Na HCO₃ R   SO₃ H R   SO₃ Na H HCO₃ H₂CO₃ H₂O CO₂ H Cl R   N(CH₃)₃OH R   N(CH₃)₃Cl HOH DOE-HDBK-1015/2-93 Principles of Water Treatment DISSOLVED GASES, SUSPENDED SOLIDS, AND pH CONTROL Rev. 0 CH-04 Page 17 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 3 - by the cation exchanger). (4-7) 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 3 - + (H CO  ).  Carbonic acid is a weak acid that will decompose to water and CO   by the following 2 3 2 reaction. (4-8) Because the carbonic acid readily dissociates, the aerator is used to remove the CO   from the 2 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 2 vented to the atmosphere.  The removal of CO   forces Reaction (4-8) to shift to the right, which 2 converts more H  CO   to CO  .  With sufficient aeration, all bicarbonate (HCO   ), and therefore 2 3 2 3 - CO    can be removed. 2, 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. (4-9) 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). 2 2