Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 215

FIGURE 16 A cell pair spacers and anion and cation membranes

with the corresponding manifolds. Water is circulating through the

paths of the spacer. (Courtesy Ionics Inc., Watertown, Mass, USA).

efficiency and an increase in pumping power requirement

and mechanical damages in some occasions.

Ion exchange membranes are very thin, about 0.5 mm

(0.020 inches), and they are supported by spacers as shown

in Figure 13. Spacers are made usually from two layers of

low density polyethylene having manifold cutouts which

match the membrane cuts as it is shown in Figure 16.

Membrane life is short, from 3 to 6 years, depending

on scale formation, fouling and the poisoning of the mem-

branes. Poisoning of the membranes occurs from chemical

agents such as chemicals for pre-treatment. The membranes

lose their characteristic properties, their volume is increased,

their water content decreased and they deteriorate easily.

Scaling, fouling and poisoning of the membranes have

influence not only on membrane life but also on membrane

efficiency, which drops considerably after long period use.

Reverse Osmosis, RO Reverse osmosis separates pure

water molecules from a salt containing solution through a

semipermeable membrane having extreme fine pores.

Osmotic flow, direct or reversed, depends on the selective

properties of some membranes to allow certain components

of a solution, usually the solvent, to pass through the mem-

brane. This intrinsic property of the membrane is termed as

semipermeability. If two solutions of different concentration,

or a pure solvent and a solution, are separated by a semiper-

meable membrane, the solvent will flow under normal con-

ditions from the less concentrated department through the

membrane into the concentrated solution, with the tendency

that both solutions reach the same concentration. This flow

is known as osmosis. Osmotic flow through the membrane

will stop when the concentrated solution reaches a suffi-

ciently higher pressure than prevailing in the less concen-

trated solution or the solvent compartment. The equilibrium

pressure difference between solvent and solution, or the two

solutions, known as osmotic pressure, is a property of the

solution. Equilibrium can also be reached by applying an

external pressure to the concentrated salt solution equal to

the osmotic pressure. Further increase of the pressure on the

concentrated solution, beyond the osmotic pressure, causes

reversal of the osmotic flow. Pure solvent passes from the

solution through the membrane into the solvent compart-

ment. By applying a pressure higher than the osmotic the

phenomenon is reversed and water flows from the con-

centrated solution to the dilute. This is the basis of reverse

osmosis, the major attractions of which, from an economic

point of view are its simplicity and the relatively low energy

consumption.

Reverse osmosis has some analogies with filtration

in that both remove substances from a liquid. As matter,

removed by reverse osmosis is in solution, the process pre-

viously was termed “hyperfiltration,” when the membranes

used were highly semipermeable with respect to low molec-

ular weight solutes. On the contrary, when the membranes

used have little or no selectivity for such solutes but they

may be applied for the separation of colloids or macromol-

ecules from low molecules solutes, the method is called

“ultrafiltration.” In fact ultra, a Latin word, is identical to

the Greek word hyper. Today high pressure, low molecular

matter separation is called “reverse osmosis” and lower pres-

sure low molecular separation is called “nanofiltration.” By

chemical engineering point of view reverse osmosis, nano-,

ultra- and micro-filtration can replace, in some extent other

unit-operation separation methods as these are shown in

Figure 17. These four methods can be applied for the puri-

fication of water of solutions related or not to desalination

methods and reverse osmosis and nanofiltration for desali-

nation of seawater brackish or natural waters, as well.

Applied pressures are higher for the reverse osmosis

method and low for microfiltration and, on the other hand,

porosity of membranes is decreasing from micro-filtration to

reverse osmosis as follow:

Applied pressure, bars Porosity

Reverse Osmosis, RO 30 to 70 5 to 20 Å

Nanofiltration, NF 20 to 40 10 to 20 Å

Ultrafiltration, UF 5 to 15 20 Å to 0.1 mm

Microfiltration, MF 1 to 4 0.1 to 2 mm

Reverse osmosis, for desalination of sea or brackish

water, by the end of 1993 had a world capacity of 6,109,250

m^3 /d per unit or 32.7% of total world capacity. This capacity

gives RO second place in fresh water production worldwide.

The vapor gap osmotic distillation process is a fore-

runner of the reverse osmosis process. It is based upon the

difference between the water vapor pressures over a saline

solution and that over pure water.

Reverse osmosis as a process was developed first with the

plate and frame concept, using the filter-press principle. This

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