This is a method of waste treatment in tall reactors having with anaerobic biological stage in the lower section of the reactor and another treatment stage in the upper section of the reactor, these stages are separated by an essentially horizontal partition. Presurized digestion gases generated in the anaerobic section are collected under the partition. The process is further improved by providing a recuperable alkalinity and stripping of biologically generated carbon dioxide. so that the free carbon dioxide concentratioon in the anaerobic mixed liquor is very low and a methane-rich digestion gas is produced. Production of pressurized methane-rich digestion gas is the major benefit of the process. Ather improvements are in the controllable gaslift mixing with gases other than digestion gases, improved process performance with increasing operating temperatures due to bioheating, and improved solid-liquid separation due to the use of the distillation techniques making use of the bioheating and heat recycle.