What Is the Methane Content of Sewage Biogas?

Sewage biogas is typically 55–65% methane (CH₄), with 35–45% carbon dioxide and small amounts of water vapor, hydrogen sulfide (H₂S), and trace nitrogen or oxygen. Sewage-derived biogas usually sits toward the upper end of that band—around 60–65% methane—because sludge is a consistent, readily degradable feedstock.

The Standard Composition

After water and H₂S are removed for use, the dry, cleaned gas is dominated by methane and CO₂ in roughly a 60:40 to 65:35 split. Methane is the combustible, energy-carrying component; CO₂ is inert and lowers the heating value, which is why upgrading strips it out for biomethane.

What Else Is Present

Beyond CH₄ and CO₂, raw biogas carries water vapor (condenses and corrodes), H₂S (toxic, corrosive, smells of rotten eggs), and trace siloxanes (from personal-care products, form abrasive deposits on engines), plus minor N₂ or O₂ from air ingress. Cleaning targets these before the gas reaches an engine or grid.

Why the Content Varies

Methane percentage moves with feedstock and digester health. A stable mesophilic digester (35–38°C) on consistent sludge holds ~60–65% CH₄; feed shocks, acidification, or temperature drift can drop it as CO₂ rises. Co-digestion with fatty or sugary feedstock can nudge it higher, while poor mixing or inhibition pushes it lower.

Comparative Data Table: Biogas Composition

Technical Considerations

Know the gas you have before specifying equipment. CHP and boilers need H₂S and water controlled; upgrading to biomethane needs CO₂ removed to reach ~98% CH₄. On-line gas analysis tracking the methane percentage is also an early-warning tool for digester stability.

Advantages and Limitations

Sewage biogas's consistent ~60–65% methane makes it a dependable fuel. The limitation is the impurity load—H₂S and siloxanes in particular—which must be managed or it will damage engines and shorten intervals between overhauls.

Best Practices / How to Manage Composition

Meter gas composition continuously, de-water and de-sulfurize before the engine, and plan siloxane removal if upgrading or firing sensitive equipment. Treat a falling methane percentage as a stability alert, not just a quality reading.

Sewage biogas runs about 55–65% methane—usually 60–65% from consistent sludge—with 35–45% CO₂ and traces of water, H₂S, and siloxanes. That composition drives both how you clean it and how you use it, and a dropping methane percentage is an early instability signal.

Frequently Asked Questions (FAQ)

Q1: What is the methane content of sewage biogas?

A: Typically 55–65% CH₄, usually 60–65% from consistent sludge, with 35–45% CO₂ plus water, H₂S, and trace N₂/O₂.

Q2: What else is in it?

A: Water vapor, hydrogen sulfide (corrosive, toxic), trace siloxanes (engine-abrasive), and minor nitrogen or oxygen from air ingress.

Q3: Why does it vary?

A: Feedstock and digester health—stable mesophilic (35–38°C) digestion holds ~60–65% CH₄; shocks or temperature drift drop it as CO₂ rises.

Q4: How is methane raised?

A: Upgrading strips CO₂ (and impurities) to reach ~98% CH₄ biomethane; better feeding and stability also keep the natural percentage high.

Q5: Does methane percentage signal problems?

A: Yes—a sustained drop below the norm, with rising CO₂, is an early warning of instability worth acting on.

Q6: Is sewage biogas good fuel?

A: Yes—its consistent ~60–65% methane makes a dependable CHP/boiler fuel, provided H₂S and siloxanes are controlled first.

Project Case Reference

Malaysia Biogas Project  —  Malaysia · 2026

5 GFS Tanks   ·   27,000 m³ Total Volume   ·   22,000 m³ Biogas/Day

A large-scale biogas project in Malaysia featuring 5 Glass-Fused-to-Steel (GFS) tanks. The project achieves approximately 80% digestibility, with each single tank producing about 4,400 m³ of biogas daily, totaling 22,000 m³ per day across all 5 tanks.

Technical Specifications

Single Tank Volume: 5,400 m³ (Ø24.46 × 12 m)

Total Effective Volume: 27,000 m³ (5 tanks)

Daily Biogas Production: 22,000 m³ total

Digestibility: ≈ 80%

Gas Production Rate: 0.45 m³ / kg COD removed

Tank Type: Glass-Fused-to-Steel (GFS)

Water Quality Data