Anaerobic Digester Souring: VFA/Alkalinity Balance

Biogas digesters run on a division of labor between fast-growing acid-forming bacteria and slow-growing methanogens, and the two can fall out of balance. This simulator models a real digester stability failure mode — souring — using Monod uptake kinetics, a VFA-to-alkalinity ratio (the operational stability metric plants actually monitor), and pH inhibition of methanogen growth. Push the organic loading rate too high, or fire a shock-load slug of feedstock, and volatile fatty acids can outpace what the methanogen population can consume; alkalinity buffering erodes, pH falls, methanogens are inhibited, and methane production collapses in a self-reinforcing loop. Dose alkalinity and manage loading rate and temperature to keep the reactor in its stable operating window.