Encyclopedia of Environmental Science and Engineering, Volume I and II

DESALINATION 197

Desalination eliminates the main elements from sea-

water, producing fresh water and concentrated brine,

almost saturated in the main salts, which are withdrawn to

the sea. There are two main reasons that these salts are not

exploited. The brine volumes are huge and cannot be han-

dled easily. The present extraction technology is expensive

for the relatively cheap materials. Nevertheless, there is

some industry exploiting, in part, the concentrated brine.

Today throughout the world more than 1,900,000 m^3 /d

(500 MUSGPO) of fresh water is produced by the various

desalination processes.^5 Usually twice to 2½ as much sea-

water is processed, so that the solids concentration of the

brine is doubled. It is estimated that the recovery from the

withdrawn brine can be:

Magnesium 2,306.000 t/y Bromine 116.100 t/y

Calcium 728.000 t/y Copper 5.385 t/y

Potassium 659.000 t/y Uranium 5.385 t/y

Sulfate 4,855.000 t/y Gold 7.2 kg/y

Calcium and magnesium are the main elements that cause

scale formation. Scales are formed and precipitate inside

desalination equipment simultaneously with other suspended

solids content in the feed water sea or brackish. These materi-

als precipitate in areas favored for deposition. In distillation

plants these are the heat exchangers and, in reverse osmosis,

the semipermeable membranes, cause the problems. These

deposits are categorised in two main types, the sludge which

is soft and can be easily washed out, and the scale which is

hard, adheres to heat transfer surfaces and can be removed

only by plant shutdown.

Brackish waters are classified as waters with total dis-

solved solids content ranging from 3,000 ppm to 20,000 ppm.

The elements vary widely, depending on the rocks and soil

coming in contact with the water. In some brackish waters

large amounts of calcium sulfate are present up to satura-

tion conditions, making the water bitter and unsuitable for

any use.

DESALINATION PROCESSES

When all other possibilities to use existing natural water

resources are exhausted or to augment fresh water supply

by conventional methods fail, then desalting of seawater, or

brackish water and/or of polluted water reserves might give

the answer to local water problems. The cost of desalting has

been drastically reduced over the past several years. This is

TABLE 1

Total Dissolved Solids in Various Seas

Ocean/Sea g/kg ppm Ocean/Sea g/kg ppm

Baltic Sea 7.0 7,000 Pacific Ocean 33.6 33,600

Caspian Sea 13.5 13,500 Atlantic Ocean 36.0 33,600

Black Sea 20.0 20,000 Mediterranean Sea 39.0 39,000

White Sea 28.0 28,000 Red Sea 43.0 43,000

Northern Adriatic 29.0 29,000 Kara Bogar (Caspian) 164.0 164,000

Dead Sea 270.0 270,000

TABLE 2

Ionic Composition of Main Elements in Seawater^6

Ions g/kg Ions g/kg

Chlorides Cl− 18.980 Copper Cu 0.003 × 10 −^3

Sodium Na+ 10.560 Uranium U 0.003 × 10 −^3

Sulfates SO^24 − 2.560

Magnesium Mg^2 + 1.270 Total TDS — 34.482

Calcium Ca^2 + 0.400 Water H 2 O 965.518

Potassium K+ 0.380

Hydrogen Seawater characteristics

Carbonates HCO 3 0.143 Salinity g/kg 34,330

Bromides Br− 0.065 Chlorinity g/kg 19,000

Boric acid H 3 BO 3 0.026 Chlorocity g/kg 19,950

Strondium Sr^2 + 0.014 Specific weight N/m^3 10,243

Fluorides F 0.001

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