gas yield (gas yield)

Gas yield is the volume of biogas (or biomethane after upgrading) produced per unit mass of feedstock fed into an anaerobic digester. It is the most direct indicator of feedstock quality and digester efficiency, and underpins the unit economics of every CBG plant: revenue is gas yield (Nm3 CBG per tonne feed) multiplied by SATAT sale price (INR per kg CBG), and feedstock cost is feedstock price (INR per tonne) — the margin between them sets gross profit per tonne processed.

Gas yield is expressed in several common formulations, each useful in different contexts:

• Nm3 biogas per tonne fresh matter (FM): most relevant for procurement contracts that quote feedstock by wet weight.
• Nm3 biogas per tonne total solids (TS): removes moisture variability.
• Nm3 biogas per tonne volatile solids (VS): the most technically rigorous unit; isolates the biodegradable fraction.
• Nm3 biogas per kg COD destroyed: used in wastewater treatment contexts.

Typical Indian achieved yields by feedstock (per tonne VS):

• Pressmud: 200-280 Nm3 biogas, 110-160 Nm3 methane (CBG yield 80-115 kg CBG per tonne dry pressmud).
• Cattle dung: 180-250 Nm3 biogas, 100-140 Nm3 methane.
• Paddy straw: 250-380 Nm3 biogas, 130-210 Nm3 methane (requires pre-treatment).
• Poultry litter: 280-400 Nm3 biogas, 170-260 Nm3 methane.
• Food waste: 380-550 Nm3 biogas, 210-330 Nm3 methane.

Achieved yield typically falls 10-30% below the theoretical biochemical methane potential (BMP) measured in lab batch tests. The gap reflects undigested fibrous material exiting at design HRT, partial mixing dead zones, occasional inhibition events, and biological variability between batches. Operators measure plant yield monthly using metered feed mass, lab-tested TS/VS, metered biogas volume corrected to standard conditions, and inline methane concentration.

The trade-offs in optimising gas yield are well known. Pre-treatment (mechanical, thermal, enzymatic) lifts yield 10-30% but adds 2-5% to operating cost. Longer HRT extracts more gas per tonne but requires larger digester volume and higher capex. Higher operating temperature (thermophilic 50-55 degC) lifts yield 15-25% over mesophilic but raises heating energy 30-50% and reduces process stability. Indian SATAT plants typically optimise for stable mesophilic operation at moderate HRT (25-35 days), accepting 85-92% of BMP as the practical operating point because squeezing the last 5-10% of yield potential rarely justifies the capex or operability penalty.