degasification, Air Stripping, Decarbonation:
These terms are often used synonymously in high purity water treatment. In this discussion, they refer to the removal of carbon dioxide from water. Carbon dioxide, along with nitrogen and oxygen, comprise the majority of atmospheric gases. Water contains these gases in solution, following Henry’s Law of gas solubility that defines a proportional relationship between the amount of a gas in a solution and its partial pressure in the atmosphere.
Carbon dioxide, however, is an exception to Henry’s Law because it reacts with the water to form carbonic acid, which then ionizes into hydrogen and bicarbonate ions. Since the bicarbonate ions are not subject to Henry’s Law, only a small amount of it can be released by aeration. Carbon dioxide in the water that does not form bicarbonates is “uncombined” and can be removed by aeration.
The pH of the water affects equilibrium between bicarbonate ions and carbon dioxide. At a pH below approximately 4.5, all of the carbon dioxide dissolved in the water is present as a gas. At a pH of about 8.5, all the carbon dioxide is ionized. For this reason, decarbonation by airstripping is only effective at low pH with the pH reduction resulting from prior process or acid addition.
High purity water often must exhibit 10 megohms resistivity or higher to meet quality specifications. Since carbon dioxide ionizes in water to form hydrogen which ionizes to hydronium ions and bicarbonate ions, it decreases the resistivity. In a deionizer, the anion resin must remove bicarbonate ions along with the other dissolved anions. In treating water having high alkalinity, a large amount of carbon dioxide is formed as the water encounters cation resin, thus putting a considerable burden on the anion capacity.
However, if two-bed deionizers are employed upstream of mixed bed polishers, air stripping may be used after the cation exchanger where the pH is 4.5 or less to remove up 90% of the carbon dioxide gas. The same principal also applies to downstream treatment of water from reverse osmosis machines. Depending on the percentage of alkalinity relative to other dissolved solids in the raw water, this can increase capacity of the anion and mixed bed exchangers by several multiples.
Air strippers or decarbonators provide a means to atomize the water, thus increasing surface area, while passing air through the droplets which has a relatively low partial pressure of carbon dioxide (i.e., atmospheric concentration) relative to that of the carbon dioxide entrained in the water. Following Henry’s Law, gas is liberated from the water containing a high concentration of carbon dioxide.
These strippers are typically large, put a lot of moisture into the air and can entrain particles of dirt or contamination in the water. For this reason, they are generally not useful for producing high purity water and have more typically been used in boiler feedwater preparation or water used in industrial processes. However, the advent of membrane degassifiers allows degasification of high purity streams, both for carbon dioxide removal as well as to remove oxygen. The WaterProfessionals® utilizes these on large and small systems to reduce customer’s operational costs.