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66
Base exchange
Raw water from the mains passes through a tank of zeolite chemicals
where a base exchange takes place:
calcium carbonate + sodium zeolite —> sodium carbonate + cal-
cium zeolite
(in water) (in tank) (in water) (left in tank)
Similar base exchanges occur between the zeolites and other hardness
salts in the raw water, turning them into non-scale-forming salts. On complete
conversion of all the sodium zeolite, the filter bed is backwashed with brine
(sodium chloride solution) which undergoes exchange with the calcium zeolite.
The normal flow direction can then be resumed. The running cost of the system
is limited to consumption of common salt, a small pump, periodic replacement
of the zeolites and a small amount of maintenance work. Steam boilers accumu-
late the salts passing through the treatment plant, and if they were allowed to
become too numerous they would be carried over into the steam pipes and clog
safety valves and pressure controllers. Either continuous or intermittent blow-
down of boiler water to the drain is designed to control salt concentration. The
high-pressure blow-down water is cooled before being discharged into the
drains, and the heat is recycled.
Demineralization
Complete removal of mineral salts is very expensive, but it is essential for
power station steam boilers, high-performance marine boilers and some manu-
facturing processes, where the presence of impurities is unacceptable. Raw wa-
ter is passed through chemical filters in several stages to complete the cycle.
Reverse osmosis
Reverse osmosis is a filtration technique in which untreated water is
pumped alongside a semi-permeable membrane in a pipe system. Clean water
passes through the membrane. This method is used to produce drinking water in
desert regions.
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