How Does pH Affect RO Membrane Performance?

pH is an indicator that measures the acidity or alkalinity of a solution, ranging from 0 to 14. A value of 7 is neutral; below 7 is acidic, and above 7 is alkaline. Most membrane elements are suitable for feed water with a pH range of 3–11.

When the water pH is too low, acidic solutions can make the membrane material brittle, potentially causing small cracks and holes, which reduces filtration efficiency.

Additionally, acidic feed water often occurs in places like plating factories, where the water may contain metal ions. When these dissolved metals contact the membrane, they can deposit on its surface, forming scale. This scale can block membrane pores, reduce water flow, and increase the pressure required for water to pass through the membrane.

For this type of acidic feed water, specific membrane products are recommended, such as our Acid-Resistant Membrane or membranes designed for metal recovery:

Acid-Resistant Membrane:

Separates divalent and multivalent metal ions in acidic solutions for purification and metal recovery. Suitable for metal ion separation in solutions containing 15% phosphoric acid, 5% nitric acid, or 20% sulfuric acid.

For ordinary RO membranes, filtering strong acidic solutions or water containing metal ions is highly inadvisable, as it can easily damage the membrane.

3. Effects of Alkaline Solutions on Membranes

On the other hand, when water has a high pH (alkaline), it also presents challenges for the membrane. Alkaline water can cause certain minerals, such as calcium carbonate and magnesium hydroxide, to precipitate. These deposits attach to the membrane surface, similar to metal scale in acidic water, but are usually harder to remove.

High-pH water can also affect the surface charge of the membrane. Most RO membranes carry a negative surface charge, which helps repel negatively charged contaminants. Under alkaline conditions, the membrane's surface charge may change, reducing its ability to repel contaminants. This can allow more impurities to pass through the membrane, resulting in lower water quality.

When treating water with high pH, it is recommended to first perform pretreatment to remove high concentrations of calcium and magnesium ions before using the RO membrane normally.