R SO3 H
R SO3 Na
Principles of Water Treatment
DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL
The cation exchanger contains resin in the hydrogen form. In this treatment step, essentially all
cations entering the ion exchanger will be held at the exchange site, and H will be released as
shown in the following typical reaction (the anions, specifically the HCO ions, are unaffected
by the cation exchanger).
The water leaving the resin is somewhat acidic (depending on the incoming ion concentration)
because it contains H ions and whatever anion was associated with the incoming cation. After
passing through the cation resin, the HCO ions combine with the H ions to form carbonic acid
(H CO ). Carbonic acid is a weak acid that will decompose to water and CO by the following
Because the carbonic acid readily dissociates, the aerator is used to remove the CO from the
makeup water at this point in the system. If we aerate the water by some means, such as spraying
it through a tower or blowing air through the water, the CO is "stripped" from the water and
vented to the atmosphere. The removal of CO forces Reaction (4-8) to shift to the right, which
converts more H CO to CO . With sufficient aeration, all bicarbonate (HCO ), and therefore
CO can be removed.
Similar reactions occur in the anion exchanger. For example, anion resin, which has hydroxide
ions at the exchange sites, will react as indicated in the following typical reaction.
In this pretreatment system, the anion resin is downstream of the cation resin, and the only cations
present are hydrogen ions. When the hydroxyl ions are released from the anion exchange sites,
they combine with the hydrogen ions to form water. As a result, pure water appears at the
effluent (this is somewhat overstated because a very small amount of other cations and anions
pass unaffected through the resin columns in actual practice).
Another method sometimes used to remove dissolved gases from water is deaeration. In this
process, the water is stored in vented tanks containing electric heaters or steam coils. The water
is heated to a temperature sufficient for slow boiling to occur. This boiling strips dissolved gases
from the stored water, and these gases are then vented to the atmosphere. Usually, the vented
gases are directed through a small condenser to limit the loss of water vapor that would escape
as steam along with the gases. This method is particularly effective in removing dissolved oxygen
as well as other entrained gases (CO , N , and Ar).