Application Note: pH Control Protects Membranes In Reverse Osmosis

Reverse osmosis is a technique for removing dissolved solids from filtered raw water. It is used in a variety of industries to condition water for plant use, or as a first step in the demineralization process.

Reverse osmosis works by pressurizing water and forcing it through a semi-permeable membrane, which is permeable to water, but relatively impermeable to dissolved solids. The effectiveness of a reverse osmosis unit is based on its percent efficiency in removing dissolved solids and, generally, the better the quality of raw water, the better the quality of the processed water.

Process
The key factor in reverse osmosis is the condition of the semipermeable membrane. The typical membrane material is cellulose acetate, which tends to be degraded by alkaline (high pH) water, resulting in a loss of efficiency. The reverse osmosis membrane can also represent a considerable capital investment.

Precipitation can occur on the process side of the membrane, when the raw water contains calcuim harness and its pH is in the alkaline range.

To protect the membrane and avoid scaling, the pH of alkaline raw water can be adjusted to the acid side (pH 5.5 is the usual target). The control action is not difficult since raw water does not typically tend to have major pH fluctuations or load changes (changes in the titration curve). To accommodate changes in flow rate, a flow measurement can be used to trim the pH control. The mode of control will be specific to the application.