Anaerobic Digestion Numerical Modelling and AI
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The OLR vs the Biogas Plant stability
The OLR vs the Biogas Plant stability
To determin whether the Organic Loading Rate (OLR) is optimal you need to involve a combination of process monitoring, performance indicators, and operational experience.
An optimal OLR is one that maximizes biogas yield and process efficiency without causing instability in the anaerobic digestion system.
I- Indicators of an Optimal OLR
1- Stable Biogas Production
Biogas volume and methane content (CH₄%) remain relatively constant.
No sudden drops in gas production.
2- Stable pH (6.8–7.5)
Indicates healthy microbial activity, particularly for methanogens.
pH drifting below 6.5 suggests overloading and acid buildup.
3- Low Volatile Fatty Acids (VFA)
VFA concentrations remain low (< 500–1000 mg/L as acetic acid).
A rapid increase indicates that acidogens are outpacing methanogens.
4- High Alkalinity and Good Buffering Capacity
Total Alkalinity (TA) supports pH stability.
VFA/Alkalinity ratio < 0.3 indicates good process balance.
5- Steady or Increasing Methane Yield per kg VS
Specific methane yield (e.g., m³ CH₄/kg VS) is consistent.
Decreasing yield despite high OLR suggests microbial stress.
6- Acceptable Ammonia and H₂S Levels
Elevated levels can indicate protein overload or imbalance.
Total ammonia > 3,000 mg/L may inhibit methanogens.
II- Methods to Determine Optimal OLR
1- Gradual Increment Testing
Start with a low OLR and gradually increase.
Monitor performance indicators after each increase.
Stop increasing once signs of instability (e.g., VFA rise, pH drop) appear.
2- Mass Balance Calculations
Use feedstock characteristics (VS, COD) and digester volume.
3- Compare actual OLR to recommended ranges:
Wet digestion: 1.0–3.5 kg VS/m³/day
Dry digestion: 4.0–10.0 kg VS/m³/day
4- Pilot Testing
Operate a smaller-scale digester under varying OLRs.
Use findings to optimize the full-scale plant.
5- Mathematical Modeling
Use models like ADM1 (Anaerobic Digestion Model No.1) to simulate digester behavior.
Predict how different OLRs will affect microbial activity and gas yield.
6- Expert Experience and Historical Data (may be integrate AI)
Past operational data can guide future OLR settings.
Experienced operators recognize instability patterns early.
III- Signs That OLR Is Too High
1- Rapid pH drop
2- Foaming and scum formation
3- VFA accumulation
4- Decrease in gas production
5- Digester souring (acidification)
IV- Summary
To know if your OLR is optimal:
1- Monitor biogas output, pH, VFA, and alkalinity regularly.
2- Look for consistency and stability in process parameters.
3- Use a stepwise approach to increase OLR, and stop at the point just before signs of stress appear.
The goal is to maximize efficiency without compromising microbial health or process stability.
🙂
- 4 Replies
Hi Hatem, thank you for sharing this information – very informative.
When are you suggesting to do this assessment?
Good Question Natalia.
Specifically, determining whether the Organic Loading Rate (OLR) is optimal best :
During the design stage of the biogas plant—closely linked with the Biochemical Methane Potential (BMP) test—as well as during start-up, after changes in feedstock, before scaling up, following operational interruptions, or when signs of instability appear (e.g., pH drop, VFA rise, reduced gas yield).
Regular monitoring of key indicators like biogas production, pH, alkalinity, VFA levels, and methane yield is essential.
Gradually increasing the OLR while observing for stress signals helps identify the optimal point where efficiency is maximized without compromising microbial stability.
Thanks for your question
At GazEoL, we support our clients in this process by applying AI and advanced numerical modelling. This allows us to simulate plant behavior, identify optimal operating conditions, and detect early signs of instability with greater accuracy and confidence.
Thanks again for your question!
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