Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 217

Ultrafiltration and microfiltration do not desalinate waters

directly but are used for pretreatment of various kinds of solu-

tion and water, depending on the molecular weight of the

dissolved or suspended matter previous to reverse osmosis

treatment.

Reverse Osmosis Membranes The reverse osmosis mem-

branes are the main and more delicate component of the

method. They are usually permeable to some species, as water

and impermeable to other, as salts. They are characterized by

two important parameters: The water flux, or water perme-

ability and the salt rejection. Concentration polarization plays

a significant role and influences both the above parameters.

The water flux at a given temperature is determined by

the membrane properties and is defined by the equation:

J v  L p (  P  s  ) m^3 /m^2 s. (6)

Salt rejection is the ability of the membrane to reject the

solution salts and leave only the solute, i.e., the water to pass

the membrane mass. A perfect membrane would reject all

salts contained in the feedwater and be highly permeable to

the flux of water, or solute. Commercial membranes are not

ideal and have certain amount of salts to move through the

membrane. The salt rejection is defined as:

% Salt rejection

product concentration

feedwater concentrat ion



100.

(7)

A module characteristic is the recovery or yield which

defines the fresh water product, m^3 /d, to the feed water flow,

m^3 /d as well.

R  m d /m s (8)

R is influenced by the required salinity in the product, fresh

water.

The required mechanical energy to drive the RO pumps

is determined by the required separation pressure, which

in turn is proportional to the salt content of the feedwaters.

It ranges between 8.0 to 12.0 kWh per m^3 freshwater pro-

duced, from seawater feed and 2 to 3.5 kWh/m^3 of brackish

water feed. The corresponding pressure is 56 to 70 bars for

seawater (812 to 1015 psi) and 20 to 28 bars (246 to 406 psi)

for brackish water.

There is a large variety of membranes prepared from

organic materials. For commercial use up to now the types

of these composite membranes in general use are the ultra-

thin composite-spiral-wound membranes and the hollow

fine fiber membranes. The best developed composite mem-

branes are: Cellulose triacetate films deposited on a cellu-

lose diacetate-cellulose nitrate support, furfuryl alcohol film

on a polysulfone support and polyamide film on a polysul-

fone support. The polyamide-film membranes provide the

best desalination performance. This development in mem-

brane preparation brought the reverse-osmosis process next

in importance to distillation during the last years so that dis-

tillation and reverse osmosis are today leading processes in

seawater desalination.

The way by which the membranes are supported and

the properties of the membranes define the reverse osmosis

system.

A way to assemble membranes is the spiral wound module

(SWM). Spiral membranes are cellulose acetate polymers or

can be made of the thin film composite type. Spiral mem-

branes have a diameter of 5.1 to 30 cm (2.0 to 11.8 inch) and

a packing density of about 600 m^2 /m^3.

Another module system is the tubular (TM) where the

membrane surface is packed in a shell and tube arrangement.

The membranes, usually cellulose acetate, are mounted

inside the tubes made of metal or plastic. The active layer,

i.e., the permeable layer, may be either on the inside or the

outside of the tubes.

The hollow fine fiber module (FHFM) are very thin fibers,

similar to human hair thickness. Their advantage is their

high surface area. Inside the modules they pre sent a packing

area exceeding 30,000 m^2 /m^3 of water produced. Millions of

fibers are assembled inside a cylindrical bundle embedded

in an epoxy resin tube sheet. Figures 18 to 20 present the

reverse osmosis modules commercially available.

Freezing Processes

All variants of the freezing processes are based on the

well-known phenomenon that, when a saline solution is

cooled to its freezing temperature, ice crystals of pure water

will form and the brine will be enclosed in the slurry.

The temperature of freezing is fixed by the concentra-

tion of the brine, while evaporation can operate over a wide

temperature range. Freezing has basic advantages:

1. much lower latent heat of phase transition in the
   solid state than for evaporation (0.33
   10 3 J/kg
   against 2.49
   10 3 J/kg or 143.2 BTU/lb against
   968 BTU/lb).


2. less heat losses (gains) because of working at
   temperature closer to the ambient;


3. no scale-formation from the usual impurities;


4. less corrosion of steel at the freezing point than at
   the boiling point of water; and


5. cheaper materials of construction may be used.

Against these there are basic disadvantages:

1. the time required for phase transition from liquid
   to solid is very much greater than that required
   from liquid to vapor;


2. handling the crystals of ice is very much more
   difficult than handling the fluids in evaporation
   processes;


3. separation of the pure water phase (ice versus
   steam) is extremely difficult;


4. the cost of removing heat energy is much more
   expensive than that of adding heat energy, and

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