Wet scrubbers are often assumed to remove virtually any pollutant in industrial air pollution control — a belief that is both widespread and dangerous.
“If there is a wet scrubber, the pollutant will be removed.”
That is not how it works.
A wet scrubber is not a universal machine. It is a gas–liquid mass transfer system.
And that means something very simple:
if the pollutant is not compatible with the physical or chemical absorption mechanism, the scrubber cannot operate effectively, regardless of its size.
The biggest misconception: gas-liquid contact does not automatically mean removal
Many systems are designed around a flawed assumption:
there is a tower,
there is water,
there is recirculation,
therefore the problem is solved.
In reality, a scrubber does not generically “capture” molecules. A wet scrubber only works if at least one of the following conditions exists:
the pollutant is soluble in the liquid;
the pollutant chemically reacts in the liquid phase;
the pollutant is associated with particulate or aerosols that can be intercepted;
thermodynamic conditions favor mass transfer.
If these conditions do not exist, the contaminant simply passes through the system.
In these cases, wet scrubbers can achieve very high removal efficiencies. But that does not mean the same approach works for every contaminant.
VOCs: the reality is far more complex
Many volatile organic compounds (VOCs) have:
low solubility,
hydrophobic behavior,
high volatility,
unfavorable absorption kinetics.
In practical terms:
simple contact with water is often insufficient.
Yet it is still common to see scrubbers designed as if every VOC could simply be “washed out.”
It cannot.
Some solvents pass through the tower almost unaffected. Others require:
specific reagents,
oxidation stages,
contact times incompatible with real tower geometry,
or completely different technologies.
Methane cannot be “scrubbed” either
Methane is one of the clearest examples. It has:
extremely low solubility,
high chemical stability,
virtually no useful reactivity in conventional wet scrubbers.
For this reason:
methane is not treated effectively with traditional wet scrubbers or biofilters.
It must either be oxidized, diluted, or managed through completely different process strategies.
Believing that methane can be removed simply by increasing water flow or chemical dosage means misunderstanding the physics of the process itself.
Chemistry matters more than the machine
One of the most common mistakes in air pollution control is treating the scrubber as a standardized product.
In reality:
pollutant chemistry comes before equipment selection.
Without understanding:
speciation,
actual concentrations,
temperature,
humidity,
aerosol presence,
process variability,
it is impossible to determine:
whether a scrubber will work,
which chemistry should be used,
what liquid-to-gas ratio is required,
what pressure drop is necessary,
or whether the selected technology is fundamentally wrong.
A scrubber does not create selectivity out of nowhere
Another common misconception is that adding:
more stages,
more chemicals,
more recirculation,
more complexity,
automatically allows treatment of any emission stream.
But no system truly “sorts” molecules.
Every compound has:
its own solubility,
its own kinetics,
its own chemical behavior,
its own equilibrium conditions.
Inside real industrial gas mixtures:
some pollutants are absorbed,
others pass through,
others interfere with each other.
This is why truly effective systems are not built from standard catalogs, but from a deep understanding of the upstream industrial process generating the emissions.
The real goal: designing around the machine instead of the process
Most failures originate here. First, the equipment is selected:
“a scrubber,”
often because it is available,
inexpensive,
or already used elsewhere.
Only afterwards does the engineering attempt to adapt it to the actual pollutants.
But the correct sequence is the opposite:
understand the industrial process;
identify the pollutants;
understand their physical and chemical behavior;
only then design the treatment system.
Conclusion
Wet scrubbers can be extremely effective technologies.
But only when:
the contaminant is compatible with the absorption mechanism,
the chemistry is correct,
mass transfer conditions are realistic,
and the system is designed around actual operating conditions.
because:
A scrubber does not remove “everything.” Removing a pollutant first requires understanding it.
Air flow rate: 15,000 Nm³/h Main pollutants: Reduced sulphur compounds (H₂S) Treatment technology: Biotrickling filtration Number of filtration units: 2 Contact time: 24 seconds per filter Design loading: 150 Nm³/h per m³ of packing
This case study presents an industrial biotrickling filter odour control system designed to treat emissions from a wastewater treatment plant. The design follows the Best Available Techniques (BAT) described in the BREF for Waste Water and Waste Gas Treatment for biological air treatment systems and VDI 3478 part 1 norm.
Overview
Wastewater treatment plants can generate odorous emissions associated with reduced sulphur compounds, particularly hydrogen sulfide (H₂S).
This project involved the design and installation of an industrial odour control system based on biotrickling filtration technology to treat contaminated air streams at a wastewater treatment facility.
The plant was designed to treat 15,000 Nm³/h of odorous air, ensuring stable removal of sulphur-based odorous compounds through biological oxidation processes.
Engineering Solution
The odour control system consists of two parallel biotrickling filters, each designed to treat 7,500 Nm³/h of contaminated air.
A centrifugal fan installed upstream conveys the polluted air to the filters through a distribution manifold.
Inside the filters, the gas stream passes through a biological packing bed, where microorganisms immobilized on the carrier oxidize hydrogen sulfide and other reduced sulphur compounds into non-odorous forms.
Biotrickling Filter Odour Control Technology
Each filtration unit is built around a polypropylene basin containing the biological packing and irrigation system, designed to ensure proper gas distribution and stable microbial activity.
Main Dimensions
Parameter
Value
Lenght:
10,000 mm
Width:
2,500 mm
Height:
3,300 mm
The filters are constructed entirely in Polypropylene (PP) to ensure resistance against corrosive gases and acidic operating conditions associated with sulphur oxidation processes.
The packing bed is supported by a reinforced polypropylene structural grid capable of supporting loads up to 2,500 kg/m².
Biological Packing Media
The filtration bed is filled with open-cell volcanic lapillus, selected for its suitability in biological filtration applications.
Packing Characteristics
Particle size: 14–20 mm
Water retention capacity: 6–10 %
Available water: 4–7 %
Neutral pH
High surface area for microbial colonization
High mechanical stability
These characteristics promote the formation of a stable microbial biofilm capable of degrading hydrogen sulfide efficiently.
Liquid recirculation system
Each filter includes an independent recirculation tank and irrigation network designed to maintain optimal moisture and nutrient conditions for biological activity.
Main equipment
Polypropylene recirculation tank
Automatic make-up water valve
Visual level indicator
Pressure level transmitter (4–20 mA)
Automatic purge valve
Two AISI 316L submersible pumps (1.1 kW) per filter
Nutrient dosing pump
Spray irrigation system covering the entire packing surface
This configuration ensures continuous wetting of the biological packing and stable microbial growth.
Air Handling System
Air movement through the treatment system is ensured by a centrifugal fan equipped with inverter control, allowing flow regulation and stable operating conditions.
Fan Specifications
Parameter
Value
Nominal air flow
15,000 Nm³/h
Maximum pressure
250 mm w.c.
Motor power
18.5 kW
Efficiency class
IE3
Noise level
< 81 dB(A)
A standby fan rated at 20,000 Nm³/h was also included to guarantee operational redundancy and maintenance flexibility.
Automation and Control
The system is managed through a PLC-based control panel designed to ensure fully automatic operation.
Control Features
PLC control (Siemens S7-1200 or Schneider M221)
HMI operator interface
pH monitoring
Conductivity monitoring
Automatic make-up water control
Inverter control for the process fan
Remote status signals
This architecture ensures reliable operation and simplified plant management.
Key Outcomes
The installed odour control system provides:
Effective removal of hydrogen sulfide and reduced sulphur compounds
Reliable odour abatement in wastewater treatment emissions
Stable long-term biological operation
Low chemical consumption thanks to biological oxidation processes
Biotrickling filtration technology enables continuous treatment of odorous air streams with low operating costs and high process stability.
This is what happens when a packed bed scrubber is used for wet dust scrubbing. Dust does not get removed. It accumulates. And over time, it transforms a process unit into a solid mass. Most industrial wet scrubbers are not designed for dust removal. Yet packed columns are still widely applied in systems where particulate matter is dominant. The result is predictable: fouling, pressure drop increase, and loss of efficiency.
What You Are Looking At
The material shown here is not simple contamination.
It is a compacted mass of:
fibrous particulate matter
fine dust
moisture-bound solids
The interaction between particles and liquid leads to:
agglomeration
adhesion to surfaces
progressive densification
Over time, the original structure of the packing disappears.
This is not fouling. This is structural failure of the packing.
Why Packed Bed Scrubbers Fail with Dust
Packed bed scrubbers are designed for mass transfer, not for solid particle removal. Their operating principles rely on:
gas-liquid contact surface
residence time
chemical absorption
However, dust removal follows completely different mechanisms:
inertial impaction
interception
diffusion
These mechanisms require high gas velocities and energy input, which are not present in packed columns.
As a result:
particles are not effectively captured
they deposit on the packing
accumulation begins immediately
The Hidden Mechanism: When Dust Meets Water
One of the most underestimated aspects of wet dust scrubbing is the behavior of particulate matter in the presence of moisture. Depending on the nature of the dust, you may observe:
hygroscopic effects
increased cohesion
formation of fibrous or paste-like structures
Instead of being washed away, particles:
stick together
attach to surfaces
grow into larger agglomerates
This is a cumulative process.
And it does not stabilize.
What Happens Inside the Scrubber
The failure does not occur suddenly. It follows a predictable sequence:
Initial deposition on the packing
Localized obstruction
Increase in pressure drop (ΔP)
Maldistribution of liquid and gas
Formation of preferential paths (channeling)
Loss of contact efficiency
Progressive blockage
At a certain point, the scrubber is no longer performing gas treatment.
It becomes a restriction in the system.
The Design Mistake
The root cause is not maintenance.
It is design.
Many systems are selected based on:
air flow rate
general assumptions about “scrubber performance”
What is often ignored:
particle size distribution
dust concentration
physical behavior of solids
interaction with water
A packed bed scrubber is simply the wrong tool for particulate-dominated streams.
The Right Approach to Wet Dust Removal
Effective wet dust removal requires mechanical capture mechanisms, not just surface area.
Hydrogen sulfide removal is not a steady-state problem
Hydrogen sulfide removal is one of the most common challenges in industrial air treatment. In many systems, caustic scrubbers are used as the primary solution to remove H2S. On paper, the chemistry is simple. In real operation, these systems often become unstable.
H2S is absorbed into the liquid phase
it reacts with sodium hydroxide (NaOH)
it forms soluble sulfide compounds
On paper, the process is stable.
In real operation, it is not.
The chemistry is predictable. The system is not
The reaction between hydrogen sulfide and caustic soda is well known:
H2S + NaOH → NaHS + H2O
NaHS + NaOH → Na2S + H2O
This is not where the problem lies.
The issue is that industrial air systems do not operate under controlled, steady conditions.
In real plants:
H2S concentration fluctuates
emissions occur in peaks, not averages
humidity changes continuously
aerosols and particulates interfere with mass transfer
Why hydrogen sulfide removal becomes unstable
A caustic scrubber is typically designed around average inlet conditions.
But real systems are defined by variability.
When peak loads occur:
the reaction accelerates locally
sulfide concentration in the liquid increases rapidly
the recirculation loop changes composition
Over time:
salinity increases
mass transfer efficiency drops
system response becomes non-linear
The scrubber still works.
But it stops working predictably.
pH control does not guarantee performance
In many hydrogen sulfide removal systems, pH is used as the primary control parameter.
The assumption is straightforward:
high pH ensures effective H2S removal.
In practice:
pH does not reflect sulfide accumulation
it does not represent real absorption capacity
it does not capture mass transfer limitations
Operators may observe a stable pH
while the system performance is deteriorating.
As a result:
caustic consumption increases
removal efficiency fluctuates
corrective actions become reactive instead of controlled
pH is an indicator. It is not a control strategy.
Field reality: transient H2S release in industrial processes
In many industrial processes, hydrogen sulfide is not released at a constant rate.
A clear example is found in tannery operations during the pickling phase.
In these conditions:
acidic environments react with residual sulfides
H2S is released rapidly
emissions occur in short, high-intensity peaks
The key issue is not the average concentration.
It is the transient load.
A caustic scrubber exposed directly to these peaks:
reacts, but not in a controlled way
accumulates reaction products quickly
loses operational stability
Hydrogen sulfide removal becomes a continuous correction process.
Hydrogen sulfide removal requires load control
The most critical mistake in H2S treatment design is assuming that the final stage can handle everything.
In reality, hydrogen sulfide removal requires load management upstream. This is where pre-treatment becomes essential.
The role of pre-treatment in hydrogen sulfide removal
Iron-based media systems (CIF) are often used as a pre-treatment stage.
They are not designed to remove all hydrogen sulfide.
Their function is to:
reduce peak concentrations
stabilize inlet conditions
protect downstream scrubbers
When pre-treatment is correctly applied:
transient peaks are dampened
load becomes more uniform
the caustic scrubber operates within a manageable range
This changes the system from reactive to controlled.
Designing hydrogen sulfide removal for real conditions
Effective hydrogen sulfide removal systems are not designed around average values.
They are designed around:
variability
peak loads
real process behavior
A caustic scrubber without pre-treatment is forced to absorb all fluctuations.
And no single stage can do that reliably.
A necessary distinction in hydrogen sulfide removal
Hydrogen sulfide removal in industrial air systems must be treated as a specific engineering problem.
Air treatment systems are characterized by:
low but highly variable concentrations
intermittent emissions
complex mixtures of contaminants
Design approaches taken from other processes do not apply directly.
Each system must be designed based on its actual operating conditions.
Hydrogen sulfide removal does not fail because the chemistry is wrong. It fails because the system is designed for average conditions instead of real variability.
Evaluate whether your hydrogen sulfide removal system is designed as a machine or as a process
Flow rate: 120,000 m³/h (two parallel lines, 60,000 m³/h each) Industry: Pet food production Technology: Variable-throat Venturi scrubber + downstream wet scrubbing stages Target pollutants: Organic vapours, fatty acids, amines, oil aerosols Process air sources: Extrusion, drying, coating and cooling
120,000 m³/h – Designing for variability, not nominal flow
Air Pollution and Odour Control in Pet Food Production requires full integration with thermal processes, airflow stability, and emission variability management.
Application context – Air emissions in pet food production
This case study concerns the design of an Air Pollution and Odour Control in Pet Food Production plant, with a total extracted air flow rate of approximately 120,000 m³/h.
The production process includes:
extrusion
drying
coating
cooling
Air emissions are characterised by:
organic vapours
fatty acids
amines
oil aerosols
In pet food production, emission profiles are not constant. Raw material composition and coating formulations significantly influence contaminant concentration and aerosol load.
For this reason, air pollution control in pet food plants cannot be designed based on nominal flow rate alone.
The real engineering constraint: no interference with the drying process
The odour control system had to be integrated without disturbing:
the thermal balance of the oven
the internal pressure profile of the dryer
the process airflow stability
Any unstable downstream pressure condition would have:
altered the internal oven temperature
affected moisture control
increased methane consumption
reduced product quality consistency
In pet food production, maintaining thermal stability is directly linked to product performance and energy efficiency.
The air pollution control system therefore had to adapt to the production process — never the opposite.
Variable throat Venturi scrubber for emission variability
Given the fluctuating contaminant load and presence of oil aerosols, a Venturi scrubber with variable throat section was selected as the primary stage of the odour control system.
The Venturi operates on inertial impaction principles: particle and aerosol removal efficiency is directly related to gas velocity and pressure drop (ΔP).
The variable throat configuration allows:
adjustment of gas velocity according to real operating conditions
controlled pressure drop
stable capture efficiency across variable loads
avoidance of efficiency loss during low-load operation
prevention of excessive ΔP during peak conditions
Unlike fixed-throat systems, the variable geometry maintains the required impact energy while adapting to process variability typical of pet food air emissions.
The Venturi stage was dimensioned based on required separation efficiency, not solely on nominal air flow rate.
Controlled fan operation and process stability
A key design principle of this Air Pollution and Odour Control system was that downstream fans must never interfere with the process fans of the oven.
The tail-end extraction fans were installed downstream of the treatment plant and equipped with variable frequency drives (VFD).
Their function is to:
continuously adjust rotational speed
maintain the required negative pressure
compensate for system pressure losses
preserve stable depression across the treatment unit
This ensures that:
the internal pressure of the dryer remains unaffected
the thermal equilibrium of the oven is preserved
process airflow conditions remain stable
The odour control system behaves as a controlled hydraulic load, not as an active disturbance to the production process.
Pressure drop and separation efficiency
In Venturi scrubbers, separation efficiency is directly related to gas velocity and pressure drop. Reducing ΔP in order to minimise energy consumption without understanding the required impact energy inevitably compromises particle and aerosol removal efficiency.
In this application, the variable throat configuration allows optimisation of ΔP according to real operating conditions, ensuring stable performance without unnecessary energy penalties.
Energy and quality impact
Maintaining stable oven conditions in pet food production results in:
consistent drying performance
stable product temperature profile
reduced methane consumption
improved product quality consistency
The air pollution control system therefore contributes not only to emission abatement, but also indirectly to energy optimisation and process reliability.
Engineering insight
In dry pet food production, effective Air Pollution and Odour Control requires full integration with process airflow and thermal dynamics.
Designing only for flow rate leads to instability.
Designing for:
emission variability
aerosol load
pressure management
process integration
ensures predictable performance.
Air pollution control in pet food production is not an add-on system. It is a process-integrated engineering solution.
In many industrial wet scrubbing systems, foam formation is often dismissed as a secondary or cosmetic issue. In reality, persistent foam is almost always the visible symptom of a deeper process inconsistency. This insight originates from real operational evidence: a recirculation tank completely filled with stable foam, generated during normal plant operation.
What foam really indicates
Foam inside a scrubber is not random. It is the result of a combination of physical and chemical conditions:
When these factors align, foam transitions from transient to persistent. At that point, it stops being a nuisance and becomes a process variable.
Why Foam Is a Problem
The consequences are rarely limited to aesthetics:
Loss of effective liquid–gas contact
Carryover of droplets and contaminants
Unstable pressure drop
Reduced mass transfer efficiency
Risk of downstream fouling
Cavitation of recirculation pump
In extreme cases, the scrubber continues to operate, but no longer performs its intended function.
The Typical Reaction (and Why It Fails)
The most common response is the addition of antifoam agents. This approach is appealing because it is immediate and simple. However, it rarely addresses the root cause. Antifoam acts on the symptom, not on the mechanism that generates foam. As a result:
Consumption increases over time
Performance becomes inconsistent
Operating costs rise without solving the instability
Engineering Perspective: Treat the Cause, Not the Symptom
Foam must be interpreted within the broader process context. The correct approach requires asking fundamental questions:
What compounds are actually present in the gas phase?
Is the chemical regime (acid/base/oxidative) coherent with those compounds?
Is the liquid recirculation accumulating by-products?
Is the hydraulic design promoting local imbalances?
Only by reconnecting the scrubber to the upstream process can the issue be solved.
Design Implications
Persistent foam often reveals deeper design limitations:
Systems designed for steady-state conditions operating under variable loads
Lack of pollutant speciation during the design phase
Oversimplified assumptions about liquid chemistry
Inadequate purge and make-up strategies
In other words, the problem is not the foam itself. The problem is that the system was never designed to handle the real process conditions.
Key Takeway
A scrubber does not fail when foam appears.
It fails when foam is treated as an isolated issue.
Understanding and controlling the origin of the emission is the only way to restore performance.
Final note
Visible phenomena in industrial systems are rarely superficial. They are signals. Ignoring them — or masking them with quick fixes — only delays the real engineering work.
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