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.
Comparison The RO+Liqui-Cel system offers many benefits over a DPRO system. The RO+Liqui-Cel system eliminates the need for a second pass RO and an RO pump. This significantly reduces capital costs and less power is required because there is not a need for a second RO pump. Also, more water is recovered as the reject water from the second pass RO is not lost in the process. There is a significant space savings as the membrane contactor system has a much smaller footprint compared to the second pass RO system. Additionally, chemical consumption at the plant can be greatly reduced as the need to increase the pH before the second stage is no longer required to facilitate CO2 removal.
The RO+Liqui-Cel Contactor system offers some other unique benefits. This system can remove both Carbon Dioxide and Oxygen. If the water is used for other applications, such as boiler feed water, the system can deliver water with a high resistivity and low levels of dissolved oxygen. If THM is an issue, the Liqui-Cel Membrane Contactor system also has superior THM removal characteristics. The capital, operating and reduction in chemical consumption costs make this an exciting and superior system design for a high alkalinity water treatment system. For more information and system sizing, please contact your Membrana representative or visit us online at www.LiquiCel.com.