Reverse osmosis membrane applications

Xu P, Drewes JE, Bellona C, Amy G, Kim T-U, Adam M, Heberer T (2005) Rejection of emerging organic micropollutants in nanofiltration-reverse osmosis membrane applications. 

Z. Amjad, ed.. Reverse Osmosis Membrane Technology, Water Chemistry and Industrial Applications, Van Nostrand Reiohold, New York, 1993. 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

The most significant application of reverse osmosis has been in the field of desalination to produce drinking water. Other important apphcations include the treatment of industrial waste water, concentration of fruit juices, and concentration of weak solutions such as aqueous ethanol [3-6]. The rest of the chapter will focus almost entirely on semi-permeable membranes used for reverse osmosis based applications. We chose this focus in view of the importance of reverse osmosis as a rather efficient separation technique for separating a wide range of solutions, especially very dilute solutions—which are usually notoriously difficult to handle using conventional techniques such as distillation. 

Membranes. Photopolymer chemistry is being applied to the design and manufacture of a variety of membrane materials. In these applications, photopolymer technology is used to precisely define the microscopic openings in the membrane as it is being formed or to modify an existing membrane. Some of the applications of photopolymer chemistry to membranes include the modification of ultrafiltration membranes (78) and the manufacture of amphiphilic (79), gas permeable (80), untrafiltration (81), ion-selective electrode (82) and reverse osmosis membranes. 

Reverse osmosis membrane process, 27 637 Reverse osmosis membrane cleaning citric acid application, 6 647 Reverse-osmosis membranes, 75 811, 825 development of, 75 797 Reverse osmosis models, 27 638-639 Reverse osmosis permeators, 76 19 Reverse osmosis seawater desalination process, 26 85 Reverse osmosis systems blending in, 26 80-81 brackish and nanofiltration, 26 80-83 Reverse osmosis technology... 

Reverse osmosis is applicable for the separation, concentration, and/or fractionation of inorganic or organic substances in aqueous or nonaqueous solutions in the liquid or the gaseous phase, and hence it opens a new and versatile field of separation technology in chemical process engineering. Many reverse osmosis processes are also popularly called "ultrafiltration", and many reverse osmosis membranes are also practically useful as ultrafilters. 

The rapid expansion of reverse osmosis technology during the past two decades has resulted in the development of a variety of new membranes. Unique polymer systems and fabrication methods have led to the production of membranes with significantly improved performance and reliability. In spite of these developments little is known about chemical sensitivity or life expectancy of reverse osmosis membranes used in desalting applications. Manufacturers are consequently reluctant to guarantee their products for long runs especially in unique chemical environments. 

Considerable activity has been generated on composite reverse osmosis membranes by Japanese researchers. Patent applications were recently published, for example, covering research at Teijin Ltd. on interfacially formed membranes prepared from polydiallylamines (17) and from amine adducts of trls-(glycidyl) isocyanurate (18). Both types of membranes were formed on micro-porous polysulfone supports. Kurihara and coworkers have developed a composite membrane, designated PEC-1000, which is formed by an... 

This paper has provided the reader with an introduction to a class of polymers that show great potential as reverse osmosis membrane materials — poly(aryl ethers). Resistance to degradation and hydrolysis as well as resistance to stress Induced creep make membranes of these polymers particularly attractive. It has been demonstrated that through sulfonation the hydrophilic/hydrophobic, flux/separation, and structural stability characteristics of these membranes can be altered to suit the specific application. It has been Illustrated that the nature of the counter-ion of the sulfonation plays a role in determining performance characteristics. In the preliminary studies reported here, one particular poly(aryl ether) has been studied — the sulfonated derivative of Blsphenol A - polysulfone. This polymer was selected to serve as a model for the development of experimental techniques as well as to permit the investigation of variables... 

Albany International Research Co. has developed an advanced hollow fiber composite reverse osmosis membrane and module under the name of Quantro II . This composite membrane is comprised of a porous hollow fiber substrate on which has been deposited a rejection barrier capable of fluxes of commercial importance at high rejection of dissolved salts at elevated temperatures. Resistance to active chlorine has been demonstrated. Proprietary processes have been developed for spinning of the fiber, establishment of the rejection barrier and processing of the fiber to prepare modules of commercial size. Prototype modules are currently in field trials against brackish and seawater feed solutions. Applications under consideration for this membrane include brackish and seawater desalination as well as selected industrial concentration processes. 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical structure. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and structural stability over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each application. Excellent discussions of RO membrane materials, preparation methods, and structures are available (8,13,16-21). 

Thermally stable POD films containing pyridine rings have potential application as reverse osmosis membranes (58). 

In other areas, POD has been used to improve the wear resistance of a rubber latex binder by incorporation of 25% of Oksalon fibers. Heat-resistant laminate films, made by coating a polyester film with POD, have been used as electrical insulators and show good resistance to abrasion and are capable of 126% elongation. In some instances, thin sheets of PODs have been used as mold release agents. For this application a resin is placed between the two sheets of POD, which is then pressed in a mold, and the sheets simply peel off from the object and mold after the resin has cured. POD-based membranes exhibit salt rejection properties and hence find potential as reverse osmosis membranes in the purification of seawater. PODs have also been used in the manufacturing of electrophotographic plates as binders between the toner and plate. These improved binders produce sharper images than were possible before. 

The chemical sensitivity or life expectancy of reverse osmosis membranes is very important for manufacturing application. Thus chlorine is the most well known reagent for water disinfection. Glaster et al. 61 inspected the influence of halogens on the performance and durability of reverse osmosis membranes. Cellulose acetate was unresponsive to halogen agents but polyamide-type membranes deteriorated rapidly when exposed to halogens. 

Pateklt. B.S. Reverse Osmosis Technology Applications of High-Purity-Water Production, Marcel Dekker, Inc.. New York. NY, 1988 Raulenbacli. R., and R. Albieclit Membrane Processes, John Wiley Sons, Inc., New York. NY, 1989... 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

Predictions of salt and water transport can be made from this application of the solution-diffusion model to reverse osmosis (first derived by Merten and coworkers) [12,13], According to Equation (2.43), the water flux through a reverse osmosis membrane remains small up to the osmotic pressure of the salt solution and then increases with applied pressure, whereas according to Equation (2.46), the salt flux is essentially independent of pressure. Some typical results are shown in Figure 2.9. Also shown in this figure is a term called the rejection coefficient, R, which is defined as... 

Dynamically formed membranes were pursued for many years for reverse osmosis because of their high water fluxes and relatively good salt rejection, especially with brackish water feeds. However, the membranes proved to be unstable and difficult to reproduce reliably and consistently. For these reasons, and because high-performance interfacial composite membranes were developed in the meantime, dynamically formed reverse osmosis membranes fell out of favor. A small application niche in high-temperature nanofiltration and ultrafiltration remains, and Rhone Poulenc continues their production. The principal application is poly(vinyl alcohol) recovery from hot wash water produced in textile dyeing operations. 

A further long-term area of research is likely to be the development of reverse osmosis membranes to recover organic solutes from water. This chapter has focused almost entirely on the separation of ionic solutes from water, but some membranes (such as the PEC-1000 membrane) have excellent organic solute rejections also. The PEC-1000 membrane was chemically unstable, but it demonstrated what is achievable with membranes. A stable membrane with similar properties could be used in many wastewater applications. 

Because of the challenging environment in which ultrafiltration membranes are operated and the regular cleaning cycles, membrane lifetime is significantly shorter than that of reverse osmosis membranes. Ultrafiltration module lifetimes are rarely more than 2-3 years, and modules may be replaced annually in cheese whey or electrocoat paint applications. In contrast, reverse osmosis membranes are normally not cleaned more than once or twice per year and can last 4-5 years. 

Electrodialysis is now a mature technology, with Ionics remaining the worldwide industry leader except in Japan. Desalting of brackish water and the production of boiler feed water and industrial process water were the main applications until the 1990s, but electrodialysis has since lost market share due to stiff competition from improved reverse osmosis membranes. Beginning in the 1990s,... 

FIGURE 30 Performance of some commercial reverse osmosis membranes for (a) seawater desalination (test conditions 56 bar 25°C 3.5% NaCI feed) (b) low-pressure desalination (15 bar 25°C 1500 mg/liter NaCI feed) and (c) ultralow-pressure nanofiltration applications (7.5 bar, 25°C 500 mg/liter NaCI feed). 

Polymeric membranes also have vast applications in several processes, such as desalination using reverse osmosis membranes. Filtration, in a wide sense, with polymeric membranes can be applied in gas separation processes, biochemical processing, wastewater treatment, food and beverage production, and pharmaceutical applications [59-61],... 

Reverse osmosis membranes for industrial applications are typically modularized using configurations that pack a large amount of membrane area into a relatively small volume. This makes the RO system more economical to use in that the system requires a smaller footprint, and membranes can be replaced in smaller modules rather than system wide. 

The deposition of a plasma polymer on an appropriate porous substrate to form a composite reverse osmosis membrane is a typical example of the second case. In both cases, however, the selection of the substrate membrane is the crucially important factor, particularly in the second case. In the application of nanofilm, the pore size of the substrate membrane must have the uniformity of pore size in nanometer scale, which is an extremely difficult requirement. 

---