Acidogenesis (acid formation stage)

Acidogenesis is the second of four biochemical stages of anaerobic digestion, in which fermentative bacteria metabolise the soluble monomers produced by hydrolysis (sugars, amino acids, glycerol, long-chain fatty acids) into a mixture of short-chain volatile fatty acids (VFAs) — primarily acetic, propionic, butyric, and valeric acid — along with alcohols (ethanol, lactate), hydrogen, and carbon dioxide. Acidogenesis is the fastest stage of the AD chain, completing in hours rather than days, which is why VFAs can accumulate rapidly when downstream methanogenesis cannot keep up.

The acidogenic bacterial community is diverse and resilient. Key genera include Clostridium, Bacteroides, Lactobacillus, Streptococcus, and Bifidobacterium. Unlike the strict anaerobes that dominate methanogenesis, many acidogens are facultative — they tolerate brief oxygen exposure — which makes the early stage of AD robust against minor air ingress during feeding.

The metabolic output of acidogenesis depends strongly on hydrogen partial pressure inside the reactor:

• Low H2 partial pressure (well-functioning digester): acidogens preferentially produce acetate, hydrogen, and CO2 — substrates directly usable by methanogens.
• High H2 partial pressure (stressed digester): acidogens shift to propionate, butyrate, and ethanol — VFAs that must first be converted by acetogens before methanogenesis can use them, adding a kinetic bottleneck.

Acidogenesis is also the stage that produces most of the CO2 in biogas. When pH drops below 6.5, methanogens are suppressed but acidogens continue to ferment, producing more acid and accelerating digester souring — a runaway feedback loop that is the classic failure mode of overloaded digesters.

Operators monitor the FOS/TAC ratio (volatile organic acids to total alkalinity) as an indicator of acidogenic versus methanogenic balance. A FOS/TAC of 0.2-0.3 is healthy; above 0.4 signals VFA accumulation and impending process upset. The control levers are reducing OLR, dosing alkalinity (sodium bicarbonate), or recirculating digestate from a healthy parallel digester. The trade-off is throughput versus stability: pushing OLR to extract higher productivity sharpens acidogenic dominance and erodes process margin, so most Indian CBG plants operate at 70-85% of theoretical maximum OLR to retain a stability buffer.