Why Is the pH Value of RO Treated Water Usually Lower Than That of the Influent?| Insights by AQUALITEK

Why RO Permeate Water Usually Has a Lower pH Than the Influent

Reverse osmosis does more than remove salts and impurities—it also changes water chemistry. One noticeable effect is a drop in pH. While this is normal, many operators are unsure why it happens. Understanding the root causes can help with troubleshooting, system optimization, and ensuring proper post-treatment.

Below is a complete explanation of why RO permeate typically has a lower pH.

1. RO Removes Buffering Ions, Making Water Less Stable

Feed water usually contains carbonate and bicarbonate ions (HCO₃⁻ / CO₃²⁻), which act as natural alkalinity buffers, stabilizing the pH.

However, RO membranes remove:

•Bicarbonate

•Carbonate

•Other alkalinity species

Result:
RO permeate has extremely low alkalinity, meaning its pH can shift easily even with very small amounts of dissolved gases.

Even a little dissolved CO₂ can significantly lower the pH.

2. CO₂ Passes Through the RO Membrane More Easily Than Alkalinity

CO₂ is:

•Neutral in charge

•Very small in molecular size

•Highly permeable through RO membranes

Because CO₂ passes through more readily than carbonate/bicarbonate, the permeate may contain relatively higher CO₂ concentrations compared to its alkalinity.

Once CO₂ dissolves in water, it forms carbonic acid:

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

This reaction lowers the pH.

3. Temperature and Pressure Changes Increase CO₂ Solubility

During RO operation:

•High pressure causes more CO₂ to dissolve

•Pressure drop after the membrane encourages CO₂ formation

•Different temperatures shift CO₂ equilibrium

The result is more free CO₂ in the permeate, which lowers pH after depressurization.

4. Permeate Tends to Be Slightly Acidic by Nature

Since RO permeate lacks mineral content, it naturally leans acidic.

Typical RO permeate pH range:

•pH 5.5–6.8, depending on

Feed alkalinity

CO₂ content

Temperature

System design

Even if the feed water pH is neutral (7.0–7.5), permeate can still be lower.

5. No Mineral Content Means No Neutralization Capacity

Minerals such as:

•Calcium

•Magnesium

•Sodium

•Potassium

are removed in RO treatment. Their absence means the water has no capacity to neutralize carbonic acid, allowing pH to shift downward easily.

6. Permeate pH Is Not a Reliable Indicator of Membrane Status

A common misconception is that low pH means membrane problems.

In reality:

•RO permeate pH varies naturally

•It is affected more by CO₂ and alkalinity than membrane performance

•Changes in feed water chemistry cause pH fluctuations

Operators should avoid using permeate pH as a main diagnostic indicator.

7. Is Low pH Harmful? What Should Operators Do?

Low pH RO permeate is generally not harmful to membranes or system components.
However:

Potential concerns:

•Corrosion in downstream pipes or tanks

•Impact on remineralization processes

•Effects on product water specifications

Common solutions:

•pH adjustment with NaOH or calcite filters

•Degasification to remove CO₂

•Post-treatment remineralization

Conclusion

RO permeate pH is usually lower than the influent mainly because:

1.RO removes alkalinity and destabilizes pH

2.CO₂ easily permeates through membranes

3.Dissolved CO₂ forms carbonic acid

4.Permeate lacks minerals and buffering capacity

This is a normal and expected characteristic of RO systems. Understanding the chemistry helps in proper system monitoring, post-treatment adjustments, and overall operational stability.