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A double pass system allows the opportunity to remove carbon dioxide gas from the permeate by injecting caustic between the first and second pass. C)2 is undesirable when you have mixed bed ion exchange resin beds after the RO. By adding caustic after the first pass, you increase the pH of the first pass permeate water and convert CO2 to bicarbonate(HCO3-‐) and carbonate (CO3-‐2) for better rejection by the RO membranes in the second pass. This can’t be done with a single pass RO because injecting caustic and forming carbonate (CO3-‐2) in the presence of cations such as calcium will cause scaling of the RO membranes.
Liqui-Cel Contactors over a forced draft deaerator for CO2 removal
