Osmosis reverse

Osmosis is the transport of a solvent through a dense membrane from the dilute 

If the liquid on one side of the membrane is a pure solvent and on the other side is a solution at a given concentration, the pressure difference required to result in zero net solvent transport is the osmotic pressure of that solution at its existing concentration and temperature. Thus, the osmotic pressure is deflned as 

0—Pure solvent, 1—Solution to be purified, 2—Low concentration solution. At equilibrium (no permeation), Pq = - JCj = 

In order for reverse osmosis to take place, P, must be greater than P2 + tti - 7t2, 

Subscripts 1 and 2 refer to the feed (or residue) side and the permeate side, respectively. If mass transfer resistances are neglected, the solvent flux is given by 

Reverse osmosis is a demineralization process that relies on a semi-permeable membrane to effect the separation of dissolved solids from a liquid. The semipermeable membrane allows liquid and some ions to pass, but retains the bulk of the dissolved solids. Although many liquids (solvents) may be used, the primary application of RO is water-based systems. Hence, all subsequent discussion and examples will be based on the use of water as the liquid solvent. 

To understand how RO works, it is first necessary to understand the natural process of osmosis. This chapter covers the fundamentals of osmosis and reverse osmosis. 

Osmosis is a natural process where water flows through a semipermeable membrane from a solution with a low concentration of dissolved solids to a solution with a high concentration of dissolved solids. 

The difference in height between the 2 compartments corresponds to the osmotic pressure of the solution that is now at equilibrium. 

Osmotic pressure (typically represented by jt (pi)) is a function of the concentration of dissolved solids. It ranges from 0.6 to 1.1 psi for every 100 ppm total dissolved solids (TDS). For example, brackish water at 1,500 ppm TDS would have an osmotic pressure of about 15 psi. Seawater, at 35,000 ppm TDS, would have an osmotic pressure of about 350 psi. 

Reverse osmosis is the process by which an applied pressure, greater than the osmotic pressure, is exerted on the compartment that 

Due to the added resistance of the membrane, the applied pressures required to achieve reverse osmosis are significantly higher than the osmotic pressure. For example, for 1,500 ppm TDS brackish water, RO operating pressures can range from about 150 psi to 400 psi. For seawater at 35,000 ppm TDS, RO operating pressures as high as 1,500 psi may be required. 

An example of reverse osmosis can be used as an illustration of mixed heterogeneous reaction. Reverse osmosis is a pressure-driven membrane process used to separate relatively pure solvents, most often water from solutions containing salts and dissolved organic molecules. The solvent passes through the membrane under the action of hydrostatic pressure leaving the dissolved materials behind. 

When an ideal semipermeable membrane separates an aqueous organic or inorganic solution from pure water, the tendency to equalize concentrations would result in the flow of pure water through the membrane to the solution. The pressure needed to stop the flow is called the osmotic pressure. If the pressure on the solution is increased beyond the osmotic pressure, then the flow would be reversed and the fresh water would pass from the solution through the membrane, therefore the name reverse osmosis. Osmotic pressure is a property of the solution and does not in any way depend on the properties of the membrane. 

Osmosis pressure data for some aqueous solutions at standard temperature are shown in Table 4.1. It shows that with an increase in NaCl concentration, the osmotic pressure increases. 

The solvent (water) flux through the membrane may be written as 

As a consequence of the passage of solvent, say, water through the membrane, the solute is carried out to the membrane surface. Hence, the concentration at the membrane surface tends to be higher than in the bulk of the liquid. This phenomenon is called concentration polarization. Due to the concentration polarization, the osmotic pressure increases because of the increase in solute concentration. Hence, the solvent flux is decreased because the effective driving pressure is reduced based on equation (4.136). Another effect is the increase in the solute concentration in the product side for leaky membranes as the flux of the solute across the membrane is proportional to the difference in solute concentration of both sides. Therefore, the concentration distribution of the solute inside the reverse osmosis channel, that is, concentration polarization, influences its performance and has been discussed in the following section. 

When miscible solutions of different concentrations are separated by a membrane that is permeable to the solvent but nearly impermeable to the solute, diffusion of solvent occurs from the less concentrated solution to the more concentrated solution, where the solvent activity is lower. The diffusion of the solvent is called osmo -sis, and osmotic transfer of water occurs in many plant and animal cells. The transfer of solvent can be stopped by increasing the pressure on the concentrated solution until the activity of the solvent is the same on both sides of the membrane. If pure solvent is on one side of the membrane, the pressure required to equalize the solvent activities is the osmotic pressure of the solution 7t. If a pressure higher than the osmotic pressure is applied, solvent will diffuse from the concentrated solution through the membrane into the dilute solution. This phenomenon is called reverse osmosis because the solvent flow is opposite the normal osmotic flow. 

In a reverse osmosis (RO) membrane-separation process, the feed is a liquid at high pressure Py No sweep liquid is used, but the other side of the membrane is kept at a much lower pressure, P2. A dense membrane such as an acetate or aromatic polyamide is used that is permselective for the solvent. The membrane must be thick to withstand the large pressure differential therefore, asymmetric membranes are commonly used. The products of RO are a permeate of almost pure solvent and a solvent-depleted retentate. Only a fraction of the solvent in the feed is transferred to the permeate. 

The most important application of RO is the desalinization of seawater. Since 1990, RO has become the dominant process for seawater desalinization. Seawater contains about 3.5 wt% dissolved salts and has an osmotic pressure of 24.1 bar. The preferred RO membrane for desalinization is a spiral-wound module of polyamide membrane operating at a feed pressure of 55 to 70 bar. With a transmembrane water flux of 365 kg/m2-day, this module can recover 45% of the water at a purity of 99.95 wt%. Atypical cylindrical module is 20 cm in diameter by 1.0 m long, containing 34 m2 of surface area. Such modules resist fouling by colloidal and particulate matter, but the seawater must be treated with sodium bisulfate to remove oxygen and chlorine. 

Although the driving force for transport of water through the dense membrane is the concentration or activity difference across it, common practice is to use a driving force based on osmotic pressure. At thermodynamic equilibrium, the solvent fugacity on the feed side of the membrane (1) must be equal to the solvent fugacity on the permeate side (2). Thus, 

For a mixture on the feed or retentate side which is dilute in the solid, 1.0. Also, 

Substitution of (6.46) into (6.31) gives us the expression for diffusion layer 

If we solve a similar problem of adsorption at a quickly reacting wall, then we 

We may then derive an expression for the diffusion flux, which turns out to be directed not from the wall, but toward it, and the thickness of the diffusion boundary layer. It is obvious that these characteristics will coincide with (6.46) and (6.47), but in these equations, Cjat should be replaced with Cq. 

The method of reverse osmosis [7] is based on filtration of solutions under pressure through semi-permeable membranes, which let the solvent pass through while preventing (either totally or partially) the passage of molecules or ions of dissolved substances. The phenomenon of osmosis forms the physical core of this method. Osmosis is a spontaneous transition of the solvent through a semi-permeable membrane into the solution (Fig. 6.4, a) at a pressure drop AP lower than a certain value n. The pressure n at which the equilibrium is estabUshed, is known as osmotic pressure (Fig. 6.4, b). If the pressure drop exceeds n, i.e. pressure p p - -K is applied on the solution side, then the transfer of solvent will reverse its direction. Therefore, this process is known as reverse osmosis (Fig. 6.4, 

In practice, however, no membrane can act as an ideal semi-permeable membrane, and some transition of the dissolved substance through a real membrane always happens - to a larger or smaller extent. With this in mind, we rewrite Eq. (6.50) as 

Sales of RO membrane products were as high as 118 million annually in the late 1980s, and continued growth is expected (10). 

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). 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-film composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has little effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich solid phase that forms the membrane and a polymer-poor liquid phase that forms the membrane pores or void spaces. 

An excellent review of composite RO and nanofiltration (NF) membranes is available (8). These thin-film, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-film composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-immiscible solvent. 

Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition) 

The RO system removes 90-95 % of the dissolved solids in the raw water, together with suspended matter (including colloidal and organic materials). The exact percent of product purity, product recovery and reject water depends on the amount of dissolved solids in the feedwater and the temperature at which the system operates. 

Influent water enters one end of the pressure vessel and is evenly distributed along the length of the vessel by a concentric distributor tube. As the water migrates out radially, some of it permeates the fibers and exits the pressure vessel via the tube sheet on the opposite end. The direction of permeate flow is from outside to inside the fibers. The concentrated solution, or reject, completes its radial flow path and leaves the vessel at the same end at which it entered. 

Basically, the spiral-wound configuration consists of a jelly roll-like arrangement of feed transport material, permeate transport material and membrane material. At the heart of the wall is a perforated permeate collector tube. Several rolls are usually placed end to end in a long pressure vessel. 

Influent water enters one end of the pressure vessel and travels longitudinally down the length of the vessel in the feed transport layer. Direct entry into the permeate transport layer is precluded by sealing this layer at each end of the roll. As the water travels in a longitudinal direction, some of it passes in radially through the membrane into the permeate transport layer. Once in the transport layer, the purified water flows spirally into the center collection tube and exits the vessel at each end. The concentrated feed continues along the feed transport material and exits the vessel on the opposite end from which it entered. 

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

For any change to occur a chemical potential gradient must exist. For a membrane system, such as the one under consideration, Haase(22) and Belfort(23) have derived the following simplified equation for constant temperature  

Incorporating the model of diffusion across the membrane, and writing Fick s law in the generalised form 24, (using /x = /x° + RT In C)  

The rejection of dissolved ions at reverse osmosis membranes depends on valence. Typically, a membrane which rejects 93 per cent of Na+ or Cl- will reject 98 per cent of Ca2+ or SO42- when rejections are measured on solutions of a single salt. With mixtures of salts in solution, the rejection of a single ion is influenced by its 

The thermodynamic approach does not make explicit the effects of concentration at the membrane. A good deal of the analysis of concentration polarisation given for ultrafiltration also applies to reverse osmosis. The control of the boundary layer is just as important. The main effects of concentration polarisation in this case are, however, a reduced value of solvent permeation rate as a result of an increased osmotic pressure at the membrane surface given in equation 8.37, and a decrease in solute rejection given in equation 8.38. In many applications it is usual to pretreat feeds in order to remove colloidal material before reverse osmosis. The components which must then be retained by reverse osmosis have higher diffusion coefficients than those encountered in ultrafiltration. Hence, the polarisation modulus given in equation 8.14 is lower, and the concentration of solutes at the membrane seldom results in the formation of a gel. For the case of turbulent flow the Dittus-Boelter correlation may be used, as was the case for ultrafiltration giving a polarisation modulus of  

Feed flow across feed channel spacer  

Although the driving potential in MF is the hydraulic pressure gradient, MF flux is often also affected by the fluid velocity along the membrane surface. 

This is invariably due to the accumulation of filtered particles on the membrane surface - in other words, the concentration polarization of particles. 

Equation 8.7 [6] was obtained to correlate the experimental data on membrane plasmapheresis, which is the MF of blood to separate the blood cells from the plasma. The filtrate flux is affected by the blood velocity along the membrane. Since, in plasmapheresis, all of the protein molecules and other solutes will pass into the filtrate, the concentration polarization of protein molecules is inconceivable. In fact, the hydraulic pressure difference in plasmapheresis is smaller than that in the UF of plasma. Thus, the concentration polarization of red blood cells was assumed in deriving Equation 8.7. The shape of the red blood cell is approximately discoid, with a concave area at the central portion, the cells being approximately 1-2.5 pm thick and 7-8.5 pm in diameter. Thus, a value of r (= 0.000257 cm), the radius of the sphere with a volume equal to that of a red blood cell, was used in Equation 8.7. 

/p is the filtrate flux (cm min ) averaged over the hollow fiber membrane of length L (cm) and is the shear rate (s ) on the membrane surface, as in Equation 8.6. The volumetric percentage of red blood cells (the hematocrit) was taken as C, and its value on the membrane surface, Cg, was assumed to be 95%. 

In the case where a liquid suspension of fine particles of radius r (cm) flows along a solid surface at a wall shear rate (s ), the effective diffusivity Dp (cm s ) of particles in the direction perpendicular to the surface can be correlated by the following empirical equation [7]  

For example, the need for a low chloride makeup water, due perhaps to the considerable use of various grades of stainless steel in heat exchangers, may warrant the use of an RO plant, with the lowering of overall TDS 

The versatility of the RO process is such that it is now used for treating municipal and some industrial waste streams, which can then be used directly for cooling system makeup water and other applications without the need for blending. 

Where blending of reclaimed RO-treated water with a low-grade source water does take place, it can often produce a zero LSI mix, which is a good starting point for designing potentially suitable cooling water treatment programs. 

2 RO Design Considerations for Cooling and Other Industrial Water Applications 

Most RO systems, however, can be categorized according to the three broad classes of water they are designed to treat, as discussed below. 

Due to the significant reduction in ionizable salt concentrations, RO systems are often used as a pretreatment method before a DI system. An RO before a DI reduces the size of the deionizer, reduces the consumption of regenerate chemicals and may reduce the length of the deionizer required service cycle. 

Even at these rejection rates, the dirty side of the membranes rapidly build up undesirable bacterial concentrations. To alleviate this potential problem, the membranes are normally automatically flushed on a continued cycle basis, say 3 to 8 minutes every four hours. Full sanitization with a sanitization chemical like phosphoric acid is required periodically based on continual monitoring of pressure drop, conductivity and bacterial count. To further reduce bacterial count, RO systems should be sized for 24 hours per day operation to minimize water stagnation. 

The two most common RO membrane configurations used in water treatment today are spiral-wound and hollow fiber. The spiral-wound elements can operate at a higher pressure and at a higher silt density index (SDI) than the hollow fiber type, and thus may require less pretreatment (and are more tolerant of pretreatment upsets). They also are easier to clean than the hollow fiber type. The main advantage of the hollow fiber configuration is that it has the highest amount of membrane area per unit volume, thus requiring less space. Since there is only one hollow fiber element per pressure vessel, it is easier to troubleshoot, and it is easier to replace membrane modules. 

Should the RO system outlet conductivity be unsatisfactory, the outlet water should be diverted automatically to drain until the problem is resolved. 

Usnally the solution-diffusion model is used to explain the flow of a solvent throngh an RO membrane.1 This explanation holds that solvent dissolves in the membrane material then diffuses across it in response lo a chemical potential gradient. Solute is presumed to pass throngh (he membrane by diffusion driven by he solute concentration difference across cbe membrane. The explanation implies that solute retention Is proportional to flux, in Ibe absence of other effects,1 

Osmotic pressure poses a practical limit to the use of RO a difference in system pressure and osmotic pressure ip - r) of approximately I MPa is required for RO membranes. Rarely are useful membranes operated above a system pressure of 5.5 MPa, giving a practical upper limit to osmotic pretenre of 4,5 MPa. This osmotic pressure is approximately that of I M NaCI and 3S wt.% sucrose solutions. 

For many common solutes at low concentrations, the van t Hoff equation 

The tendency of the retained solutes to concentrate at the membrane, that is, concentration polarization, is of less importance than it is in UF.  

Tbe next mejor comasereial success was the family of composite membranes. They feature a very thin RG membrane on a suitable substrate, usually a UF membrane. Most of the RO composite membranes are polyamide cnatings in which the separating layer is produced by interfacial polymerization of a diamine and a multibasic acid chloride. The most snecessfol recent memhrane is based on an inteifacial polymer of 1,3-diamino benzene and 1,3,5-benzene tricarboxylic acid chloride coated on a polysulfone membrane substrate. 

RO membranes have a very small molecular weight cut-off (MWCO) and are expected to retain a large fraction of LMW compounds, such as amino acids or sugars, and are therefore useful for extracting a representative mixture of NOM from surface waters. For nanofiltration smdies it is important to retain the LMW fraction, as these molecules could be major contributors of pore pluming in membrane filtration. Pure HS solution cannot be obtained from such a mixture with a the salt content of the sample depending on its origin. 

Serkiz and Perdue (1990) were the first to report the successful use of RO to isolate NOM from river water. Polyamide (PA) membranes on a polysulphone (PS) support were used. Freshwatcrs were pretreated with a cation exchange resin and, due to a low salt content, no further desalination was required. Concentrates were then freeze dried. Recoveries of about 90% were achieved. The method appears to be well suited for NOM removal with the possibility of subsequent use of XAD for separation of FA and HA from the NOM. The method can be applied to freshwatcrs containing DOC in the range of 3 to 40 mgL h Problems of the method include the presence of residual inorganics such as H2SO4 and Si(OH)4. 

In applying reverse osmosis for NOM extraction, the choice of an appropriate river water, preferably high in DOC and low in salt content, is important. Also the choice of a suitable membrane with maximised rejection is critical to avoid the loss of small organics with the permeate. The drawbacks of this method are the concentration of salts and other contaminants. This generally results in a product of high ash content and the salt can lead to precipitation in the concentrate. Particles in the surface water also need to be removed prior to concentrations however this can be achieved with microfiltration (MF). 

The overlap of the definitions for RO and UF membranes arises from the following considerations. The pores in die skin of a membrane intended for removal of salt by RO are generally larger (e.g., 10-40 A) than the hydrated ions (e.g., Na Cl, Ca, SO4 ) diey are intended to repulse. However, diese pores are filled with water that is strongly influenced by die polymeric walls of the pores. Such water becomes ordered water , which, because of its ordering, has too low a dielectric constant to 

UF and MF membranes are generally a few mils in thickness however, the discriminatory layer may be either a tight skin supported by an open substructure (i.e., a very thin effective thickness and, thus, low frictional resistance to flow) or it may be the entire thickness of the membrane or gel involved in the pass/rejection mechanism. In the latter case, the friction fhctors are much higher, i.e., die entire thickness equals the effective thickness. 

Osmotic pressure across a semipermeable membrane arises from differences in concentration, which in turn arise from relative ratios of the numbers of impermeable individual ions or molecules on the two sides of the membrane. These osmotic pressures are dominant when salts are to be removed by RO. Osmotic pressures vary from 3.5 psi for good tapwater to 350 psi with average seawater, as the number of ions per unit volume is very high (35,000 pi n). At the other extreme (MF), diere are essentially no dissolved species that cannot permeate through die membrane it follows that the osmotic pressures are minimal. UF membranes lie in between, usually with very few impermeable species of very high molecular wei t and therefore, much lower osmotic pressures exist across die membrane. Exceptions can exist involving UF and may be circumvented, e.g., the pervaporation process. 

Of medical and biotechnical importance are the thicker homogeneous gel membranes, such as Cuprophane , which are used in the artificial kidney and/or concentration dialysis. With the Cuprophane membranes, diffosional nugration, driven by concentration differences across the membrane, effects flie transport of the various species across the membrane and little, if any, pressure differential is applied. 

In kidney dialysis, toxic middle molecules diffuse across flie Cuprophane membrane and out of die blood, while the larger desirable species are retamed. Almost as much salt diffuses out of die blood as diffuses into the blood from die dialysate during diis procedure. A small pressure is imposed that depletes die patient of a few pounds of accumulated water over a period of hours. Such processes are considered to be primarily concentration-driven. 

Particles, single-cell microorganisms Small colloids, viruses Dissolved organics, divalent ions (Ca Mg ,SOi-) 

A problem common to all membrane processes is that posed by the retentate in which impurities are concentrated. In some cases, retentate can be discharged with wastewater. Other options include evaporation of the water followed by disposal or incineration of the residue, reclamation of chemicals from industrial wastewater, and disposal in deep saline water aquifers. For the special case of desalination of seawater by reverse osmosis, the retentate is returned to the ocean, which has the potential to cause problems due to excess salinity. 

FIGURE 5.5 Solute removal from water by reverse osmosis. 

FIGU RE 5.6 A reverse osmosis system for the removal of ions and other impurities from water. Highly pressurized water is forced through a membrane selectively permeable to water so that desalinated purified water penetrates the membrane and a waste brine is rejected. Membrane filtration processes that employ filters with 

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Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

Reverse osmosis is used for desalination of seawater, treatment of recycle water in chemical plants and separation of industrial wastes. More recently the technique has been applied to concentration and dehydrogenation of food products such as milk and fruit juices. See ultrafiltralion. 

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