acid-forming bacteria (acidogenic bacteria)

Acid-Forming Bacteria are the microorganisms that drive Stage 2 of the four-stage anaerobic digestion sequence — known as acidogenesis — converting the simple soluble compounds produced by hydrolysis (sugars, amino acids, fatty acids) into volatile fatty acids (acetic, propionic, butyric, valeric), short-chain alcohols, hydrogen, and carbon dioxide. The principal genera include Clostridium, Bacteroides, Lactobacillus, Streptococcus, and Bifidobacterium, all classified as facultative or strict anaerobes that thrive across a wide pH range (4.5–7.5) and temperature window (20–50 °C).

Acid-forming bacteria are the fastest-growing population in the digester, with doubling times of 1–2 hours, compared with 5–16 days for downstream methanogens. This kinetic mismatch is the source of most digester instability. When feedstock loading is increased too quickly, acidogens can produce volatile fatty acids faster than methanogens can consume them, leading to VFA accumulation, falling pH, and digester souring.

Key operational characteristics:

• Substrate range — readily degrade carbohydrates, proteins, and lipids; cannot efficiently break down lignin or recalcitrant fibre
• Energy yield — acidogenesis liberates only 20–30% of the chemical energy in the substrate; the remaining 70–80% is locked in VFAs awaiting methanogenesis
• Hydrogen sensitivity — high partial pressure of hydrogen inhibits propionate and butyrate degradation, biasing the VFA mix toward harder-to-degrade species
• Robustness — tolerate temperature shocks, oxygen exposure, and ammonia better than methanogens, which is both a strength (system resilience) and a weakness (they keep working even when methanogens are dying)

For Indian CBG operators, the practical implication is that acidogens are never the limiting population — methanogens are. Operating decisions (loading rate, mixing, alkalinity addition, trace nutrient supplementation) are aimed at protecting methanogens from being overwhelmed by acidogen output. Lime, sodium bicarbonate, or urea additions buffer the pH and give methanogens time to catch up when feed quality shifts unexpectedly.