process balance and efficiency (process balance)

Process balance and efficiency, in anaerobic digestion, refers to the operational state in which the four biological stages — hydrolysis, acidogenesis, acetogenesis and methanogenesis — proceed at compatible rates so that intermediates produced by each stage are consumed by the next without dangerous accumulation. A balanced process realises a high fraction of the theoretical biogas potential of the feedstock; an unbalanced process accumulates VFA, drops pH and progressively kills methanogens.

Balance is monitored through three primary indicators. VFA-to-alkalinity ratio (also called FOS/TAC in German notation) — under 0.3 the process is stable; 0.3-0.4 is a warning zone; above 0.4 is stress requiring corrective action; above 0.6 indicates near-failure. pH trend — a stable digester holds pH between 6.8 and 7.6; a falling trend, even within range, signals VFA build-up before the ratio gives a hard warning. Specific gas yield — m³ biogas per kg of volatile solids fed should track to within ±10% of the feedstock's known potential; persistent under-yield without other instability is usually a sign of partial inhibition.

Efficiency is the second axis. Theoretical biogas potential of a feedstock — calculated from its biochemical composition (lipids 1,000-1,400 L/kg VS, proteins 600-700, carbohydrates 350-450) and Biochemical Methane Potential (BMP) test — sets the ceiling. Realised yield in a continuous plant typically reaches 70-85% of BMP at well-tuned operation, dropping to 50-60% in poorly managed plants. The gap is attributable to short-circuiting of feed (HRT too short for full conversion), inhibition at high OLR, low temperature in winter, or under-utilised digester volume from settled solids and dead zones.

The corrective levers when balance slips are straightforward in principle but require disciplined operation. Reduce OLR by 30-50% for 5-10 days to let VFA decline; add buffer (sodium bicarbonate or lime) to lift alkalinity; check temperature and mixing; verify feedstock composition has not shifted. The trade-off is the lost gas during recovery. A well-instrumented plant with online VFA and pH sensors catches drift in 24-48 hours and recovers within a week; a poorly-instrumented plant catches it only after lab analysis of weekly grab samples and may take 3-6 weeks to recover, during which 20-40% of expected gas output is lost.