pH and volatile fatty acids

Changes in volatile fatty acids (VFA) levels were shown to be a good parameter describing anaerobic digester process stability. Volatile fatty acids (VFA) concentration will change according to the changes in hydraulic loading, organic loading or temperature.


Bacteria involved have different requirements regarding the biochemical environment. Acid forming bacteria are much more tolerant to low pH concentrations than methanogens. Acidogenic bacteria metabolise still at high levels at pH 4.5 whereas methanogens face decreasing rates at pH 6.0.

Fatty acid concentration and subsequent pH levels in anaerobic digesters play a vital role operating a biogas system. Production of VFAs leads to decrease in pH. Normally the drop in pH is counterbalanced and buffered by formation of alkalinity through CO2 production.