How Does a Wet Scrubber Work? A Practical Guide for Plant Managers
If you manage a manufacturing facility that produces acid gases, odorous emissions, or water-soluble pollutants, a wet scrubber is likely one of the most effective air pollution control technologies available to you.
But understanding how the wet scrubber process works and what separates a well-designed system from one that underperforms is essential before you invest.
This guide explains how wet scrubbers work from a practical engineering perspective, covers the main types used in the UK industry, and outlines what to look for when specifying, operating, and maintaining a system.
What is a wet scrubber?
A wet scrubber is an air pollution control device that uses a liquid, typically water, or water mixed with a chemical reagent, to remove contaminants from an industrial gas stream. The contaminated exhaust is brought into direct contact with the scrubbing liquid, and pollutants are either absorbed into the liquid, chemically neutralised, or physically captured as the gas passes through the system.
Wet scrubbers are widely used across UK manufacturing for treating acid gases such as hydrochloric acid (HCl), sulphur dioxide (SO₂), and ammonia (NH₃), as well as for odour control and particulate removal. They are a recognised Best Available Technique (BAT) for these applications under the European Commission’s BREF guidance documents.
How does the wet scrubber process work?
The fundamental principle behind every wet scrubber is mass transfer: moving pollutant molecules from the gas phase into the liquid phase. The effectiveness of this transfer depends on three things: the contact area between gas and liquid, the time the gas spends in contact with the liquid, and the chemical driving force (how strongly the pollutant wants to dissolve into the scrubbing liquid).
Here is how a typical counter-current packed bed wet scrubber operates, step by step.
Step 1: Contaminated gas enters the scrubber
The polluted exhaust stream, drawn from your process by a centrifugal fan, enters the scrubber tower at the base through a gas inlet duct. At this stage, the gas may contain acid fumes, volatile organic compounds, odorous substances, or fine particulate matter, depending on your manufacturing process.
Step 2: Gas rises through the packing media
The gas flows upward through a bed of packing media structured or random-packed shapes designed to maximise the surface area available for gas-liquid contact. Common packing materials include polypropylene, ceramic, or metal saddles, rings, and structured sheet configurations.
This packing bed is the heart of the scrubber. It is where the mass transfer happens. As the gas passes over the wetted surfaces of the packing, pollutant molecules cross from the gas phase into the liquid film coating the packing.
Step 3: Scrubbing liquid is distributed from above
While the gas rises, a scrubbing liquid is sprayed downward from distribution nozzles mounted above the packing bed. The liquid flows down through the packing under gravity, counter-current to the rising gas, meaning the cleanest liquid meets the cleanest gas at the top, and the most contaminated liquid meets the dirtiest gas at the bottom.
This counter-current arrangement maximises removal efficiency because it maintains the strongest chemical driving force across the full height of the packing bed.
For acid gas applications, the scrubbing liquid is typically a solution of sodium hydroxide (NaOH) or lime. For odour control, oxidising agents such as sodium hypochlorite may be added. For dust or particulate removal, water alone may be sufficient.
Step 4: Mist eliminator removes liquid droplets
At the top of the tower, above the spray nozzles, a mist eliminator captures any fine liquid droplets that have been entrained in the upward gas flow. Without this stage, scrubbing liquid would carry over into the exhaust stack, wasting reagent and potentially creating a visible plume.
Step 5: Clean gas exits to atmosphere
The treated gas, now stripped of its target pollutants, exits through the top of the scrubber and is discharged to atmosphere via an exhaust stack. A well-designed and properly maintained wet scrubber typically achieves removal efficiencies of 95% to 99% for its target compounds.
Step 6: Spent liquid is collected and recirculated
The scrubbing liquid collects in a sump at the base of the tower. From here, a recirculation pump returns it to the spray nozzles at the top. A pH monitoring and chemical dosing system maintains the correct reagent concentration in the recirculating liquid.
Over time, the liquid accumulates dissolved pollutants and must be partially or fully replaced. The spent liquid, which is now a waste stream, requires appropriate treatment or disposal in line with UK environmental regulations.
What are the main types of wet scrubbers?
While the packed bed tower described above is the most common configuration for gas absorption applications, there are several other wet scrubber designs, each suited to different pollutant types and process conditions.
Packed bed scrubbers provide the highest gas-liquid contact area per unit volume and are the standard choice for acid gas removal (HCl, SO₂, NH₃) and odour abatement. They are the type most frequently designed and maintained by process engineers in the UK manufacturing sector.
Venturi scrubbers use a converging-diverging throat section to accelerate the gas stream and atomise the scrubbing liquid into very fine droplets. The high-velocity impact between gas and liquid makes venturi scrubbers particularly effective at capturing fine particulate matter and aerosols, though they are less efficient than packed beds for gas absorption.
Spray tower scrubbers are the simplest design, the gas passes upward through an open chamber while liquid is sprayed downward. They have lower pressure drop than packed beds but also lower removal efficiency, making them suitable for pre-treatment stages or applications with high-solubility pollutants.
Multi-stage scrubbers combine two or more stages, for example, a first stage with acidic scrubbing liquid to capture ammonia, followed by a second stage with alkaline liquid to capture acid gases. Multi-stage designs are common in food processing, pharmaceutical, and waste treatment facilities where the exhaust contains multiple pollutant types.
What pollutants can wet scrubbers remove?
Wet scrubbers are most effective for water-soluble gases and compounds that react readily with chemical reagents. The main applications include:
Acid gases: HCl, SO₂, HF, and NOₓ are efficiently captured using alkaline scrubbing solutions. This is one of the highest-volume applications for wet scrubbers in UK industry, particularly in chemical manufacturing, pharmaceutical production, and metal finishing.
Ammonia and amines: Readily absorbed by acidic scrubbing solutions (typically dilute sulphuric acid). Common in food processing, waste handling, and composting operations.
Odorous compounds: Hydrogen sulphide, mercaptans, and organic odour compounds can be neutralised using oxidising scrubbing solutions. Wet scrubbers are a standard odour control technology for bakeries, ready-meal production, wastewater treatment, and in-vessel composting facilities.
Particulate matter: Venturi scrubbers and high-energy spray scrubbers can capture dust and fine particulate from exhaust streams, though fabric filters or electrostatic precipitators may be more cost-effective for dry dust applications.
Water-soluble VOCs: Some volatile organic compounds dissolve readily in water or can be chemically oxidised in the scrubbing liquid. However, for most industrial solvent streams, particularly in print and packaging, textiles, and coating operations, a Regenerative Thermal Oxidiser (RTO) is the more appropriate and efficient abatement technology for VOC destruction.
How Operational Group approaches wet scrubber projects
At Operational Group, we apply our four-stage engineering approach to every wet scrubber project, whether it is a new installation or the optimisation of an existing system.
Stage 1: Survey and understand. We carry out a detailed process survey to establish your exhaust gas characteristics: flow rates, temperatures, pollutant composition, and concentration levels. We work with your engineering and production teams to understand the operational variability of your process, because a scrubber designed for steady-state conditions will underperform if your actual emissions fluctuate significantly.
Stage 2: Design and engineer. Using the survey data, we develop a bespoke scrubber design specification covering tower sizing, packing selection, liquid-to-gas ratio, chemical dosing regime, materials of construction, and instrumentation. Where the scrubber forms part of a wider abatement system, for example, upstream of or alongside an RTO, we design the central ductwork and control integration to ensure both systems operate as a coordinated whole.
Stage 3: Install and commission. We deliver a turnkey installation covering fabrication, ductwork, pipework, pumps, instrumentation, controls, and commissioning. Our project managers provide a single point of contact throughout, with full project management from our local team.
Stage 4: Service and maintain. A scrubber that is not properly maintained will lose efficiency, waste reagent, and risk non-compliance. Our planned preventative maintenance programmes cover the full-service scope: interior and exterior housing cleaning, fan removal and cleaning, baffle and panel inspection, packing media condition assessment, exit duct cleaning to roof level, cowl grating replacement, pH system checks, and pump inspection. Reports are issued digitally via our cloud-based reporting system, providing full traceability for your maintenance, safety, and quality records.
Common wet scrubber problems and how to avoid them
From our experience servicing and maintaining scrubber systems across multiple industries, the most frequent issues we encounter are:
Fouled or degraded packing media. Over time, dissolved solids, biological growth, or chemical deposits can coat the packing surfaces, reduce the effective contact area and increase pressure drop. Regular inspection and cleaning or replacement where necessary is essential.
Incorrect pH dosing. If the scrubbing liquid pH drifts outside the optimal range for your target pollutant, removal efficiency drops sharply. Automated pH monitoring with redundant sensors and properly calibrated dosing pumps prevents this.
Nozzle blockage. Spray nozzles can become blocked by precipitated solids or scale, leading to uneven liquid distribution across the packing bed. Periodic nozzle inspection should be part of every maintenance visit.
Fan performance degradation. The process fan is critical to maintaining the correct gas flow through the scrubber. Bearing wear, impeller fouling, and belt tension issues all reduce fan performance over time. Vibration monitoring during maintenance visits catches these problems early.
Inadequate liquid blowdown. If spent scrubbing liquid is not replaced frequently enough, dissolved pollutant concentrations build up and reduce the chemical driving force for absorption. This is one of the most common causes of gradually declining scrubber performance.
Is a wet scrubber the right choice for your process?
Wet scrubbers are the right technology when your primary pollutants are water-soluble gases, acid fumes, ammonia, or odorous compounds. They carry no combustion-related explosion risk, can handle corrosive gas streams that would damage thermal oxidation systems, and can be designed to target multiple pollutants through multi-stage configurations.
However, they are generally not the best choice for high-volume solvent-based VOC destruction, which is where RTOs excel. And they produce a liquid waste stream that must be managed, which adds an ongoing operational consideration.
The right answer often involves a combination of technologies. A food manufacturer, for instance, might use a wet scrubber for odour control on one process line and an RTO for solvent abatement on another, with both systems feeding into a shared central ductwork system designed to comply with DSEAR and the Explosive Atmosphere Directive.
If you’re unsure which air pollution control technology suits your process, contact Operational for a free site survey. We’ll assess your exhaust characteristics, regulatory requirements, and operational priorities and recommend the right solution, engineered around you.
Operational (UK) Limited designs, installs, and maintains wet scrubber systems, Regenerative Thermal Oxidisers, carbon adsorbers, and integrated air pollution control solutions for manufacturers across the UK and Europe. With over 30 years of engineering experience and a team of 32 specialist engineers, we deliver bespoke systems that meet your permit requirements and reduce your operating costs.