The pH of the water into the RO membrane
The pH of the water into the RO membrane can be raised to shift the equilibrium equations to favor the carbonate side. In this process there is little carbon dioxide gas present in the water. The RO membrane will reject the ionic species and little or no carbon dioxide gas will be present downstream of the RO membrane.
When adjusting the pH of the water, chemicals are added to the water. This adds to the contamination of the reject water that needs to be treated. Water that has high alkalinity may also foul the RO membrane. In order to prevent this fouling, anti-scalants are typically used. This again increases the chemicals added to the water.
Since water quality changes seasonally the control of the chemical addition must be designed to take this into account. This adds to the complexity of the pH control system.
The major drawback of pH control is that the user must add additional chemicals into the water stream. This adds to the chemical, handling, storage and treatment of the waste streams generated from the usage of these chemicals.