What Reverse Osmosis Is

When pure water and a salt solution are introduced on opposite sides of a semipermeable membrane in a vented container, the pure water diffuses through the membrane and dilutes the salt solution, At equilibrium, the liquid level on the saline water side of the membrane will be above that on the freshwater side; this process is known as osmosis and is depicted in Figure 9. The view on the left illustrates the commencement of osmosis and the center view presents conditions at equilibrium. The effective driving force responsible for the flow is osmotic pressure. This pressure has a magnitude dependent on membrane characteristics, water temperature, and salt solution properties and concentration. By applying pressure to the saline water, the flow process through the membrane can be reversed. When the applied pressure on the salt solution is greater than the osmotic pressure, fresh water diffuses in the opposite direction through the membrane and pure solvent is extracted from the mixed solution; this process is termed reverse osmosis (RO). The fundamental difference between reverse osmosis and electrodialysis is that in reverse osmosis the solvent permeates the membrane, while in electrodialysis the solute moves through the membrane.

Reverse osmosis is a means for separating dissolved solids from water molecules in aqueous solutions as a result of the membranes being composed of special polymers which allow water molecules to pass through while holding back most other types of molecules; since true "pores" do not exist in the membrane, suspended solids are also retained by superfiltration. In an actual reverse osmosis system, operating in a continuous-flow process, feed water to be treated or desalinated is circulated through an input passage of the cell, separated from the output product water passageway by the membrane.

Nornul Ojmotic

Oimoui Equilibrium

Nornul Ojmotic

Oimoui Equilibrium

Figure 9. Principle of Reverse Osmosis

The feed stream is split into two fractions - a purified portion called the product water (or permeate) and a smaller portion called the concentrate' containing most of the impurities in the feed stream. At the far end of the feed-water passage, the concentration (dewatered) reject stream exits from the cell. After permeating the membrane, the product (fresh-water) flow is collected. The percentage of product water obtained from the feed stream is termed the recovery, typically around 75 percent.

The ratio (F-P)/F, or the concentration of a solute species in the feed (F) minus that in the product (P) over the concentration in the feed, is called the rejection of that species. Rejections may be stated for particular ions, molecules, or conglomerates such as TDS or hardness. Solids rejection depends on factors such as types and forms of solids, membrane types, recovery, pressure, and pH. Suspended solids (typically defined as particles larger than 0.5 micron mean diameter, and including colloids, bacteria, and algae) are rejected 100 percent; that is, none can pass through the membrane. Weakly ionized dissolved solids (usually organics, but may include other materials such as silicates) undergo about a 90 percent rejection at normal recoveries for certain membranes. Although pH can strongly influence the rejection, when the molecular weight of these solids is less than 100, rejection decreases appreciably. Ionized solids, or salts, are rejected independent of molecular weight and at molecular weights considerably below 100. At 75 percent recovery and pressures greater than 250 pounds per square inch, overall rejection of total dissolved solids (TDS) is about 90 percent. Rejections vary with pressure because the actual salt flow through the membrane remains fairly constant, but the water permeation depends nearly linearly on pressure, affecting the ratio of concentrations. For example, rejection of sodium chloride can fall from 90 percent at 300 pounds per square inch to 20 percent at 50 pounds per square inch, indicating the need to operate at the highest pressures possible.

Cellulose acetate is a common membrane material, but others include nylon and aromatic polyamides. The mechanism at the membrane surface involves the influent water and impurities attempting to pass through the pressurized side, but only pure water and certain impurities soluble in the membrane emerge from the opposite side.

Various configurations of membranes with different surface-to-volume ratios and different flux capabilities (gallons per day per square foot, or gpd/ ft') have been developed. Each type of membrane is a flexible plastic filmno more than 4 to 6 mils thick, firmly supported. Basic designs include the plate and frame, the spiral-wound module (jellyroll configuration), the tubular, and the newest of the process designs, the hollow-fine fiber. Fibers range from 25 to 250 microns (0.001 to 0.01 in.) in diameter, can withstand enormous pressure, are self-supporting, and can be bundled very compactly within a containment pipe. While product flow per square foot of fiber surface is less than that for an equivalent area of flat membrane, the difference in surface area more than compensates for the reduced unit flux.

Major problems inherent in general applications of RO systems have to do with (1) the presence of particulate and colloidal matter in feed water, (2) precipitation of soluble salts, and (3) physical and chemical makeup of the feed water. All RO membranes can become clogged, some more readily than others. This problem is most severe for spiral-wound and hollow-fiber modules, especially when submicron and colloidal particles enter the unit (larger particulate matter can be easily removed by standard filtration methods). A similar problem is the occurrence of concentration-polarization, previously discussed for ED processes. Concentration-polarization is caused by an accumulation of solute on or near the membrane surface and results in lower flux and reduced salt rejection.

The degree of concentration that can be achieved by RO may be limited by the precipitation of soluble salts and the resultant scaling of membranes. The most troublesome precipitate is calcium sulfate. The addition of polyphosphates to the influent will inhibit calcium sulfate scale formation, however, and precipitation of many of the other salts, such as calcium carbonate, can be prevented by pretreating the feed either with acid or zeolite softeners, depending on the membrane material.

Hydrolysis of cellulose acetate membranes is another operational problem and occurs whenever the feed is too acid or alkaline; that is, the pH deviates beyond designed range limits. As may readily happen, whenever C02 passes through the membrane, the resultant permeate has a low pH. The operational solution is to remove the gas from the permeate by deaerators, by strong-base anion resins or a complementary system-for example, RO and ion exchange, in series. Aromatic polyamide or nylon membranes are much less sensitive to pH than cellulose acetate. Compounds such as phenols and free chlorine that are either soluble in the membrane or vice versa will be poorly rejected and may damage the membrane. Procedures to improve feed-water makeup and thus reduce such membrane damage include acid pretreatment of the feed water, dechlorination, periodic cleaning or replacement of the membrane, sequestration of cations, coagulation and filtration of organics, and use of alternative, more durable membrane materials.

Reverse osmosis process is applied-or undergoing evaluation for imminent application-to a number of water-upgrading needs including high-purity rinse water production for the electronics industry (semiconductor manufacturing), potable municipal water supplied for newly-developed communities (for example, large coastal plants to upgrade brackish well water contaminated by seawater intrusion), boiler feed-water supplies, spent liquor processing for pulp and paper mills, and treatment of acid mine drainage.

In desalting operations, distillation plants have provided the major portion of the world's capacity. As the world's requirements for treated water increase, however, and water quality standards become more stringent, the membrane treatment processes in general and commercial RO processes in particular have been undergoing appreciable development. Important factors in the expansion of commercial RO applications are their favorably low power requirements and the realization of continuous technical improvements in membranes which are used in RO systems. A general guideline in water benefication is that RO is most frequently considered for cases in which the TDS is greater than 2,000 to 3,000 ppm; ED generally applies when the TDS is less than 2,000 to 3,000 ppm. However, many exceptions exist, based on feed-water species and product requirements.

One of the most important applications of RO is in the reclamation of large volumes of municipal and industrial wastewaters and the concentration of the solids for

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