How Does the Presence of Dissolved Gases Affect a Residential RO Membrane?
When people discuss residential reverse osmosis (RO) systems, they often focus on contaminants such as heavy metals, dissolved salts, bacteria, and organic matter. However, dissolved gases in water can also influence the performance of an RO membrane and the overall efficiency of the purification system.
Understanding how these gases interact with RO membranes can help homeowners improve water quality, extend membrane lifespan, and avoid common operational issues.
What Are Dissolved Gases in Water?
Natural water sources contain various dissolved gases that enter the water through contact with the atmosphere, biological activity, and industrial pollution. The most common dissolved gases found in residential water supplies include:
- Oxygen (O₂)
- Carbon dioxide (CO₂)
- Hydrogen sulfide (H₂S)
- Ammonia (NH₃)
- Chlorine (Cl₂)
Although these gases are usually present at relatively low concentrations, they can still affect membrane performance and water quality in different ways.
Carbon Dioxide Can Increase Product Water TDS
Carbon dioxide is one of the most common dissolved gases found in groundwater and municipal water supplies. Unlike dissolved salts, CO₂ molecules are very small and can pass through RO membranes more easily.
After passing through the membrane, carbon dioxide reacts with water to form carbonic acid, which partially dissociates into ions. As a result, homeowners may notice that the TDS reading of RO water remains higher than expected even when the membrane is functioning properly.
This often leads users to believe that the membrane has failed, when in reality the elevated TDS may be caused by dissolved carbon dioxide rather than poor membrane rejection.
Dissolved Oxygen May Promote Fouling and Corrosion
Oxygen itself does not directly damage the RO membrane. However, high dissolved oxygen levels can accelerate the oxidation of iron and manganese present in the feed water.
The resulting iron oxides and manganese oxides may accumulate on the membrane surface, causing fouling and reducing water production. In addition, oxygen can contribute to corrosion of metal components within the water treatment system, generating particles that further contaminate the membrane.
Proper pretreatment and regular maintenance can help minimize these issues.
Hydrogen Sulfide Causes Odor Problems and Biological Growth
Hydrogen sulfide is commonly associated with a rotten egg smell. It is frequently found in private wells and groundwater sources.
When hydrogen sulfide enters an RO system, it can create unpleasant odors and contribute to biological contamination. Sulfur-oxidizing bacteria may use hydrogen sulfide as an energy source, leading to biofilm formation on the membrane surface.
Biofouling is one of the most difficult types of membrane contamination to remove and can significantly reduce membrane performance if left untreated.
Activated carbon filtration or oxidation pretreatment is often recommended when hydrogen sulfide is present.
Ammonia May Reduce Overall Water Quality
Ammonia exists in water as either dissolved ammonia gas or ammonium ions. Because uncharged ammonia molecules are small, they can pass through RO membranes more easily than many dissolved salts.
As a result, some ammonia may remain in the permeate water, affecting taste and water quality. In areas with agricultural runoff or wastewater contamination, ammonia levels should be monitored carefully and appropriate pretreatment measures should be considered.
Chlorine Is the Most Dangerous Dissolved Gas for RO Membranes
Among all dissolved gases, chlorine poses the greatest risk to residential RO membranes.
Most residential RO membranes are manufactured from thin-film composite (TFC) polyamide materials, which are highly sensitive to chlorine. Even low concentrations of free chlorine can gradually oxidize the membrane surface, reducing salt rejection and shortening membrane life.
For this reason, activated carbon filters are typically installed before the RO membrane to remove residual chlorine from municipal tap water.
If chlorine breakthrough occurs, membrane damage can become irreversible.
How to Minimize the Impact of Dissolved Gases
The best approach is to identify the specific gases present in the feed water and apply appropriate pretreatment methods.
For example, activated carbon filters can effectively remove chlorine and improve odor problems. Aeration systems can help release excess carbon dioxide and hydrogen sulfide. In some cases, degassing equipment may be installed to improve overall RO performance.
Regular water testing is also important, especially for households using private well water, where dissolved gas concentrations can vary significantly over time.
Although dissolved gases are often overlooked, they can have a noticeable impact on residential RO systems. Carbon dioxide can affect TDS readings, hydrogen sulfide can create odor and biofouling issues, dissolved oxygen can contribute to fouling and corrosion, ammonia may reduce water quality, and chlorine can permanently damage RO membranes.
By understanding these effects and implementing suitable pretreatment measures, homeowners can improve membrane performance, maintain better water quality, and extend the service life of their residential RO membranes.