A plug flow digester is a type of anaerobic digester that uses a long, narrow horizontal tank in which a material (manure) is added at a constant rate and forces other material to move through the tank and be digested. Typically a plug flow digester vessel is five times longer than it is wide, is insulated and heated, and is made or reinforced concrete, steel, or fiberglass. A gas tight cover traps the biogas which is given off. This biogas is used to heat the digester to the required temperature and run a generator.

A plug flow digester has no means of agitation. The term "plug flow" derives from the fact that the manure in principle flows through the digester vessel as a "plug," gradually being pushed toward the outlet as new material is added. In fact, the situation is more complicated and this and some parts of the manure travel faster than others on their way through the vessel, or may even settle or float and remain in the digester.