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. Methane from the digester is usually the fuel for heating.