Deionizer / Deionization/Demineralization
The words are derived from de (remove) and ions (inorganic mineral elements dissolved in water that are clear and colorless). Examples would be the ions of sodium (Na+) and chloride (Cl-) which result from dissolving of salt or sodium chloride (NaCl). Another would be the ions of calcium (Ca+) and carbonate (CO3-) which result from dissolving limestone (CaCO3).
Depending on the design of the equipment the deionizer will remove 99% to 99.999% of all ions in the raw water. The conventional measurement of the quality of deionized water is ohms of resistance across a 1 centimeter sample of water. At 99% removal a typical value of resistance is 100,000 ohms and at 99.999% the reading will be 18,000,000 ohms which is usually referred to as 18 Meg. water. To put 18 Meg water into perspective, the theoretical maximum resistance across a 1 cm sample of water is 18,200,000 ohms or 18.2 Meg. water.Sometimes, the water user wishes to have their water quality expressed as conductivity with the units being micromhos (umho) or microsiemens (uS). Nothing fancy, just divide 1,000,000 by the resistance and presto you have the conversion. 100,000 ohm water is therefore 10 umho / 10 uS, 1,000,000 water is 1 umho / 1 uS and 18 Meg. Water is 0.055 umho / 0.055 uS.
To achieve deionized water, two resins similar to softening resins are used. One is in the Hydrogen (H) or cation form and the other is in the Hydroxyl (OH) or anion form. “Form” indicates what is given up or exchanged when water with other ions pass by. Positive charged ions (those with a +) are removed and replaced with H+ and negative charged ions (those with a -) are removed and replaced with OH-. You’re ahead of me aren’t you — that’s correct, the H and OH combine and we have H2O or water. Dissolved ions go in and extra water comes out.
The resins must be regenerated to replace the Hydrogen and the Hydroxyl. The process is the same as a softener except the chemicals replace the salt in a softener. The source of hydrogen is hydrochloric acid HCl or sulfuric acid H2SO4 and the source of the hydroxyl is sodium hydroxide NaOH which is also called caustic soda. Both chemicals are dangerous to handle and often restricted in use because of drain discharge issues. Units therefore are normally sold to companies with trained personnel. The drain issue can be addressed by balancing the acid and caustic use and then mixing to neutralize prior to discharge.
Lower resistance values are achieved with two column deionizers—water passes through a acid/cation tank first and then through a hydroxyl/anion tank last. Higher resistance values require mixed bed deionizers where as the name implies a ratio of about 40% cation and 60%anion resin are in the same pressure vessel.
Because of chemical costs, safety issues and discharge problems many engineers remove 95% or more of the dissolved ions with a reverse osmosis unit before the deionizer thus increasing the capacity of the deionizer by 20X and reducing the chemical use by the same factor. For further study of the deionization/demineralization process we recommend “Demineralization by Ion Exchange” by Applebaum.