Engineering Evaluation Case: A Groundwater Air Stripper Works in Summer but VOC Removal Drops During Winter

Packed air-stripping towers remove volatile contaminants from water by transferring them from the liquid phase into an air stream.

The process can be highly temperature sensitive.

A tower that meets the outlet requirement in warm summer conditions may perform differently when the incoming groundwater becomes much colder.

The equipment has not changed.

The equilibrium and mass-transfer conditions have.

Project Situation

Consider a packed air stripper treating VOC-contaminated groundwater.

The system includes:

• water distributor;
• random packing;
• upward air flow;
• downstream off-gas treatment.

During summer:

• inlet water is relatively warm;
• VOC removal meets the target.

During winter:

• water temperature falls significantly;
• airflow and water flow remain similar;
• outlet water VOC concentration increases.

The initial suspicion may be fouling or packing damage.

Seasonal temperature should be checked first.

Temperature Changes Volatility

Air stripping depends on the tendency of the contaminant to move from water into the gas phase.

For many volatile compounds, this tendency changes with temperature.

Colder water can hold the contaminant differently than warmer water.

The gas-liquid equilibrium therefore shifts.

A tower designed only from a warm-water case may have insufficient transfer capacity during winter.

Liquid Properties Change Too

Lower temperature can also increase water viscosity slightly and influence:

• film thickness;
• mass-transfer coefficients.

These effects may be smaller than equilibrium changes for some VOCs, but they are part of the complete performance picture.

Air Flow May Need to Be Evaluated Seasonally

The plant may consider increasing air-to-water ratio during cold operation.

That may improve stripping.

But higher airflow also increases:

• packing gas velocity;
• pressure drop;
• fan duty;
• downstream off-gas volume.

The off-gas treatment system must therefore be checked before simply increasing the blower speed.

Off-Gas Load Can Shift

More stripping transfers more contaminant into the gas phase.

The downstream system may include:

• activated carbon;
• thermal oxidation;
• another absorber.

Its capacity must be evaluated at the increased air volume and contaminant mass load.

Optimizing the packed stripper alone can overload the off-gas control system.

Water Distribution Must Remain Uniform

If winter operation uses a different water flow, distributor performance should be checked.

A reduced liquid rate can create:

• incomplete wetting;
• poorer use of packing area.

The seasonal operating case should include both temperature and flow.

Packing Does Not Need to Be Replaced Automatically

If the tower works well in summer, the packing may be mechanically sound.

Before changing it, review:

• design water temperature;
• actual winter temperature;
• air-to-water ratio;
• inlet VOC loading;
• off-gas conditions.

The existing bed may simply need a different operating strategy.

Seasonal Fouling Can Still Exist

Cold weather can also influence:

• biological growth;
• precipitation;
• upstream water chemistry.

Therefore, pressure-drop trends remain useful.

If pressure drop stays stable but VOC removal changes with water temperature, thermal/process effects become more likely.

Design Should Use the Worst Seasonal Case

A reliable air stripper should be rated over the actual environmental range.

The project should define:

• minimum water temperature;
• maximum water temperature;
• water flow range;
• contaminant concentration range;
• required treated-water target.

The most difficult case may not be the maximum flow case.

Can Bed Height Be Increased?

If winter process calculations show insufficient mass transfer and no operational adjustment is practical, options may include:

• more packing;
• changed air rate;
• changed water rate;
• additional treatment stage.

The correct solution depends on the full system.

Mechanical Materials Still Need Water-Chemistry Review

Groundwater can contain:

• iron;
• hardness;
• suspended solids.

These can produce scaling or fouling independently from VOC stripping performance.

The final tower design should therefore address both seasonal mass transfer and water quality.

Engineering Takeaway

A packed air stripper should be designed for the coldest relevant water condition, not only an annual average.

Seasonal temperature can change stripping performance even when the tower hardware is unchanged.