While the organisms that make the organic acids are fast and rugged, the organisms that feed on the acids and make methane are slow, delicate, and stop functioning when the pH strays below neutrality. When the acid formation is balanced by methane formation to keep acid concentration low and the pH neutral, digestion performs well. If the feed rate to a digester is too great, acid accumulation drives the pH down so than methane formation halts. This accelerates the rate of acid accumulation and gives a sour or stuck digester. A failed digestion tends not to recover by itself.
Possible remedies are neutralization with lime and reinoculation with a vigorous culture.
If the waste treatment plant has several digesters, fluid from one that is operating well can be added to one that has failed. Plants that have no digester operating properly can send a tank truck to a neighboring plant for inoculum. Anaerobic digestion tends to be slow. If the only mixing comes from adding and withdrawing sludge and from the bubbling of the gases that are produced, residence time is 30 to 60 days. Continuous mixing can reduce the residence time to 10 to 30 days.