Biological H₂S removal uses a controlled oxygen supply to support oxidation of hydrogen sulphide in a treatment process. Even at relatively low concentrations, H₂S can cause corrosion, damage engines and gas-treatment equipment, increase maintenance requirements and produce sulphur oxide emissions during combustion. At higher concentrations, it also presents a serious toxic-gas hazard.
Biological desulphurisation uses sulphur-oxidizing microorganisms to convert hydrogen sulphide into elemental sulphur, sulphate or a combination of sulphur compounds. These microorganisms require a carefully controlled supply of oxygen. An air blower may therefore be used to provide the oxidation air required by the biological process.
In this application, the blower normally handles atmospheric air rather than raw biogas. This is an important technical distinction. The blower supplies a measured air stream to a digester headspace, biological filter, biotrickling filter or associated liquid-treatment stage. Because this air ultimately comes into contact with methane-containing gas, its flow must be limited, monitored and automatically isolated under unsafe operating conditions.
Biological Desulphurisation Methods
Biological hydrogen sulphide removal can be integrated directly into the anaerobic digester or installed as an external gas-treatment system.
In-situ biological desulphurisation introduces a small, controlled quantity of air or oxygen into the upper section of the digester. Sulphur-oxidizing microorganisms grow on available surfaces, support media or the inner structure of the gas space. In the presence of oxygen, these microorganisms oxidize H₂S and form elemental sulphur or sulphate.
This approach can reduce the H₂S concentration before the biogas leaves the digester. It may offer relatively simple operation, but the oxygen concentration must be tightly controlled. Excessive air addition introduces nitrogen into the biogas, reduces its calorific value and can create an unsafe gas mixture. Sulphur deposits may also accumulate on surfaces, pipework and gas-handling components.
An external biotrickling filter provides a more controlled treatment environment. Raw biogas passes through a packed vessel containing media colonized by sulphur-oxidizing bacteria. A circulating nutrient solution maintains moisture, supplies nutrients and helps remove acidic reaction products.
Air can be introduced into the biogas stream, the circulating liquid or a designated section of the treatment vessel, depending on the process design. The microorganisms consume the supplied oxygen and convert the absorbed hydrogen sulphide.
A complete biotrickling filter system may contain:
- Raw-biogas inlet and distribution piping
- Packed biological treatment vessel
- Air blower or air-compression unit
- Precision air-flow control equipment
- Nutrient-liquid circulation pump
- Irrigation distribution system
- pH and conductivity monitoring
- Demister or droplet separator
- H₂S analysers upstream and downstream
- Oxygen measurement in the treated gas
- Liquid bleed and make-up system
The biological reactor must be sized for both maximum biogas flow and maximum H₂S loading. A filter designed only for average H₂S concentration may be unable to treat short-duration concentration peaks. Where large fluctuations are expected, a polishing stage such as activated carbon or another approved treatment technology may be installed downstream.
Biological removal does not eliminate every contaminant in biogas. Moisture, siloxanes, ammonia, carbon dioxide and particulates may require separate treatment depending on whether the gas is intended for a CHP engine, boiler, biomethane upgrading plant or pipeline injection.
Oxygen Demand and Airflow Control
The amount of oxygen required depends partly on the final sulphur product. Simplified biological oxidation reactions can be represented as follows.
For oxidation to elemental sulphur:
H₂S + 0.5 O₂ → S + H₂O
For complete oxidation to sulphate:
H₂S + 2 O₂ → SO₄²⁻ + 2 H⁺
The theoretical oxygen requirement therefore ranges from approximately 0.5 to 2 moles of O₂ per mole of H₂S, depending on the dominant reaction pathway.
For a preliminary volumetric calculation at equivalent reference conditions:
Theoretical Oxygen and Air Flow for H₂S Removal
- QO2,theoretical = theoretical oxygen flow
- Qair,theoretical = theoretical dry-air flow
- νO2 = stoichiometric oxygen coefficient, between 0.5 and 2 for the simplified reactions
- Qbiogas = biogas flow at the same reference conditions
- CH2S = H₂S concentration in parts per million by volume (ppmv)
- 0.2095 = approximate oxygen volume fraction in dry atmospheric air
These equations provide only a chemical starting point. The actual air requirement also depends on oxygen-transfer efficiency, biological activity, treatment-vessel design, liquid circulation, temperature, pH, gas residence time and the desired balance between elemental sulphur and sulphate formation.
The theoretical value must not be used directly as the blower setpoint without validation by the biological-process supplier.
Producing elemental sulphur usually requires less oxygen than producing sulphate. Excessive oxygen availability can shift the process towards sulphate formation, increase acid production and raise the chemical demand required for pH control. Insufficient oxygen, on the other hand, can reduce H₂S removal efficiency.
Air dosage should ideally respond to measured process conditions rather than remain fixed. Useful control inputs include:
- Incoming biogas flow
- Inlet H₂S concentration
- Outlet H₂S concentration
- Oxygen concentration in treated biogas
- Biotrickling-filter pH
- Oxidation-reduction potential
- Nutrient-liquid flow
- Differential pressure across the packed bed
A feed-forward signal based on biogas flow and H₂S concentration can establish the approximate oxygen demand. Feedback from the outlet H₂S and O₂ analysers can then trim the air-flow setpoint.
This combined strategy responds more effectively to changing sulphur loads than controlling the blower from biogas flow alone.
Selecting an Air Blower for H₂S Removal
A positive-displacement roots blower can provide a stable volume of air against changes in system resistance. This characteristic may be useful for large biological filters, centralized air systems or installations supplying several treatment vessels.
Roots blowers offer:
- Predictable air delivery
- Effective variable-speed control
- Oil-free air within the compression chamber
- Suitability for continuous operation
- Capability to overcome packed-bed and pipeline resistance
- Relatively simple industrial maintenance
- Duty-and-standby package options
However, not every biological H₂S removal system requires a roots blower. In-situ microaeration may need only a small air quantity. For very low flows, an appropriately sized metering compressor, diaphragm pump or side-channel blower may provide better controllability than an oversized roots machine.
The equipment should be selected from the actual required flow range, including minimum stable dosage—not simply the maximum theoretical airflow.
The approximate pressure requirement can be expressed as:
Blower Differential Pressure for Air Injection
- Δpblower = required blower differential pressure
- Δpinlet filter = pressure loss across the inlet filter
- Δppipeline = air-pipeline pressure loss
- Δpcontrol valve = pressure loss through the control valve
- Δptreatment system = pressure loss through the treatment system
- ρliquid = liquid density
- g = gravitational acceleration
- h = submerged injection depth
- Δpvalves = losses through control and isolation valves
- Δpinjector = pressure required across the injection device
For a biotrickling filter, the treatment-system loss can include the packing, gas distributor, demister and connecting pipework. Biological growth and sulphur deposition can gradually increase the differential pressure across the packed bed. The blower should be selected for the clean and expected fouled conditions, while excessive differential pressure should initiate maintenance rather than simply be overcome with a larger blower.
Airflow from a positive-displacement blower is most efficiently controlled by varying its speed. Throttling the discharge does not substantially reduce the volume displaced by the machine; it primarily raises pressure and absorbed power. A variable-frequency drive can establish the main capacity range, while a flow-control valve or mass-flow controller provides fine dosage where necessary.
Glossary
Biological desulfurization: Removal of hydrogen sulfide through biological oxidation.
Biotrickling filter: Packed reactor where a liquid film and biofilm treat contaminated gas.
Elemental sulfur: Solid sulfur product formed by partial oxidation of H2S.
Oxygen dosing: Controlled addition of air or oxygen to support biological treatment.
Oxygen utilization: Fraction of supplied oxygen consumed by the biological desulfurization process.
Sulfate: Oxidized sulfur ion produced by further sulfide oxidation.
Sulfur loading: Mass flow of sulfur compounds applied to the treatment system.
Sulfur yield: Mass of recovered elemental sulfur relative to the sulfur removed from the gas.