Why is the pH of RO permeate lower than the feedwater pH?
This question can be best answered when you understand the equilibrium relationship between CO2, HCO3, and CO3. In a closed system, the relative amount of each of these CO2, HCO3, and CO3 vary with pH. At lower pH levels, CO2 is the predominant species. HCO3 is the predominant species at mid pH levels, and CO3 is present at higher pH levels. Since RO membranes will reject dissolved ions and not reject dissolved gases, the RO permeate and RO feed will contain roughly the same amount of CO2. The HCO3 and CO3, however, are often reduced by 1-2 orders of magnitude. This upsets the CO2, HCO3, CO3 equilibrium that was established in the feed. In a series of equilibrium reactions, CO2 will combine with H2O driving reactions similar to that shown below, until a new equilibrium is established. CO2 + H2O à HCO3 + H+ The new equilibrium will always result in a lowering of permeate pH if there is CO2 present in the feedwater. There is typically a 1-2 unit pH drop in the permeate for most RO system
