digester souring (souring)

Digester Souring is the severe operational failure in a biogas plant in which volatile fatty acids accumulate in the digester faster than methanogens can consume them, dropping the pH below 6.5 and progressively below 6.0, at which point methane-producing archaea are catastrophically inhibited and gas production collapses. A soured digester is recognisable from outside: gas yield falls 50–95% within 3–7 days, the headspace shows rising CO₂ and falling CH₄ percentage, and the slurry develops a sharp, vinegar-like odour from accumulated acetic acid.

The cascading mechanism makes souring particularly dangerous:

• Initial trigger — overloading, temperature drop, toxic pulse, or trace-nutrient deficiency slows methanogens
• VFA accumulation — acid-forming bacteria, which are 10–100x faster-growing than methanogens, continue producing acids unchecked; total VFA rises from 500 mg/L to 5,000–10,000 mg/L within days
• Alkalinity collapse — bicarbonate buffering is consumed by acid neutralisation; alkalinity drops from 3,000–5,000 mg/L (as CaCO₃) to under 1,500 mg/L
• pH crash — once buffering is gone, pH drops from 7.0 to 6.0 in 24–48 hours
• Methanogen die-off — at pH below 6.0, methanogens stop reproducing and begin to die; recovery now requires reseeding

Recovery options in order of severity and cost:

• Mild souring (pH 6.5–6.8) — halt feed for 1 HRT, add lime or sodium bicarbonate at 1–3 kg/m³, resume feeding at 50% rate
• Moderate souring (pH 6.0–6.5) — halt feed for 2–3 HRTs, dewater and discharge top portion, add fresh dilution water and alkalinity, restart with reduced load
• Severe souring (pH below 6.0) — partial draining, inoculation with 10–20% volume of seed sludge from a healthy digester, restart from low loading over 4–6 weeks

For an Indian CBG plant, a severe souring event typically costs ₹15–40 lakh in lost gas revenue, seed sludge transport, alkalinity chemicals, and plant downtime, plus a 6–8 week recovery period. The standard prevention is weekly VFA/alkalinity testing (₹500 per sample at a third-party lab), automated pH and ORP monitoring on the digester, and a written feeding protocol that caps the daily loading-rate increase at 10% to avoid surprise overloads.