Using membrane contactors for CO2 removal to extend resin bed life


In today’s economic environment when prof its are reduced due to rising costs, it is necessary to find ways to save time and money. City Public Service, San Antonio, Texas, had a need to be more cost effective with their water system. The utility had an older system that could not be completely upgraded. Budget constraints prevented a complete system replacement; therefore, efforts were focused on improving operating efficiency of the existing system.


After a water analysis was performed, it was determined that a high amount of bicarbonate alkalinity present in the water was having a negative effect on the anion bed life. They investigated ways to remove this high amount of alkalinity from the system. After evaluating alI of the options, City Public Service made the decision to put in a membrane degassing system between the cation and anion beds to remove the free carbon dioxide (CO ).


Ion Exchange Fundamentais

The process of ion exchange (IX) removes positive and negative ions from water and replaces them with a selected positively or negatively charged ion. As water moves through a cation bed, the cation-exchange resin will attract positively charged ions (e.g., calcium [Ca] or magnesium [Mg]) to negatively charged sites on the resin. The positive ion then bonds to the resin, and the hydrogen (H) ion that was there is released to the water. The cation-exchange process lowers water pH with the release of the H ions into the water stream.


Water entering an anion bed passes over positively charged anion-exchange resin, attracting negative ions (e.g., ni- trate [NO3] or bicarbonates [HCO3 ]). At this stage, the anion-exchange resin releases hydroxyl radicals (OH). The addition of OH groups increases the water pH level to approximately neutral (pH - 7).

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