Engineering Evaluation Case: A CO₂ Degassing Tower Removes Carbon Dioxide but the Outlet Water pH Still Does Not Reach the Target

Packed degassing towers are widely used to strip dissolved CO₂ from water.

A customer may define the desired result as:

“Raise outlet pH to 8.0.”

This sounds like a direct tower-performance target.

But pH is not determined by CO₂ concentration alone.

The relationship between:

• dissolved CO₂;
• alkalinity;
• mineral chemistry;
• temperature

means the packing supplier should be careful about guaranteeing a final pH from tower geometry alone.

Project Situation

Consider a water-treatment system using a packed forced-draft degasser.

The feed contains elevated dissolved CO₂.

The tower uses:

• plastic random packing;
• water distributor;
• upward air flow.

After commissioning:

• dissolved CO₂ decreases substantially;
• outlet pH rises;
• but the final pH remains below the desired value.

The client asks whether adding more packing will solve the problem.

Possibly—but first the water chemistry must be understood.

CO₂ Removal Can Raise pH

Dissolved CO₂ participates in the carbonate system.

Removing CO₂ reduces carbonic-acid influence and can cause pH to rise.

That is why packed degassers are useful in:

• groundwater treatment;
• RO systems;
• demineralization pretreatment.

But the final pH depends on the remaining chemical buffering of the water.

Alkalinity Controls the Response

Two waters with the same dissolved CO₂ concentration can respond differently after degassing if their alkalinity differs.

A low-alkalinity water may show a different pH response from a highly buffered water.

Therefore, one cannot reliably convert:

packing height → outlet pH

without the full water chemistry.

The Tower Should Be Rated for CO₂ Transfer

A more defensible process basis uses:

• inlet dissolved CO₂;
• required outlet dissolved CO₂;
• water flow;
• air flow;
• temperature;
• pressure.

The resulting pH can then be evaluated from the water chemistry.

This separates the mass-transfer duty from the chemical-equilibrium result.

Air-to-Water Ratio Matters

CO₂ stripping requires adequate gas flow.

Increasing air can increase the driving force.

However, more air also means:

• more blower energy;
• greater packing gas velocity;
• greater noise and off-gas volume.

The tower should not be pushed beyond hydraulic limits just to chase a pH number.

Temperature Changes the Duty

Gas solubility and transfer behavior change with water temperature.

Cold water may be more difficult to degas to the same level.

A tower designed on warm summer water should therefore be checked at the minimum expected temperature.

Water Distribution Remains Critical

Degassing towers often operate with:

• large water flow;
• relatively low-pressure air.

Poor liquid distribution creates areas where water receives insufficient air contact.

A taller bed cannot fully compensate for severe distributor maldistribution.

Excessively Fine Packing Can Increase Blower Cost

High-area media can improve transfer.

It can also increase:

• pressure drop;
• fouling sensitivity.

The optimum packing balances:

• transfer efficiency;
• blower energy;
• water quality;
• maintenance.

Scaling Can Develop as CO₂ Is Removed

CO₂ removal can change carbonate equilibrium.

In mineral-rich water, this may increase the tendency for certain scales to form.

Therefore, successful degassing can create:

• packing deposits;
• distributor deposits

under some water chemistries.

Water analysis should include scaling potential.

The Outlet pH Target May Need Additional Treatment

If the plant needs a precise pH after degassing, additional:

• chemical dosing;
• mineral conditioning

may sometimes be required.

The correct solution depends on the downstream process.

It is not always economical to achieve the entire pH adjustment through air stripping alone.

What Should Be Measured?

Useful data include:

• inlet and outlet dissolved CO₂;
• inlet and outlet pH;
• alkalinity;
• temperature;
• water flow;
• air flow;
• bed differential pressure.

These measurements identify whether the tower is failing to remove CO₂ or whether the water chemistry simply produces a different pH than expected.

Engineering Takeaway

A CO₂ degasser removes a chemical species.

It does not directly “manufacture” a guaranteed pH.

The final pH is the result of the complete carbonate and mineral system.