1320 WATER TREATMENT
adsorbed on the negatively charged calcium carbonate par-
ticles and a stable colloidal suspension may result. Iron and
manganese are objectionable constituents of water supplies
because they impart a brown colour to laundry goods and
frequently will stain household plumbing fittings.
Precipitation of iron and manganese can also be satisfac-
torily accomplished by using the lime-soda-ash process as
described above for softening.Ion ExchangeIon exchange units are most frequently used for softening
waters, but are also used by certain industries for the production
of de-ionized water. This is quite common in the brewery
industry, where an attempt is made to strip the water down
to its most pure constituents so that water in one part of the
world is similar to water in other parts of the world. Following
de-ionization, breweries and often distilleries will reconsti-
tute the water so that the water used for the production of a
certain type of beer will be the same all over the continent
and not have the variations which were characteristic of beers
when native waters were used for their production.
The chemistry of the ion exchange process is shown below,
where a cation resin which will exchange the sodium (Na^ ^ )
for the calcium and magnesium (Ca^2 ^ , Mg^2 ^ ). When the resin
is saturated with calcium and magnesium, a regeneration is
required such as is used in household water softening units,
when a very strong brine solution is forced back through the
resin and in turn displaces the calcium and magnesium into the
backwash line and restores the sodium on the resin for further
softening.(^)
Softening Na
Ca
Mg
(HCO ) Ca
SO
Cl Mg
2
32
4
2
→
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
(1)
R
2NaHSO
Na SO
2NaCl
4
24
⎡
⎣
⎢
⎢
⎢
(^)
Regeneration
Ca
Mg
R 2Na Cl Na R
Ca
Mg
22 Cl
⎤
⎦
⎥ →
⎤
⎦
⎥
(2)
Desalination
Although the principles of desalination were fully known in
Julius Caesar’s time, the energy requirements of this process
are presently so high that these will be usually considered as
a last resort after all other water sources have been explored.
Water quality is frequently referred to as fresh, brackish, sea
water or brine. Fresh water normally contains less than 1000
mg/liter of dissolved salts, while brackish water ranges from
1000–35,000 mg/liter of dissolved salts. Sea water contains
35,000 mg/liter of dissolved salts, whereas brine contains very
much more from salt water by a semi-permeable membrane,
the fresh water will tend to flow into the salt water to equal-
ize the concentration of salts on both sides of the membrane.
Bearing in mind that the membrane will not allow the salts to
pass back, it is clear that a certain pressure which is known
as the osmotic pressure is forcing the fresh water through
to the brine side of the membrane. If a force greater than
this osmotic pressure is applied on the sea water side, then
fresh water will flow backwards through the semi-permeable
membrane at a rate proportional to the incremental pressure
over the osmotic pressure. In practice, quite high pressures
are required in order to get a useful volume of water to pass
through the membrane—such pressures as 40–100 kg per
square centimeter. This has been shown to work for waters
of fairly high dissolved solids, but the structural properties of
the membranes must be fairly well developed and of course
the membranes must be very well supported. Membranes
used for this type of process are frequently cellulose acetate
or some derivatives thereof. The power requirement for this
process is considerably less than electrodialysis, but it is a
S.W.
14
IN
18
HEAT EX.
10
19
11
24 28
13 16
VAPORIZATION-COOLING
STEAM
GENERATOR
SOLUTION
HEAT EX.
27 VENT
ABSORBER
CONC
WATER
SEA
OUT
FRESH WATER
OUT
FIGURE 14 Conventional multistage
flash evaporation – MSF evaporating
and cooling of hot feed brine (vertical
arrows down) on left side at succes-
sively lower pressures after heating
to highest temperature in prime heater
(PH) at top; vapors (horizontal ar-
rows) from MSF, passing to preheat
the sea water by condensation-heating
on right side; fresh water condensate
passing stagewise from top to dis-
charge at bottom; additional sea water
coolant (dotted line) rejecting heat in
lower stages, withdrawing of vapors
from prime heater to be condensed in
half-stage (dashed lines) increasing
the production of fresh water.
C023_006_r03.indd 1320C023_006_r03.indd 1320 11/18/2005 1:32:35 PM11/18/2005 1:32:35 PM