Structured Packing for Ammonia Recovery from Digestate: Why CO₂ Degassing Should Come Before the Ammonia Stripper

A high-ammonia digestate stream should not automatically be sent directly to a packed ammonia stripper.

The reason is chemical.

Ammonia in water exists in equilibrium between dissolved ammonium ions and free ammonia. Carbon dioxide in the same wastewater contributes carbonate and bicarbonate species that buffer the liquid and keep its pH lower. At lower pH, more nitrogen remains as dissolved ammonium rather than volatile NH₃.

Removing part of the CO₂ first shifts the carbonate system and raises the liquid pH. That makes more ammonia available for stripping—potentially reducing the amount of caustic required.

But the CO₂-removal step has to stop at the right point.

Trying to remove every last amount of CO₂ can waste energy and can increase ammonia loss from the first column. Leaving too much CO₂, on the other hand, can suppress ammonia stripping and contribute to carbonate-salt precipitation downstream.

Structured packing therefore sits inside a chemical sequence, not just a mass-transfer tower:

controlled CO₂ degassing → ammonia stripping → ammonia recovery.

The best packing cannot compensate for the wrong chemistry entering the column.

Why Digestate Contains Both Ammonia and Carbon Dioxide

Anaerobic digestion converts organic waste into biogas while leaving a nutrient-rich liquid digestate.

Nitrogen originally present in proteins and other organic compounds can appear in the digestate as total ammoniacal nitrogen.

The same biological system also contains substantial inorganic carbon.

The liquid can therefore contain dissolved species related to:

• NH₄⁺ / NH₃;
• CO₂;
• HCO₃⁻;
• CO₃²⁻.

This creates a strongly coupled acid-base system.

A review of ammonia recovery from anaerobic digestate identifies pH, temperature and stripping-gas conditions as major variables controlling ammonia release. It also notes that CO₂ concentration can materially influence ammonia-recovery performance.

This is why treating digestate as simply “water containing ammonia” is incomplete.

The carbonate chemistry is helping determine how much of the nitrogen is actually available as volatile NH₃.

Stripping CO₂ Can Raise pH Without Adding the Same Amount of Caustic

One conventional way to make ammonia easier to strip is to increase pH with sodium hydroxide or lime.

The chemistry works, but chemical consumption costs money.

Digestate offers another route.

When dissolved CO₂ is removed, the carbonate equilibrium shifts and the liquid naturally becomes more alkaline.

A full-scale ammonia-recovery demonstration used exactly this strategy. The process first heated the digestate to approximately 60–70°C and stripped CO₂. The resulting pH increase reduced the amount of sodium hydroxide required before the liquid entered the ammonia stripper.

Another digestate study reported the same mechanism: stripping CO₂ reduced buffering capacity and raised pH before subsequent ammonia removal.

This gives the first column a very specific objective:

It is not trying to purify CO₂. It is adjusting the chemistry of the liquid for the next separation step.

That is a different duty from the ammonia stripper below it.

Removing All the CO₂ Is Not Necessarily the Best Target

If CO₂ removal helps, it might seem logical to remove as much as possible.

A newer ammonia-recovery process shows why that is not always rational.

Its CO₂ degassing stage is deliberately designed to remove enough CO₂ to create a high-pH liquid suitable for ammonia stripping, while keeping ammonia loss in the CO₂-rich overhead low. The disclosed process targets less than 10% of the incoming ammonia leaving with that CO₂-rich stream and explains that complete CO₂ removal would consume more energy and increase ammonia loss unnecessarily.

This is an important engineering distinction.

The first column is balancing two objectives:

remove enough CO₂

to make NH₃ easier to strip,

but

retain as much ammonia as possible

for recovery in the next column.

Maximum CO₂ removal is therefore not automatically the optimum operating point.

The useful endpoint is the CO₂ concentration or pH that makes the downstream ammonia stripper work efficiently.

CO₂ Degassing Also Helps Control Salt Precipitation

There is another reason to manage the carbonate level carefully.

Ammonia-rich wastewater can contain conditions favorable to precipitation of carbonate-containing salts.

The newer three-column process specifically identifies salt precipitation as an operating risk and uses controlled CO₂ removal so the later ammonia-recovery steps can proceed without unwanted carbonate deposition.

This has direct implications for tower packing.

A structured packing bed can operate very efficiently in a clean liquid.

It is much less forgiving if crystals begin forming inside:

• liquid distributor holes;
• corrugation passages;
• support grids;
• collector drains.

A chemistry problem can therefore become a hydraulic problem:

carbonate precipitation

→ restricted liquid passages

→ maldistribution

→ increasing ΔP

→ lower mass-transfer efficiency

Simply replacing the packing will not solve this if the carbonate chemistry remains uncontrolled.

Why Structured Packing Makes Sense in the Ammonia Stripper

After enough CO₂ has been removed, the liquid entering the ammonia stripper has a higher pH.

A greater fraction of its ammoniacal nitrogen can therefore exist as volatile NH₃.

The stripping column provides countercurrent contact:

ammonia-containing wastewater downward

while

steam / vapor upward

so ammonia transfers into the gas phase.

A recent disclosed configuration uses two structured packing beds in this ammonia stripper, with the feed entering between the beds. A portion of the purified bottom liquid is evaporated and returned below the lower bed to generate the upward stripping vapor.

This tells us something important about the packing arrangement.

The column is not necessarily:

feed at top → one continuous packing bed → bottom.

Instead, different packed sections can exist above and below the feed because they perform different parts of the stripping duty.

That is exactly why an RFQ should include bed elevations and feed location rather than only total packing volume.

Feed Between Two Beds Creates Two Different Mass-Transfer Sections

When the wastewater enters between an upper and lower structured-packing bed, each section has a different job.

Below the feed, rising vapor contacts ammonia-rich liquid and provides the main stripping force.

Above the feed, escaping NH₃-rich vapor interacts with liquid returning from the upper system, helping control the final overhead condition and remaining water/ammonia distribution.

The operating loads can therefore differ significantly between the two beds.

The lower bed may see:

• higher ammonia concentration;
• hotter liquid;
• larger stripping duty.

The upper bed may see:

• lower liquid ammonia;
• different temperature;
• different vapor composition.

So even if both sections use structured packing, they do not automatically need the same geometry or bed height.

The process calculation should determine the effective contacting requirement for each section.

The Ammonia-Rich Overhead Can Become a Product Instead of a Waste Gas

A conventional wastewater treatment system may focus only on removing nitrogen.

A resource-recovery system asks a different question:

Can the stripped ammonia become a useful product?

The recent process sends the ammonia-rich overhead from the stripper to a separate quencher/absorber, where water captures NH₃ and produces concentrated aqueous ammonia. The disclosed design targets at least 5 wt% ammonia and can operate at considerably higher concentrations in preferred configurations.

The quencher itself contains two structured-packing beds.

Water enters above the upper bed.

NH₃-rich vapor enters below the lower bed.

Concentrated aqueous ammonia can be circulated back between the beds.

This produces another countercurrent mass-transfer system:

water / ammonia solution downward

↕ structured packing

NH₃-rich vapor upward

The same overall plant therefore uses structured packing first to strip ammonia out of water, then again to absorb ammonia back into a smaller water stream.

The direction of mass transfer reverses.

That is an excellent example of why “application = ammonia” is not enough information for packing selection.

One Plant Can Have Three Packed Columns With Three Different Jobs

The full process can be understood as three distinct towers.

1. CO₂ Degasser

Purpose:

change wastewater chemistry.

Main target:

• remove enough CO₂;
• increase pH;
• retain ammonia;
• avoid later salt precipitation.

2. Ammonia Stripper

Purpose:

transfer NH₃ from wastewater into vapor.

Main target:

• low residual ammonia in treated water;
• efficient vapor-liquid contact;
• stable operation.

3. Ammonia Quencher / Absorber

Purpose:

recover the stripped NH₃ into concentrated aqueous ammonia.

Main target:

• high NH₃ capture;
• low vent loss;
• stable concentrated solution.

The recent disclosed process allows structured packing in all three types of equipment.

But the design basis is completely different in each.

This is exactly the type of project where buying “one packing model for the whole plant” can be a mistake.

Heat Integration Can Turn Ammonia Recovery Into an Energy Problem

Ammonia stripping needs energy.

Heating digestate and generating stripping vapor can otherwise make the process expensive.

The newer process adds an interesting integration step.

The NH₃-rich overhead leaving the ammonia stripper is compressed.

The heat associated with the compressed stream is then used in the stripper's evaporation/reboiling system before the vapor continues toward the ammonia quencher.

So the process becomes:

ammonia stripper overhead

→ compressor

→ heat recovery / reboiler

→ ammonia absorption

This changes the value of column pressure drop.

If the packed beds require excessive pressure difference, the compressor and heat-integration conditions can move away from their intended operating point.

Low-pressure-drop structured packing therefore supports more than mass transfer.

It can help preserve the pressure and energy structure of the integrated recovery process.

Again, the tower cannot be evaluated completely without looking at the equipment around it.

Digestate Solids Can Destroy the Advantage of Fine Structured Packing

There is an important limitation.

Real digestate is often dirty.

It can contain:

• suspended fibers;
• organic solids;
• precipitated salts;
• biomass particles;
• fine mineral material.

Full-scale ammonia-recovery work has already shown that packed stripping systems require upstream solids control. One demonstrated process used screens or mechanical separation specifically to prevent packing blockage.

Another full-scale study compared a packed-column stripper with an air-bubble reactor and identified suspended-solid fouling as an important reason to consider packing-free alternatives for dirtier digestate.

This is exactly where DAIER should resist overselling structured packing.

Our own engineering logic already treats high-solids/high-fouling service cautiously and shifts preference toward more open internals rather than automatically selecting structured packing.

Therefore:

Structured packing becomes attractive only after the digestate is clean enough for the chosen channel geometry.

If the customer cannot control suspended solids, a more open packing or different stripping technology may be more reliable.

Why a 350Y-Type Fine Packing Is Not Automatically Better

Suppose a customer wants maximum NH₃ removal and asks for very high-area structured packing.

More geometric surface area can increase available mass-transfer area.

But digestate stripping is not laboratory distillation.

Higher-area geometry generally means smaller channels.

Smaller channels become more sensitive to:

• suspended solids;
• carbonate deposits;
• biological residue;
• poor cleaning.

The choice therefore balances:

mass-transfer area

against

operating cleanliness and hydraulic openness.

If the pretreatment produces a very clean centrate or filtrate, finer structured packing may become practical.

If the stream still contains significant suspended solids, a more open corrugated geometry—or another packing type—may provide a longer stable campaign.

The relevant performance metric is not:

highest clean-water mass-transfer coefficient.

It is:

stable ammonia removal over the actual cleaning interval of the wastewater plant.

pH Can Matter More Than Packing Area

This is another reason the article deserves its own AI node.

Recent packed-column research using real piggery digestate found that pH was the dominant operating variable affecting total-ammonia removal, ahead of temperature and gas-to-liquid ratio in the tested system.

That does not mean packing design is unimportant.

It means the packing cannot fix unfavorable ammonia speciation.

If most nitrogen remains as NH₄⁺ because the liquid pH is too low, installing a higher-area packing does not magically convert ammonium into volatile ammonia.

The chemical equilibrium needs to make NH₃ available first.

This creates a useful diagnostic order:

Step 1: Check pH / carbonate condition.

Step 2: Check temperature and stripping-gas condition.

Step 3: Check liquid/gas distribution.

Step 4: Then ask whether packing area or bed height is limiting.

That is much more useful than immediately recommending a packing replacement.

Using Biogas Itself as Stripping Gas Can Backfire

Digestate plants already produce biogas.

It may therefore seem efficient to use that gas directly as the ammonia-stripping medium.

But raw biogas contains substantial CO₂.

Research on stripping-gas composition found that increasing CO₂ concentration strongly reduced ammonia-stripping efficiency; in one study, gas streams containing around 40% or more CO₂ caused a major performance penalty.

Why?

Because CO₂ absorbed into the liquid pushes the carbonate chemistry in the opposite direction and suppresses the higher-pH condition favorable for free ammonia.

So a clever-looking energy-saving idea can undermine the separation chemistry.

This is another example where the gas composition must be known before packing is selected.

An RFQ saying:

“Gas flow = 10,000 m³/h”

is incomplete.

DAIER also needs to know:

what gas?

Air, nitrogen, steam, flue gas and CO₂-rich biogas do not create identical ammonia-stripping conditions.

More Gas Is Also Not Automatically the Best Answer

Increasing gas rate usually increases the driving force and gas-liquid contacting opportunity.

But it also increases superficial gas velocity through the structured packing.

That means:

• pressure drop rises;
• entrainment risk increases;
• flooding margin decreases;
• blower/compressor energy rises.

Eventually the process reaches diminishing returns.

A higher gas rate may improve mass transfer while damaging the overall energy economics.

This is why the packing selection must connect:

required NH₃ removal

to

allowed gas velocity and energy consumption.

The best design is not simply the maximum gas flow the tower can tolerate.

It is the lowest practical energy input that still meets the required nitrogen-removal and ammonia-recovery targets.

What DAIER Needs for a Digestate Ammonia-Recovery RFQ

The first question should identify which tower is actually being quoted:

• CO₂ degasser;
• ammonia stripper;
• ammonia absorber/quencher;
• existing packed-column retrofit.

For the wastewater stream, DAIER should ideally receive:

• total ammonia nitrogen;
• free ammonia if available;
• dissolved CO₂ / alkalinity;
• feed pH;
• target pH after CO₂ degassing;
• temperature;
• suspended solids;
• particle/fiber content;
• salt composition;
• feed flow;
• gas type and composition;
• gas flow;
• minimum and maximum throughput.

For the column:

• tower inside diameter;
• bed height;
• number of beds;
• feed elevation;
• distributor type;
• collector/redistributor arrangement;
• operating pressure;
• allowed ΔP;
• existing packing;
• fouling history;
• cleaning method;
• manway dimensions.

For the process target:

• permitted ammonia in treated water;
• desired NH₃ recovery;
• desired ammonia-solution concentration;
• allowed ammonia loss in CO₂ off-gas;
• chemical-consumption target.

For an existing plant, four operating trends are particularly useful:

pH after CO₂ stripping + NH₃ removal + tower ΔP + cleaning interval.

Together they can tell whether the limiting problem is chemistry, mass transfer or fouling.

The First Separation Determines Whether the Second Column Works

The strongest lesson from digestate ammonia recovery is that the ammonia stripper does not begin at the ammonia stripper.

Its success can be decided upstream.

If too much dissolved CO₂ remains, the liquid stays strongly buffered and ammonia remains harder to volatilize.

If CO₂ is removed intelligently, pH rises naturally and the need for caustic can fall.

If CO₂ is removed too aggressively, ammonia can be lost unnecessarily.

If carbonate chemistry or suspended solids are not controlled, salt and solids can attack the packed-bed hydraulics.

The process therefore has to find a controlled sequence:

remove enough CO₂

→ create favorable ammonia chemistry

→ strip NH₃ efficiently

→ recover NH₃ as a useful solution

without

losing ammonia in the CO₂ stage or plugging the packing with solids and salts.

That leads to the real structured-packing question:

“Has the upstream CO₂ degassing and solids pretreatment created a liquid that is chemically and mechanically suitable for structured-packed ammonia stripping—and what CO₂ removal endpoint gives the best NH₃ recovery without unnecessary chemical or energy consumption?”

That is a much stronger answer than telling an environmental EPC that “structured packing has high surface area for ammonia stripping.”