Encyclopedia of Environmental Science and Engineering, Volume I and II

216 DESALINATION

configuration is now coming back, having many improve-

ments and makes use of a rigid plate with the membranes

mounted on opposite sides and sealed to the plate. Salt water

under pressure is following on the outer side of the mem-

brane and the product water is forced through the membranes

into the interior of the porous plate, which serves as mem-

brane support.

Two other configurations, are in use: The tubular and the

spiral wound. Both, as the plate and frame concept, use flat

sheet membranes, and another concept uses membranes in

the form of hollow fibers.

All membranes are assembled in units called modules.

Each module is self-contained and can be used as an indepen-

dent reverse osmosis unit. Most often, many modules con-

nected in series are used to increase capacity. Semipermeable

membranes are very thin, as are ion-exchange membranes

and fragile, and they need special support to withstand the

pressure applied for the separation. Assembling the vari-

ous types of modules special care must be taken to support

the membranes properly. The fact that water flow through

a membrane is proportional to the membrane area and

inversely proportional to its thickness renders a module a

device requiring the maximum possible area per unit volume

of water passing through the membrane. The membranes

are vulnerable to mechanical damage, fouling and scaling as

well as to concentration polarization phenomenon. A perfect

module has the following advantages.

• Minimizes the effects of concentration polarization.


• Has high resistance in seawater corrosion and in
  the cleaning chemicals.


• Continuous operation at high pressure.


• Almost constant quality of freshwater produced.

Reverse osmosis is a method used not only for desalina-

tion of sea or brackish water, but also for the purification of

a large variety of various solutions in the chemical industry

by using suitable semipermeable membranes.

EVAPORATION

DISTILLATION

ADSORPTION/REGENERATION

ADSORPTION/REGENERATION

ION-EXCHANGE CENTRIFUGATION

FLOCCULATION/PRECIPITATION

CONVENTIONAL FILTRATION

DECANTING

35

30–60 Bar

100 1000 100,000 1.0^10

20–40 Bar 5–10 Bar 1–4 Bar

Pressure

Particle size

Porosity

Separation Process

102 46103 46104 46 10^54610624

A μm μm

RO NF UF MF

°

A°

FIGURE 17 Particle size separation by various membrane and the range where they can replace the conventional

chemical engineering separation methods. The particle size range for each membrane separation method depends on

the porosity of the membrane, increasing with the particle size, and on the pressure which is decreasing proportionally

with porosity increase.

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