In high alkalinity feed water applications one economical option is to remove carbon dioxide from the water. Carbon dioxide gas dissociates in water to form HCO3 - and CO3 2-. These ionic species will contribute to the total anionic load to the EDI.
Historically, the conventional method for CO2 removal was to use a forced draft degassing tower (FDA) downstream of the RO. However, this is not a recommended practice today as the environmental air used in the tower comes into
direct contact with the RO water which could cause significant contamination.
Liqui-Cel® Membrane Contactors are the superior solution for CO2 removal. They operate under the same principle
as a conventional FDA except the air and water contact is minimized with a membrane. Since the membrane pores are 0.02 microns in diameter, particles and other debris cannot pass through the pores to contaminate the RO water.
The contactor operates by flowing gas on one side of the membrane and liquid on the other. Gasses are removed from the liquid by lowering the partial pressure of the gasses in contact with the liquid. Filtered room air is drawn into the contactor using a vacuum pump. Since the room air and vacuum pump are low in carbon dioxide they create the driving force to remove the gas from the water.
A chemical plant in China is using a Liqui-Cel Membrane Contactor system to remove CO2 downstream of a single
pass RO system. This design was selected over conventional two pass RO with a caustic injection system due to savings in capital and operating costs. The system was designed, built and installed by Changzhou Xianfeng Water Treatment Equipment Co., Ltd. The Deionization (DI) system is a fully integrated membrane system and operates
without any chemical addition.