Biological Desulphurisation for AD Energy Plants

The Challenge: Controlling H₂S in Biogas

Hydrogen sulphide (H₂S) is a natural by-product of anaerobic digestion, but if left untreated it creates serious operational risks:

• Corrosion of CHP engines, pipework and gas systems
• Increased maintenance costs and unplanned downtime
• Reduced efficiency in gas upgrading and combustion
• Risk of non-compliance and environmental issues

Traditional methods such as ferric dosing or activated carbon filtration are widely used, but they introduce ongoing chemical costs, maintenance burden and operational inefficiencies.

 

The Solution: Biological Desulphurisation

Biological desulphurisation is one of the most effective and cost-efficient methods of reducing hydrogen sulphide (H₂S) in anaerobic digestion plants. Rather than relying solely on chemical dosing or consumable filtration systems, biological processes use naturally occurring bacteria and controlled oxygen environments to convert H₂S into elemental sulphur and water.

Depending on the plant size, gas quality requirements and downstream process equipment, biological desulphurisation can be achieved using several proven approaches:

• Biological desulphurisation towers
• Controlled air injection into digesters
• Automated oxygen dosing systems

All methods aim to reduce H₂S concentrations, improve biogas quality and protect downstream equipment, while significantly reducing operating costs compared to traditional chemical treatment systems.

Biological Desulphurisation Towers

Biological desulphurisation towers, sometimes referred to as biological scrubbers or trickling filters, provide highly effective downstream H₂S treatment for anaerobic digestion and gas upgrading applications.

In these systems, biogas passes through a packed media tower where sulphur-oxidising bacteria biologically convert hydrogen sulphide into elemental sulphur under carefully controlled oxygen conditions.

These systems are particularly well suited to:

• Large-scale anaerobic digestion plants
• Gas-to-grid upgrading systems
• Applications requiring stable, high-efficiency H₂S removal
• Sites looking to minimise chemical consumption and maintenance

Key advantages include:

• High H₂S removal efficiency
• Extremely low water and energy consumption
• Reliable continuous operation
• Reduced operating costs compared to consumable media systems

Air Injection (Microbial Desulphurisation)

One of the most widely adopted biological desulphurisation methods is controlled air injection into the digester headspace. Small volumes of air, typically less than 0.5% vol, are introduced to support aerobic bacteria naturally present within the digester.

These bacteria convert H₂S into sulphur deposits, reducing hydrogen sulphide levels before the gas reaches CHP engines or upgrading systems.

• Low capital cost solution
• Ideal for CHP-based AD plants
• Simple and cost-effective
• Widely adopted across the anaerobic digestion industry

Oxygen Injection (PSA / VSA Systems)

For higher performance applications, particularly gas-to-grid and upgrading systems, oxygen injection using PSA or VSA oxygen generators provides improved control and gas quality.

Compared to standard air injection, oxygen dosing minimises nitrogen dilution within the biogas stream while providing highly efficient biological desulphurisation performance.

Key advantages include:

• Improved biomethane quality
• Lower nitrogen carryover
• On-site oxygen generation up to ~95% purity
• Fully automated dosing capability
• Ideal for upgrading and grid injection applications

Regardless of the biological desulphurisation method used, maintaining the correct dosing and process control is critical to achieving stable and efficient H₂S reduction.

The Key Challenge: Getting Control Right

Biological desulphurisation is highly sensitive and depends entirely on maintaining the correct oxygen or air dosing rate.

• Too much air or oxygen can disrupt anaerobic digestion and create safety concerns
• Too little dosing allows H₂S concentrations to rise rapidly
• Feedstock variability can cause significant fluctuations in gas composition

Historically, many systems have relied on manual adjustment and operator experience, making them inconsistent and difficult to optimise.

The real solution is automated, closed-loop control based on continuous gas analysis and intelligent dosing control.

How It Works: Automated Desulphurisation Control

Allison Engineering delivers fully integrated systems that combine:

• Continuous biogas analysis (H₂S, O₂, CH₄, CO₂)
• Intelligent control algorithms
• Automated air or oxygen dosing
• Flow measurement and process monitoring
• Integration with biological desulphurisation towers and upgrading systems

The system continuously monitors gas composition and automatically adjusts dosing rates in real time to maintain stable H₂S reduction without constant operator intervention.

Typical operation includes:

• Air or oxygen setpoint control
• Adaptive control responding to H₂S fluctuations
• Multi-digester sampling and control
• Integrated alarm and safety functionality

Result: stable H₂S reduction, improved efficiency and reduced operational risk.

Desulphurisation Methods

Air Injection (Microbial Desulphurisation)

• Low capital cost solution
• Ideal for CHP-based AD plants
• Injects small volumes of air into the digester headspace
• Proven method for reducing H₂S by up to 90%

✔ Simple and cost-effective
✔ Widely adopted across AD plants

 

Oxygen Injection (PSA / VSA Systems)

• On-site oxygen generation (up to ~95% purity)
• Eliminates nitrogen dilution in biogas
• Ideal for gas-to-grid and upgrading systems

✔ Improved gas quality
✔ Lower nitrogen carryover
✔ Fully automated dosing control

Biological Desulphurisation Towers

• Biological trickling filter technology
• Suitable for downstream gas treatment and upgrading

✔ Extremely low water and energy consumption
✔ High performance H₂S removal
✔ Suitable for large-scale installations

 

Recommended Products & Technologies

Biogas Analysers (Core Control Technology)

• AwiFlex / AwiFlex XL
• Multi-gas measurement (CH₄, CO₂, H₂S, O₂, H₂)
• Multi-point sampling across digesters
• Automated desulphurisation control
• Integration with plant control systems
• AwiEco
• Cost-effective solution for smaller installations
• Reliable monitoring of key gas parameters

Oxygen Generation Systems

• PSA / VSA oxygen generators
• Modular, low maintenance systems
• Designed for continuous operation

 

Flow Measurement (Supporting Optimisation)

• Thermal mass flow meters for biogas
• No moving parts → high reliability in wet, dirty gas
• Accurate measurement across wide flow ranges

 

Technical Insight: Why Automation Matters

The biggest shift in biological desulphurisation is moving from:

Manual dosing, to Intelligent, automated control systems.

Automated systems:

• Prevent over- or under-dosing
• React instantly to process changes
• Improve consistency of H₂S removal
• Reduce operator workload

This is particularly important in AD plants where:

• Feedstock variability impacts gas composition
• H₂S levels can fluctuate rapidly
• Stability is critical for CHP and upgrading systems

 

Our Experience in Biogas Applications

Allison Engineering has extensive experience delivering biogas solutions across the UK, including:

• Hundreds of biogas analyser installations
• Integrated desulphurisation control systems
• CHP and gas-to-grid projects

We support customers through:

• System design and specification
• Installation and commissioning
• Ongoing service and optimisation

See also:

Gas-to-Grid Upgrading

Biogas Flow Measurement

Biogas Analysis

Biogas Flare Gas Monitoring

 

Key Benefits

• Effective reduction of H₂S to protect CHP units and upgrading systems
• Improves biogas quality for energy generation and grid injection
• Low operating cost compared to chemical or alternative treatment methods
• Supports stable digester performance with controlled oxygen dosing
• Reduces corrosion, maintenance and equipment downtime
• Environmentally friendly process using biological treatment