Hydrolysis (hydrolysis stage)

Hydrolysis is the first of four biochemical stages in anaerobic digestion, in which complex organic polymers in fresh feedstock — cellulose, hemicellulose, proteins, lipids, starch — are broken down into simpler soluble monomers (sugars, amino acids, long-chain fatty acids, glycerol) by extracellular enzymes secreted by hydrolytic bacteria. Because polymers cannot cross bacterial cell walls intact, hydrolysis is the gateway step that makes substrate accessible to the downstream stages of acidogenesis, acetogenesis, and methanogenesis.

The enzymes involved are substrate-specific:

• Cellulases: degrade cellulose into glucose; rate-limited by lignin shielding in lignocellulosic feeds.
• Hemicellulases: hydrolyse hemicellulose into xylose, arabinose, mannose.
• Proteases: cleave proteins into amino acids and peptides.
• Lipases: split lipids into glycerol and long-chain fatty acids.
• Amylases: convert starch to glucose and maltose.

For most easily-digestible feedstocks — food waste, molasses, dairy manure — hydrolysis is fast and not rate-limiting; the whole AD process completes in 20-25 days. For lignocellulosic feedstocks such as paddy straw, sugarcane bagasse, Napier grass, and wood chips, hydrolysis becomes the bottleneck because lignin physically and chemically shields cellulose from enzyme access. Yield drops by 30-60% versus theoretical, and HRT extends to 40-60 days.

Operators accelerate hydrolysis of stubborn feedstocks through pre-treatment, each method with a distinct trade-off:

• Mechanical (milling, shredding): increases surface area; low cost (0.5-2 kWh per tonne) but limited benefit (10-25% yield uplift).
• Thermal hydrolysis (140-170 degC, 6-9 bar steam): dramatic uplift (30-50%) but high energy cost (200-400 MJ per tonne).
• Chemical (alkali/NaOH or acid): effective but generates salt-laden process water and corrosion risk.
• Enzymatic / biological pre-digestion: targeted and effective; enzyme cost 50-200 INR per tonne feedstock.

Indian CBG plants increasingly bundle a separate hydrolysis tank (HRT 1-3 days, slightly acidic) upstream of the main digester for fibrous feedstocks. This two-phase configuration lifts overall yield 15-25% at a 10-15% capex premium and is now standard design for paddy-straw-based plants.