VFA (VFA)

Volatile Fatty Acids (VFAs) are short-chain organic acids — primarily acetic, propionic, butyric and valeric — produced as intermediates during the second and third stages of anaerobic digestion (acidogenesis and acetogenesis). They are the substrates that methanogens consume to make methane: every kilogram of methane produced has passed through a VFA stage in the digester. In a well-balanced system VFA concentrations are low because production by acidogens matches consumption by methanogens; in a stressed system VFAs accumulate, and the accumulation pattern reveals which stage of the process is failing.

The healthy operating envelope for total VFA in a CBG digester is below 1,500-2,000 mg/L as acetic acid equivalent. Concentrations of 2,000-4,000 mg/L indicate stress; above 4,000 mg/L the digester is in active souring and methane production is collapsing. Equally important is the composition: acetic acid alone tends to dominate in healthy operation and during mild overload; elevated propionic acid above 1,000 mg/L is the canonical sign of severe inhibition, because syntrophic propionate oxidation is thermodynamically sensitive to hydrogen partial pressure and stops first when methanogenesis slows.

VFA accumulation results from four mechanisms. Overloading — OLR pushed beyond microbial capacity, generating VFA faster than methanogens consume. Temperature drop — winter cold-shock slowing methanogenesis. Toxicity — ammonia above 3,000 mg N/L, sulphide above 200 mg/L, or LCFA from high-fat feedstock inhibiting methanogens directly. Trace nutrient deficiency — cobalt, nickel, selenium shortages reducing methanogen activity, more common on monoculture feedstocks like distillery effluent.

VFA is the most sensitive process-stability indicator available. Measured weekly through a simple titration or distillation, or daily through online sensors (FOS/TAC ratio with alkalinity), VFA trend gives 5-10 days of warning before pH starts to fall and 15-30 days before gas output drops visibly. Operationally, when VFA crosses 2,500 mg/L the response is to cut OLR by 30-50% for 7-14 days, add sodium bicarbonate to lift alkalinity by 500-1,000 mg CaCO₃/L, verify temperature and mixing, and wait for the ratio to drop. Plants that ignore VFA monitoring until pH falls have already lost 4-6 weeks of normal gas output and face a longer recovery period.