Raw biogas production volume is the most commonly monitored gas-phase parameter in operating digester, and the least sensitive early warning indicator available. A system can accumulate VFAs sufficient to drive the VFA/alkalinity ratio above 0.4 while total gas production declines by only 10–15%. Normalised biogas yield removes the masking effect of loading variation and reveals methanogenic efficiency directly.
What Normalised Biogas Yield Measures
Normalised biogas yield is the volume of biogas produced per unit of organic matter removed, expressed as m³ of biogas per kg of COD removed. The word ‘normalised’ means that the effect of organic loading variability has been removed: if loading increases and gas production increases proportionally, the yield is unchanged. If loading increases but gas production does not keep pace, the yield declines, signalling that methanogenic efficiency is falling.
It is the equivalent of measuring fuel efficiency in kilometres per litre rather than total kilometres driven. Total kilometres tells you how far you went. Kilometres per litre tells you how efficiently the engine is performing.
The Calculation
| Step | Formula / Value |
|---|---|
| COD removed (kg/day) |
[Influent COD (mg/L) − Effluent COD (mg/L)] × Flow rate (m3/day) ÷ 1,000 |
| Normalised biogas yield |
Total biogas produced (m3/day) ÷ COD removed (kg/day) |
Worked Example
| Parameter | Value |
|---|---|
| Influent COD | 8,000 mg/L |
| Effluent COD | 1,200 mg/L |
| COD removed | 6,800 mg/L |
| Flow rate | 500 m3/day |
| COD removed per day | 6,800 × 500 ÷ 1,000 = 3,400 kg/day |
| Biogas produced | 2,040 m3/day |
| Normalised biogas yield | 2,040 ÷ 3,400 = 0.60 m3/kg COD removed |
Benchmark Values and Their Interpretation
Theoretical maximum biogas yield from COD removal is approximately 0.35 m³ methane per kg COD at standard conditions. For total biogas at 60–65% methane content, the equivalent yield is approximately 0.50–0.65 m³ per kg COD in a well-operated system.
| Normalised Yield (m3/kg COD removed) | What It Indicates |
|---|---|
| 0.55 – 0.65 | Healthy system — methanogenesis efficient |
| 0.45 – 0.55 | Moderate efficiency — investigate VFA accumulation or temperature |
| 0.35 – 0.45 | Poor efficiency — methanogenic activity significantly below potential |
| Below 0.35 | Serious underperformance — active biological disturbance likely |
These ranges vary with wastewater composition: lipid-rich wastewaters produce more methane per kg COD than carbohydrate-rich ones. The plant’s own historical baseline is therefore more meaningful than generic benchmarks — the trend matters more than the absolute value.
Why It Detects Problems That Raw Gas Volume Misses
Consider two scenarios that appear identical on a raw gas flow meter but represent opposite biological realities:
| Scenario | Raw Gas Volume | Normalised Yield | What Is Actually Happening |
|---|---|---|---|
| A: Loading up 20%, gas up 15% | Looks acceptable — flow increasing | Dropped from 0.60 to 0.53 | VFAs accumulating; system under early methanogenic stress masked by loading increase |
| B: Loading down 20%, gas down 20% | Looks alarming — flow decreasing | Stable at 0.61 | Biology healthy; production lower because less substrate available |
Without normalisation, Scenario A looks acceptable and Scenario B looks worrying. With normalisation, the reality is exactly reversed. Raw gas volume without normalisation is a loading signal, not a performance signal.
How to Implement It
The required data is already available in every plant measuring influent COD, effluent COD, flow rate, and gas production. The calculation takes two minutes per week from a simple spreadsheet. Plot the result as a weekly trend line alongside the VFA/alkalinity ratio and OLR.
A downward trend in normalised yield sustained over three to four consecutive weeks — even if absolute gas production appears stable or rising because loading is increasing — is the signal to measure VFA/alkalinity ratio more frequently and investigate whether methanogenic efficiency is declining. The yield trend will typically show the problem 2–3 weeks before the VFA/alkalinity ratio reaches an intervention threshold.
Divergence Between OLR and Normalised Yield — The Critical Pattern
The most diagnostically significant pattern is a divergence between organic loading rate (OLR) and normalised yield: loading increasing or stable, yield declining. This indicates that the methanogenic community is not keeping pace with the organic load being applied — the biological gap that, if not identified and corrected, proceeds to VFA accumulation and eventual acidification.
| OLR Trend | Normalised Yield Trend | Interpretation | Action |
|---|---|---|---|
| Stable | Declining | Methanogenic efficiency falling — loading unchanged |
Investigate temperature, nutrients, pH; increase VFA/alk measurement frequency |
| Increasing | Declining | Methanogenic population cannot keep pace with load |
Reduce OLR increase rate; verify sludge inventory; check SRT |
| Increasing | Stable | System absorbing additional load efficiently |
Monitor; continue planned loading increase |
| Declining | Increasing | Load reduction improving methanogenic performance |
Positive signal; biology recovering |
What Operators Should Do
- Calculate normalised biogas yield regularly using COD removed rather than only total gas generated.
- Correct gas volume to standard conditions (Nm³) for meaningful comparison.
- Trend gas yield daily, weekly, and monthly to identify gradual deterioration.
- Investigate declining methane yield even if total gas volume appears stable.
- Check gas flowmeter calibration and sensor reliability periodically.
- Avoid increasing feed solely because total gas production rises temporarily.
- Monitor VFA trends when gas efficiency starts declining.
References
- Schievano, A. et al. (2014). Toward a predictive model of biodegradability and biogas production potential in anaerobic digestion. Energy & Fuels, 28(8), 5243–5250.
- Angelidaki, I. et al. (2009). Defining the biomethane potential (BMP) of solid organic wastes and energy crops: a proposed protocol for batch assays. Water Science & Technology, 59(5), 927–934.
- Melse, R.W. & Timmerman, M. (2009). Sustainable intensive livestock production demands manure and exhaust air treatment technologies. Bioresource Technology, 100(22), 5506–5511.
- Lossie, U. & Pütz, P. (2008). Targeted control of biogas plants with the help of FOS/TAC. Practice Report, Hach-Lange GmbH.
- Batstone, D.J. et al. (2002). Anaerobic Digestion Model No. 1 (ADM1). IWA Publishing, London.
- Green Pulse™ Anaerobic Treatment Module — Internal Technical Documentation (2024).