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.
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.
Application: Sludge drying Industrial sector: Wastewater treatment Air flow rate: 6,000 Nm³/h Air characteristics: humid process air Main pollutants: H₂S, ammonia, sulfur compounds, VOCs Treatment lines: 2
This sludge dryer odour control system is designed for wastewater treatment plants handling complex industrial emissions.
Project Overview
Sludge drying processes used in wastewater treatment plants generate complex gaseous emissions containing hydrogen sulfide, ammonia and various odorous organic compounds released during thermal treatment of sewage sludge. For the sludge drying units, two dedicated air treatment systems were engineered and installed to ensure reliable odour abatement. Each system was designed to treat process air extracted directly from the sludge dryer and combines multiple treatment stages to address the complex chemical composition of the emission stream.
Engineering Challenges
Air emissions generated by sludge dryers present several challenges for air pollution control systems:
variable pollutant composition
high humidity and condensable vapours
presence of aerosols and fine droplets
coexistence of sulfur compounds, nitrogen compounds, VOCs and dust
These conditions require a multi-technology treatment approach, capable of handling different pollutant classes within the same emission stream.
Sludge dryer odour control System Configuration
Each treatment line integrates several sequential technologies.
Venturi Scrubber + Packed Column
The first treatment stage consists of awet scrubber system composed of a Venturi section followed by a packed column. The Venturi section improves gas–liquid contact and promotes removal of dust and soluble compounds. The packed column increases gas–liquid contact time and allows efficient absorption of odorous pollutants. Main design parameters:
Air flow rate: 6,000 Nm³/h
Pressure drop: < 1,500 Pa
Contact time: 2 s
Column diameter: up to 2,000 mm
Column height: up to 7.5 m
Construction material: polypropylene
Biofiltration Stage
Downstream of the chemical scrubber, the air stream is treated through a biofilter designed to biologically oxidize residual odorous compounds. The biofilter uses wood chips as filtering media, providing a suitable environment for microbial populations capable of degrading sulfur compounds.
Main characteristics:
Filter media volume: 75 m³
Media type: wood chips
Moisture range: 55–85 %
Air Conditioning System
To maintain optimal operating conditions for the biofilter, the system includes an air conditioning section with electric heating. This stage stabilizes temperature and humidity before biological treatment.
Activated Carbon Polishing Filters
The final stage consists of activated carbon adsorption filters designed to remove trace pollutants remaining after biological treatment. Each filter contains approximately 2,500 kg of activated carbon, complemented by impregnated alumina for enhanced removal of sulfur compounds.
Results
The sludge dryer odour control system ensures stable performance and reliable odour abatement over time.
The installed systems provide stable treatment of emissions generated by sludge drying operations, ensuring reliable odour abatement and compliance with environmental regulations.
The integration of chemical scrubbing, biofiltration and adsorption allows progressive removal of pollutants with different chemical properties, increasing operational robustness.
Polypropylene scrubbers are widely used in industrial air pollution control systems due to their excellent chemical resistance and relatively low cost. A Polypropylene scrubber is typically used in wet scrubbing applications where corrosive gases, odours or pollutants must be removed from air streams. However, in real industrial environments, Polypropylene scrubbers are often selected based only on chemical compatibility. This is where most design mistakes begin.
Why Polypropylene Scrubbers Are So Common
The success of scrubbers made in Polypropylene comes from three main factors:
High chemical resistance
Cost-effectiveness compared to stainless steel
Ease of fabrication
For many standard applications, a polypropylene scrubber is a reliable and efficient solution.
But this is only part of the story.
The Real Limit of Polypropylene Scrubbers
The main limitation of scrubbers made in Polyptopylene is not chemical resistance.
It is mechanical behaviour under temperature.
Polypropylene has a high thermal expansion coefficient:
0.15–0.20 mm/m·°C
In industrial scrubbers, this means that even small temperature variations can generate significant structural movement.
Thermal Expansion in Polypropylene Scrubbers
Thermal expansion is often underestimated during design.
In real operation, it leads to:
Misalignment between scrubber and piping
Stress on flanges and connections
Progressive deformation of the structure
Long-term mechanical fatigue
These effects are not immediate, but they become critical over time.
In constrained layouts, like long duct runs with fixed supports, thermal expansion cannot dissipate. It accumulates.
Why Polypropylene Scrubbers Fail in Operation
When scrubbers made in Polypropylene fail, the cause is rarely chemical.
Typical failure mechanisms include:
Rigid piping connected to the scrubber
Lack of expansion compensation
Poor mechanical support design
Temperature fluctuations not considered
In many cases, the problem is attributed to installation errors, while the real issue is design-related.
Polypropylene vs Other Materials
Polypropylene is often compared with other materials such as HDPE or stainless steel.
Each material behaves differently:
Polypropylene scrubbers → excellent chemical resistance, high thermal expansion
HDPE scrubbers → better mechanical resilience, similar expansion issues
Stainless steel scrubbers → low expansion, but limited chemical resistance in aggressive environments
There is no universally “best” material. The correct choice depends on the process conditions.
When Polypropylene Work Well
Polypropylene perform reliably when:
Temperature varies significantly
Large structures are mechanically constrained
The system is rigid and over-constrained
Long-term dimensional stability is critical
In these cases, alternative materials or different design approaches should be considered.
Conclusion: Understanding Polypropylene Scrubbers
Polypropylene scrubbers are not a bad choice. They are often a misunderstood one.
The key is not only to evaluate chemical resistance, but to understand how Polypropylene scrubbers behave under real operating conditions.
In industrial air treatment, material selection is not a chemical decision. It is an engineering decision.
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.
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.
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