Overcoming Oxygen Challenges in Biogas Desulphurisation: Solutions for Europe’s Growing Biomethane Market
Discover how to overcome oxygen challenges in biogas desulphurisation for biomethane production. As the EU tightens O₂ limits in gas grids, choosing the right desulphurisation technology is crucial. Explore efficient solutions to ensure compliance and optimize biomethane output.
Introduction : Why is developing the Biomethane market relevant for the EU ?
The EU is facing very high energy costs for gas and remains heavily dependent on the import of fossil gas. Driven by the war in Ukraine, the EU has actively worked to reduce its dependency on Russian gas. However, even after three years of war, as of 2025, 33% of the EU’s gas supply still comes from Russia.
Secondly, the EU faces significant challenges in meeting its CO₂ footprint reduction goals (EU Green Deal). Currently, 70% of the EU's total energy demand (electricity, heat, and transport) is met by fossil oil and fossil gas. While solar and wind power have increased the share of renewable electricity, their overall impact on reducing the EU's CO₂ footprint remains limited.
Thirdly, the EU is significantly behind in decarbonizing transport compared to its stated goals. Even in the most optimistic projections by the European Commission, 66% of all cars in 2040 will still have combustion engines, and by 2050, the figure will still be 45%.
The most recent official EU models (end of 2024, RED III) indicate that biomethane will play a crucial role in achieving CO₂ reduction targets in both global energy and transport, surpassing biodiesel and bioethanol.
Biomethane offers more than a 95% CO₂ savings compared to fossil gas. This explains the projected annual growth rate of 7–10% for biomethane in the EU until 2050 (REPowerEU).
The process of converting waste(water) into biomethane involves three key steps:
- Anaerobic digestion to produce biogas from waste,
- Biogas desulphurisation, and
- Biomethane upgrading.
A high-performing biomethane project requires an efficient biogas desulphurisation system. This post highlights the key challenges for biogas desulphurisation in a rapidly evolving biomethane market.
Oxygen challenges for biogas desulphurisation in a rapidly developing biomethane market in EU
Key Technical Challenge: Low Oxygen Concentration Limits in the EU Gas Grid
The EU’s methane grid must adhere to very low oxygen levels to prevent corrosion, combustion risks, and the formation of black powder within the grid. EU gas networks enforce strict maximum O₂ limits (source: ENTSOG).
Unfortunately, many biogas desulphurisation systems struggle to maintain these low O₂ concentration limits. As the proportion of biomethane in the natural gas grid increases, the oxygen problem is becoming more pronounced.
In the early years of biomethane, limits of <0.8% to <1.0% vol O₂ (1 vol% = 10,000 ppm O₂) were typical. However, an increasing number of projects now impose stricter limits of <0.5% vol O₂ (5,000 ppm), and in some cases, <0.2% vol O₂ (2,000 ppm). As the biomethane share in the grid continues to grow, this issue will only become more critical.
Biomethane Upgrading Technologies That Worsen the Oxygen Problem
Membrane-based upgrading units separate desulphurised biogas into two flows: a methane-rich flow and a CO₂-rich flow.
Approximately two-thirds of the oxygen remains in the purified methane flow. Since CO₂ is removed, the oxygen concentration in biomethane increases significantly.
Challenges in Achieving Low Oxygen Levels in Biogas Desulphurization
Biological trickling filters with O₂ injection use specific bacteria to convert H₂S into sulphuric acid:
- Step 1: H₂S (S as -2) + O₂ → S₀ (elemental sulphur)
- Step 2: S₀ + O₂ → SO₄²⁻ (sulphate)
For optimal performance and reduced clogging, these systems typically require a high O₂ concentration (>1.0% vol O₂, often ≥1.5% vol O₂). The high oxygen concentration drives the reaction towards sulphate (SO₄²⁻).
However, in biomethane applications—where stricter O₂ limitations apply—biological trickling filters typically operate at 0.6–0.7% vol O₂.
Lower O₂ levels push the reaction kinetics toward elemental sulphur (S₀), leading to:
- Higher clogging rates
- Preferential channeling in the trickling filter
- Reduced H₂S removal efficiency (300–450 ppm H₂S outlet)
- Increased activated carbon costs for final purification
- More downtime for cleaning
Solutions: Biogas Desulphurisation achieving very low O₂ Levels
Three biogas desulphurisation systems can achieve very low oxygen concentrations.
1. Ferric Dosing in the Digester, Followed by Activated Carbon
This method involves reducing sulphur content with iron chloride in the digester, followed by activated carbon filtration.
Advantages:
- Low CAPEX, high OPEX
- Simple operation
- No oxygen dosing → Very low O₂ levels in desulphurised gas
Disadvantages:
- Best suited for small to mid-scale plants; OPEX becomes too high for large-scale operations
- H₂S levels after ferric dosing remain at ~400 ppm, leading to high activated carbon consumption
2. External High-pH Washing with Biological Regeneration Using Thiothrix Bacteria
This system consists of two steps:
- High-pH chemical washing (pH 8.5)
- A caustic scrubber removes H₂S without injecting oxygen, ensuring very low O₂ levels in the desulphurised gas.
Biological regeneration of NaS solution
- Specialised, slow-growing Thiothrix bacteria convert NaS into elemental sulphur (S₀) and NaOH, in a delicate balance of specific oxygen and redox conditions.
- The recovered caustic soda (NaOH) is reused in Step 1.
Advantages:
- Achieves very low O₂ levels
- Low chemical consumption
Disadvantages:
- High CAPEX (best suited for large-scale plants)
- Complexity of the biological regeneration – due to dedicated slow growing bacteria quickly upset by any variation - requiring a high presence of highly skilled process personnel (1,5 hour’s/day).
- Potential downtime (5–10%) to be considered in system TCO
- The disadvantages of the system are the high process complexity of the biological regeneration – due to dedicated slow growing bacteria quickly upset by any variation - requiring a high presence of highly skilled process personnel (1,5 hour’s/day) and potential downtime of 5 to potentially 10 %, to be considered in the TCO of the system.
3. Physical Biogas Washing with Mixed Liquor from an Aerobic Wastewater Treatment Plant (BIOTIM® Scrubber)
H₂S dissolves in slightly alkaline (pH >7.6) water, enabling a straightforward alternative:
- Most aerobic wastewater treatment plants operate at light alkaline pH, making them excellent sources of wash water.
- A multi-stage scrubber effectively removes both H₂S and O₂ from the biogas.
Advantages:
- Achieves very low O₂ concentrations (<0.2% vol O₂)
- Efficient H₂S removal
BIOTIM® Scrubber : Efficient H2S removal at optimal Cost of Ownership
As the biomethane sector continues to grow in the EU, stricter low O₂ requirements pose increasing challenges for biogas desulphurisation. Many traditional systems struggle to meet these demands, but solutions like ferric dosing with activated carbon, high-pH biological regeneration, and physical washing with wastewater treatment plant liquor offer promising alternatives. The choice of system depends on project scale, budget, and operational complexity. The BIOTIM®scrubber is particularly adapted for H2S removal, achieving very low oxygen concentrations. Read more on BIOTIM® scrubbing technology.
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