwater heat exchanger, in addition to permeate and brine to feedwater heat exchangers to recover its heat content. 2) Feedwater pressure regulating facility, consisting of hydraulic coupling and by pass throttling valve for fine tuning. Beachwells (two) were chosen for raw water feeding to minimize the plant cost and the pretreatment requirements. The control supervisory functions for the plant were designed to be implemented using a distributed control and data acquisition system.

Boron reduction performance of reverse osmosis seawater desalination process

In recent years, seawater desalination systems using reverse osmosis (RO) membranes have been constructed to settle the lack of drinking water. RO desalination membranes have high rejection for most of solutes in seawater. Japanese drinking water standards for the water quality of the permeate can be achieved except for boron. Therefore, the boron rejection needs to be considered in the design of the RO process and during the operation of the plant. Luckily, there is a simple and easy method to estimate boron concentration. In this paper, we report measured boron permeabilities and their relation to salt permeabilities using cross-linked polyamide membranes. Chemical degradation of the membranes affected these permeabilities to different degrees. Boron concentrations in the permeate were then calculated using a computer program that was based on the boron permeabilities calculated from the measured salt permeabilities. Results obtained were compared with actual data taken at a RO plant of Toray Industries, Inc., Ehime. The model data fitted the experimental result, well. It was also found that a relationship existed in the permeate between salt and boron concentrations and that the boron concentration can be obtained from measurement of the salt concentration.

XPS characterization of poly(methylhydrosiloxane)-modified cellulose diacetate membranes

The surface composition of a series of poly(methylhydrosiloxane) (PMHS)-modified cellulose diacetate (CDA) membranes was examined by X-ray photoelectron spectroscopy (XPS). The Si2p signal is a unique diagnostic feature for PMHS, thus allowing a straightforward determination of the relative proportion of this modifier on the surface of CDA. The surface compositions of the membranes under test reach readily a steady composition as a result of various treatments (initial PMHS concentration, solvent volume and reaction time). This parallels the performance of the membranes for reverse osmosis desalination. Chlorides were detected in the untreated CDA and the modified membrane for which salt rejection was poor. The Si2p binding energy indicates possible covalent grafting of the PMHS to CDA via an Si–O–C linkage.

Characterization of foulants by autopsy of RO desalination membranes

A study was undertaken to identify various types of scales that were responsible for shortening the useful life span of the membrane permeators in a commercial reverse osmosis (RO) desalination plant. Compositions of the raw and treated feed water and of the reject brine were determined using the inductively coupled plasma (ICP) spectrometry and ion chromatography (IC). Various scaling index calculations showed that the feed and brine were non-scale forming with respect to CaCO 3 (calcite), SrSO 4, CaSO 4.2H 2O (gypsum), and silica (SiO 2). Two completely fouled membrane permeators, retired from stage 1 and stage 2 of a commercial plant, were subjected to membrane autopsy using scanning electron microscopy (SEM), X-ray diffractometry (XRD), optical microscopy (OM), and energy dispersive x-ray florescence (XRF). The deposits were predominantly amorphous in nature. The membrane autopsy showed that CaCO 3, SrSO 4, and CaSO 4.2H 2O (gypsum) scales did not constitute a serious problem in the plant. The advanced phosphonate+polyacrylate based scale inhibitor had itself formed Ca phosphonate sludge, but the amount was quite small. Though below saturation, silica is believed to have been precipitated due to the catalyzing effect of trivalent Al 3+ and Fe 3+ ions. Iron fouling was the major cause of reduced life span of the membranes and, to a lesser extent, calcium-alumino-silicates.

Hollow fine fiber vs. spiral-wound RO desalination membranes Part 1: Pilot plant evaluation

The HFF and SW membrane permeators were installed in parallel units of a reverse osmosis (RO) pilot plant. Brackish feed water to the two units was treated with identical scale control treatment consisting of 6 ppm of sodium-hexa-meta phosphate (SHMP) and about 46 ppm (98% basis) of sulfuric acid. The pH of the acidified feed was about 6.4. Unit #1, employing the HFF membrane, was operated at feed pressure of 27.6 bar (400 psig) and unit #2, employing the SW membrane, was operated at 15.2 bar (220 psig). The water recovery in both units was 70%. The pilot plant trial lasted for 7,500 h. The techno-economic evaluation showed that performance of the SW membrane was far superior; its product TDS in 7500 h was 86–98 ppm whereas that of the HFF membrane increased from 80 ppm to 468 ppm. Water output of both membranes declined appreciably, but output of the SW membrane declined 12% less. The average electrical energy consumed by high pressure pump of the SW membrane was nearly half the energy consumed by the HFF membrane (0.70 kWh/m 3 product vs. 1.51 for HFF). Variations in the pressure drops across the HFF and SW permeators were modest (5–10 psi) due to amorphous nature of the scale deposits found on fibers of both membranes and due to significant decrease in feed flows experienced by the permeators. The pretreatment, the scale control treatment cost and water recovery being the same for the two units, the SW membrane produced excellent quality water (86 ppm) at half the electrical energy cost compared with the HFF membrane which produced 468 ppm water. The SW membrane out-performed the HFF membrane.

Post radiation grafting of vinyl acetate onto low density polyethylene films: Preparation and properties of membrane

Reverse osmosis membranes were prepared by the post radiation grafting of vinyl acetate onto low density polyethylene films. The factors affecting the grafting process such as radiation dose, monomer concentration and temperature on the grafting yield were studied. It was found that the dependence of the grafting rate on radiation intensity and monomer concentration was found to be of 0.64 and 1.4 order, respectively. The activation energy for this grafting system was calculated and found to be 4.45 kcal/mol above 30°C. Some properties of the grafted films such as specific electric resistance, water uptake, mechanical properties and thermal and chemical stability were investigated. An improvement in these properties was observed which makes possible the use of these membranes in some practical applications. The use of such membranes for reverse osmosis desalination of saline water was tested. The effect of operating time, degree of grafting and applied pressure on the water flux and salt rejection were determined. The results showed salt rejection percent over 90% and a reasonable water flux. A suitable degree of grafting of the membrane was determined as well as the optimum applied pressure.

Radiation effects on polymers—XI. Radiation grafting of acrylamide on to cellulose acetate reverse osmosis membranes

With the aim of improving properties of cellulose acetate membranes for reverse osmosis desalination, grafting was performed using high energy electrons. In this paper, the grafting parameters (radiation dose and method, monomer concentration, solvents, chain transfer agent and redox system) are considered; conclusions are drawn as to the grafting content and mode of grafting.

Microstructure and ordering in cellulose acetate and reverse osmosis membranes

AbstractThe performance of cellulose acetate reverse osmosis membranes for desalination or purification is greatly affected by the microstructure of the membrane. It is, therefore, highly desirable to characterize the microstructure and its dependence on preparation conditions and past history. In this study, various types of cellulose acetate powders, flakes, and solvent cast films have been characterized by differential scanning calorimetry and thermo‐optical analysis. It is shown that ordered microstructures exist in many of these samples and that this ordering can be intensified or diminished by suitable treatments. It is conjectured that a similar microordering occurs in the dense layer of asymmetric cast membranes as a result of solvent evaporation, gelation and annealing and that the extent of orientation and chain packing in the ordered regions greatly affects the performance of reverse osmosis membranes.