Swk

Porous tube .-d^.Membrane

Feed flow

Permeate

Large tube

Feed flow

Permeate

Large tube

Feed water

Hollow fibers

Feed water

Hollow fibers

Product water

Hollow fine fibers

Product water

Hollow fine fibers

Roll to assemble Feed side spacer

Feed flow

Roll to assemble Feed side spacer

Feed flow

Spiral wound

Permeate out

Permeate side' backing material with membrane in each side and glued around edges and to center tube

Spiral wound

FIGURE 8.4 Reverse osmosis module designs.

The basics of a normal osmosis process are shown in Figure 8.5a. A bag of semipermeable membrane is shown placed inside a bigger container full of pure water. Inside the membrane bag is a solution of sucrose. Because sucrose has osmotic pressure, it "sucks" water from outside the bag causing the water to pass through the membrane. Introduction of the water into the membrane bag, in turn, causes the solution level to rise as indicated by the height n in the figure. The height n is a measure of the osmotic pressure. It follows that if sufficient pressure is applied to the tip of the tube in excess of that of the osmotic pressure, the height n will be suppressed and the flow of water through the membrane will be reversed (i.e., it would be from inside the bag toward the outside into the bigger container); thus, the term "reverse osmosis."

Sucrose in a concentration of 1,000 mg/L has an osmotic pressure of 7.24 kNa (kiloNewtons absolute). Thus, the reverse pressure to be applied must be, theoretically, in excess of 7.24 kNa for a sucrose concentration of 1,000 mg/L. For NaCl, its osmotic pressure in a concentration of 35,000 mg/L is 2744.07 kNa. Hence, to reverse the flow in a NaCl concentration of 35,000 mg/L, a reverse pressure in excess of 2744.07 kNa should be applied. The operation just described (i.e., applying sufficient pressure to the tip of the tube to reverse the flow of water) is the fundamental description of the basic reverse osmosis process.

(a)

FIGURE 8.5 (a) Osmosis process; (b) reverse osmosis system.

FIGURE 8.5 (a) Osmosis process; (b) reverse osmosis system.

UF, NF, MF, and RO and are all reverse osmosis filtration processes; however, when the term reverse osmosis or RO is used without qualification, it is the process operated at the highest pressure range to which it is normally referred. Figure 8.5b is a schematic of an RO plant. Figure 8.2 is a photograph of a bank of modules in the Sanibel-Captiva RO Plant in Florida. This plant treats water for drinking purposes.

Take careful note of the pretreatment requirement indicated in Figure 8.5b. As mentioned before, the RO process is an advanced mode of filtration and its purpose is to remove the very minute particles of molecules, ions, and dissolved solids. The influent to a RO plant is already "clean," only that it contains the ions, molecules, and molecular aggregates that need to be removed.

After pretreatment the high-pressure pump forces the flow into the membrane module where the solutes are rejected. The flow splits into two, one producing the product water and the other producing the waste discharge. The waste discharge has one drawback in the use of RO filtration in that it may need to be treated separately before discharge.

8.2.1 Membrane Module Designs

Over the course of development of the membrane technology, RO module designs, as shown in Figure 8.4, evolved. They are tubular, plate-and-frame, spiral wound, and hollow fine-fiber modules. In the tubular design, the membrane is lined inside the tube which is made of ordinary tubular material. Water is allowed to pass through the inside of the tube under excess pressure causing the water to permeate through the membrane and to collect at the outside of the tube as the product or permeate. The portion of the influent that did not permeate becomes concentrated. This is called the concentrate or the reject.

The plate-and-frame design is similar to the plate-and-frame filter press discussed in the previous chapter on conventional filtration. In the case of RO, the semipermeable membrane replaces the filter cloth. The spiral-wound design consists of two flat sheets of membranes separated by porous spacers. The two sheets are sealed on three sides; the fourth side is attached to a central collector pipe; and the whole sealed sheets are rolled around the central collector pipe. As the sheets are rolled around the pipe, a second spacer, called influent spacer, is provided between the sealed sheets. In the final configuration, the spiral-wound sealed membrane looks like a cylinder. Water is introduced into the influent spacer, thereby allowing it to permeate through the membrane into the spacer between the sealed membrane. The permeate, now inside the sealed membrane, flows toward the central pipe and exits through the fourth unsealed side into the pipe. The permeate is collected as the product water. The concentrate or the reject continues to flow along the influent spacer and is discharged as the effluent reject or effluent concentrate. This concentrate, which may contain hazardous molecules, poses a problem for disposal.

In the hollow fine-fiber design, the hollow fibers are a bundle of thousands of parallel, self-supporting, hair-like fibers enclosed in a fiberglass or epoxy-coated steel vessel. Water is introduced into the hollow bores of the fibers under pressure. The permeate water exits through one or more module ports. The concentrate also exits in a separate one or more module ports, depending on the design. All these module designs may be combined into banks of modules and may be connected in parallel or in series.

8.2.2 Factors Affecting Solute Rejection and Breakthrough

The reason why the product or the permeate contains solute (that ought to be removed) is that the solute has broken through the membrane surface along with the product water. It may be said that as long as the solute stays away from the membrane surface, only water will pass through into the product side and the permeate will be solute-free; However, it is not possible to exclude the solute from contacting the membrane surface; hence, it is always liable to break through. The efficiency at which solute is rejected is therefore a function of the interaction of the solute and the membrane surface. As far as solute rejection and breakthrough are concerned, a review of literature revealed the following conclusions (Sincero, 1989):

• Percentage removal is a function of functional groups present in the membrane.

• Percentage removal is a function of the nature of the membrane surface. For example, solute and membrane may have the tendency to bond by hydrogen bonding. Thus, the solute would easily permeate to the product side if the nature of the surface is such that it contains large amounts of hydrogen bonding sites.

• In a homologous series of compounds, percentage removal increases with molecular weight of solute.

• Percentage removal is a function of the size of the solute molecule.

• Percentage removal increases as the percent dissociation of the solute molecule increases. The degree of dissociation of a molecule is a function of pH, so percentage removal is also a function of pH.

This review also found that the percentage removal of a solute is affected by the presence of other solutes. For example, methyl formate experienced a drastic change in percentage removal when mixed with ethyl formate, methyl propionate, and ethyl propionate. When alone, it was removed by only 14% but when mixed with the others, the removal increased to 66%. Therefore, design of RO processes should be done by obtaining design criteria utilizing laboratory or pilot plant testing on the given influent.

8.2.3 Solute-Water Separation Theory

The sole purpose of using the membrane is to separate the solute from the water molecules. Whereas MF, UF, and NF may be viewed as similar to conventional filtration, only done in high-pressure modes, the RO process is thought to proceed in a somewhat different way. In addition to operating similar to conventional filtration, some other mechanisms operate during the process. Several theories have been advanced as to how the separation in RO is effected. Of these theories, the one suggested by Sourirajan with schematics shown in Figures 8.6a and 8.6b is the most plausible.

Sourirajan's theory is called thepreferential-sorption, capillary-flow theory. This theory asserts that there is a competition between the solute and the water molecules for the surface of the membrane. Because the membrane is an organic substance, several hydrogen bonding sites exist on its surface which preferentially bond water molecules to them. (The hydrogen end of water molecules bonds by hydrogen bonding to other molecules.) As shown in Figure 8.6a, H2O molecules are shown layering over the membrane surface (preferential sorption), to the exclusion of the solute ions of Na+ and Cl-. Thus, this exclusion brings about an initial separation. In Figure 8.6b, a pore through the membrane is postulated, accommodating two

Bulk of the solutions

H20 Na+Cl H20 Na+Cl H20 Na+Cl H20 Na+Cl H20 Na+Cl-0"TNa++Cl"^R2"0""Na++CF

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