acid formation (acid build-up)

Acid Formation is the production of volatile fatty acids (VFAs) — primarily acetic, propionic, butyric, and valeric acids — during the acidogenesis and acetogenesis stages of anaerobic digestion. Under stable operation, the VFA pool is in dynamic equilibrium: acid-forming bacteria generate VFAs at the same rate methanogens consume them, keeping the digester pH between 6.8 and 7.5. The total VFA concentration sits at 100–500 mg/L as acetic acid, and the system runs steadily for years.

When acid formation outpaces acid consumption, the digester drifts into instability. The progression is predictable:

• Stage 1 — early warning — VFA concentration rises to 1,000–2,000 mg/L; alkalinity-to-VFA ratio falls below 1.4; pH still neutral due to buffering
• Stage 2 — buffering depletion — VFAs reach 3,000–5,000 mg/L; alkalinity consumed; pH drops to 6.5–6.8; gas production declines 10–30%
• Stage 3 — methanogen inhibition — pH below 6.5 directly inhibits methanogens; VFAs exceed 5,000 mg/L; gas yield collapses
• Stage 4 — souring — pH falls below 6.0; methanogens die off; digester effectively becomes a fermentation tank producing only acids and CO₂, not methane

Triggers for runaway acid formation in Indian CBG plants:

• Sudden organic overload — feeding too much, too fast, especially highly biodegradable food waste or molasses
• Temperature shock — heating system failure dropping mesophilic digester from 37 °C to 30 °C kills methanogens faster than acidogens
• Toxin pulse — antibiotics in dairy waste, disinfectants in food waste, or heavy metals can selectively kill methanogens
• Trace nutrient deficiency — nickel, cobalt, iron deficiency in maize-silage-only diets slows methanogenesis

Recovery from acid formation events requires reducing or halting feed for 1–3 HRTs, adding alkalinity (lime, sodium bicarbonate at 2–5 kg/m³), and waiting for methanogens to consume the accumulated VFAs. Severe cases require seed inoculum from a healthy digester. Routine monitoring of VFA and alkalinity (weekly) is the cheapest insurance against acid-formation events that can shut a CBG plant for 3–6 weeks.