Abstract: An apparatus for the anaerobic digestion of natural organic waste comprises a feed tank, a digester and an outlet tank. There is further provided feed means which feeds waste to be digested from the feed tank to the digester at an adjustable continuous rate. A gas pump may be used to recirculate from the top of the digester through a diffuser to the bottom of the digester. A suitable gas pump comprises a container partially filled with liquid so as to provide a space above the liquid surface and within the container sealed by the liquid from the exterior of the container, pumping means being provided in the said space.