Abstract: An animal-supporting platform receives solid waste excreted by the animals. A mixture of the waste and a carrier liquid are treated in airtight holding tanks where ammonia gas is produced at ambient temperatures. The material is then fed to an airtight chamber provided with liquid overflow means and a liquid outlet passage which leads downwardly from the overflow means and then upwardly to a discharge point located outside the chamber. Combustible methane gas generated by decomposition of the waste material is discharged through a gas outlet opening located in an upper portion of the chamber, and undigested solids are moved through the chamber and then removed from the chamber by a conveyor means leading through a submerged chamber outlet opening.