Why Most Biogas Plants Fail – A Technical Reality Check

 

Biogas plants are often assumed to be simple systems: prepare feedstock, pump slurry into the digester, mix, wait for gas, purify, compress, and sell.

In practice, this assumption is the root cause of failure.

A biogas plant is fundamentally a biological reactor, not just a mechanical one. Gas production depends on the stability and performance of microbial populations. When biological limits are ignored, process efficiency collapses—regardless of equipment quality or automation.

This will not happen initially. The actual affect will knock the door in a year or two.

Why Methane Percentage Drops and CO₂ Increases

Low methane content is not a gas purification problem; it is a process imbalance issue.

Common technical causes include:

• Feedstock imbalance
High proportions of Napier grass or press mud or rice straw relative to dung increase volatile fatty acid (VFA) formation beyond methanogenic conversion capacity.

Now, understand what is VFA (Volatile Fatty Acid): Inside a digester, the process happens in stages:

1. Hydrolysis – complex materials (fiber, carbohydrates, proteins) are broken into simpler compounds
2. Acidogenesis – these compounds are converted into VFAs
3. Acetogenesis – VFAs are further converted into acetic acid, hydrogen, and CO₂
4. Methanogenesis – methane-producing microbes convert these into CH₄

VFAs sit right in the middle of this chain.

Common VFAs in Biogas Digesters

Acetic acid (most important – directly converts to methane), Propionic acid, Butyric acid and Valeric acid

Among these, acetic acid is “good”, while high levels of propionic and butyric acids usually are troublemakers.

VFAs are normal and necessary, but when they accumulate faster than methanogens can consume them, problems start.

High VFA levels cause:

• pH drop
• Methanogen inhibition
• Lower methane percentage
• Higher CO₂ in biogas
• Process instability or digester souring

• Organic overloading
Excessive amount of organic matter (usually measured as Volatile Solids) fed per cubic meter of digester volume per day leads to Volatile Fatty Acid accumulation, pH depression, and inhibition of methanogens.

• High lignocellulosic content

Lignocellulosic content means Cellulose – long sugar chains (energy source) plus Hemicellulose – shorter, branched sugars and Lignin – a rigid, woody polymer that protects cellulose

Materials with high lignin and fibre digest slowly, resulting in incomplete methanogenesis.

• pH deviation
Methanogens operate optimally between pH 6.8–7.5. Values outside this range significantly reduce methane formation.

• Temperature instability
Methanogenic activity is highly temperature sensitive. Even short-term fluctuations disrupt gas quality.

• Inadequate mixing
When mixing is poor, some parts of the digester stop working properly. Gas is then produced only in a small active area, leading to low methane and more CO₂.

It’s like cooking food only on half the stove—output drops and quality suffer. In a digester, poor mixing leaves part of the volume unused, resulting in weak, CO₂-heavy gas.

Technical Consequences of Low Methane %

• Reduced calorific value of biogas
• Increased load on CO₂ removal and upgrading systems like VPSA
• Higher operational cost per Nm³ of gas
• Lower CBG/CNG recovery
• Acidic digestate with reduced agronomic value

Operations: Where Design Meets Reality

Even with consistent daily feed supply, methane yield depends on biological conversion efficiency, not tonnage fed. When microbial balance is lost, increased feeding only accelerates failure.

The Core Truth

A biogas digester behaves like a living biochemical system, not a storage tank with agitators. Mechanical mixing cannot compensate for biological stress.

Therefore, Sustainable biogas performance requires:
• Stable microbial ecology
• Controlled organic loading
• Balanced feedstock composition
• Tight control of pH and temperature
• Continuous process monitoring

Ignore these fundamentals, and even well-designed plants with premium equipment will underperform—or fail entirely.