Engineering Evaluation Case: Ammonia Is Being Stripped from Wastewater but Packing Changes Do Not Fix Poor Removal

Ammonia stripping is the reverse of an ammonia scrubber.

Instead of absorbing ammonia from gas into liquid, the tower tries to move ammonia:

from wastewater into air.

This means process chemistry can control the result just as strongly as packing selection.

A plant can install better packing and still see poor ammonia removal if most ammonia remains in a nonvolatile ionic form.

Project Situation

Consider a packed air-stripping tower treating ammonia-containing wastewater.

The system includes:

• air blower;
• water distributor;
• random packing;
• treated-water sump.

The customer observes insufficient ammonia removal and considers replacing the packing with a higher-surface-area product.

Before doing so, the process conditions should be reviewed.

Ammonia Exists in Different Chemical Forms

In water, ammonia-related species exist in equilibrium between:

• un-ionized ammonia;
• ammonium ion.

The relative amount depends strongly on:

• pH;
• temperature.

Air stripping primarily removes the volatile un-ionized form.

If the water chemistry favors ammonium ion, increasing packing area alone may provide limited improvement.

pH Can Be the Main Process Lever

Raising pH shifts more of the total ammonia toward the volatile form.

That can increase stripping effectiveness.

However, pH adjustment introduces:

• chemical cost;
• scaling risk;
• downstream neutralization requirements.

The tower design and water chemistry should therefore be evaluated together.

Temperature Also Matters

Higher temperature generally increases volatility and can improve stripping.

Cold wastewater may therefore be more difficult to treat.

This can create seasonal performance differences similar to other stripping systems.

A tower that meets the requirement in warm conditions may struggle in winter.

Air-to-Water Ratio Remains Important

Once the chemistry makes ammonia available for transfer, the tower still needs enough air.

Increasing airflow can improve stripping up to process and hydraulic limits.

But higher air rate affects:

• pressure drop;
• blower power;
• off-gas volume.

The released ammonia must also go somewhere.

Off-Gas Treatment May Be Required

Ammonia stripped from water becomes an air-emission load.

Depending on the application, the off-gas may require:

• acid scrubber;
• another treatment stage.

Improving liquid treatment can therefore increase the duty of downstream gas treatment.

This is a system-level trade-off.

Packing Surface Area Is Still Important

Once the chemical and operating conditions are appropriate, packing controls:

• gas-liquid contact;
• pressure drop;
• effective transfer area.

A more efficient packing can reduce required bed height or improve performance.

But it cannot convert ammonium ion into volatile ammonia.

Scaling Can Become a New Limitation at High pH

Alkaline conditions can promote precipitation of certain minerals present in wastewater.

This can create deposits on:

• packing;
• distributor;
• support.

A high-efficiency fine packing may therefore suffer from the chemistry used to improve stripping.

Water composition should be reviewed.

Distributor Performance Matters at Low Water Flow

Some stripping systems operate at relatively modest liquid rates over a large tower area.

Uniform wetting remains essential.

The designer should verify the distributor over:

• minimum;
• normal;
• maximum water flow.

Do Not Judge Tower Performance from Total Ammonia Alone

The relevant transfer potential depends on the fraction present in volatile form.

Process calculations should therefore consider:

• pH;
• temperature;
• ammonia equilibrium.

A simple total-ammonia concentration is not enough to explain performance.

What Data Should Be Collected?

Useful inputs include:

• total ammonia concentration;
• pH;
• water temperature;
• water flow;
• air flow;
• target outlet;
• alkalinity;
• scaling components.

These data help determine whether the main limitation is:

• chemistry;
• air rate;
• packing area;
• fouling.

Engineering Takeaway

Ammonia stripping is a chemistry-plus-mass-transfer problem.

The packing can improve contact only after the water chemistry makes ammonia available to be stripped.