Why is My Reverse-Osmosis Water Not pH Neutral?

Reverse osmosis (RO) is a popular water purification method that uses a partially permeable membrane to remove contaminants from water. Many people use RO systems to purify tap water, expecting the resulting water to have a neutral pH of 7. However, sometimes the pH of reverse-osmosis water can be slightly acidic or alkaline. In this article, we'll explore the reasons behind this phenomenon and how it can be addressed.

What is pH and Why is It Important?​

pH is a measure of the acidity or alkalinity of a solution, with values ranging from 0 to 14. A pH of 7 is considered neutral, while values below 7 are acidic and above 7 are alkaline. The pH of water is important for several reasons, including its impact on the taste of water, its compatibility with certain plumbing materials, and its effects on human health.

Factors Affecting the pH of Reverse-Osmosis Water​

There are several factors that can contribute to reverse-osmosis water not being pH neutral. Some of these factors include:

Carbon Dioxide and the Carbonic Acid Equilibrium​

Reverse-osmosis membranes effectively remove most ions and dissolved solids from water, but they may not remove all dissolved gases, including carbon dioxide. When carbon dioxide dissolves in water, it forms carbonic acid, which can lower the pH of the water. The amount of carbon dioxide present in the feed water can influence the pH of the purified water.

Membrane Material and Performance​

The material and performance of the RO membrane can also affect the pH of the water. Some membranes may allow certain ions to pass through, altering the pH of the purified water. Over time, the performance of the membrane may also decline, leading to a change in the water's pH.

Post-Treatment Processes​

Some reverse-osmosis systems include post-treatment processes, such as adding minerals back into the water or using an additional filtration stage. These processes can alter the pH of the purified water, depending on the minerals added or the type of filtration used.

How to Address pH Imbalance in Reverse-Osmosis Water​

If the pH of your reverse-osmosis water is not neutral, there are several steps you can take to address the issue:

Check the RO System​

Ensure that your RO system is functioning correctly and that the membrane and filters are in good condition. Replace any worn or damaged components as needed.

Add a pH Neutralization Filter​

You can install a pH neutralization filter after the RO membrane to correct the pH of the water. These filters typically use calcite or other minerals to raise the pH of acidic water or use a blend of minerals to balance the pH of both acidic and alkaline water.

Use a Remineralization Cartridge​

A remineralization cartridge can be added to the RO system to reintroduce essential minerals removed during the purification process. Adding minerals like calcium and magnesium can help balance the pH of the water and improve its taste.

Monitor and Adjust​

Regularly monitor the pH of your reverse-osmosis water using a pH testing kit or meter. If necessary, make adjustments to your RO system or post-treatment processes to maintain the desired pH level.

Conclusion​

Although reverse-osmosis water is expected to be pH neutral, various factors can cause the pH to deviate from neutrality. By understanding the causes of pH imbalance and implementing appropriate measures, you can ensure your reverse-osmosis water maintains a pH level that is suitable for your needs.