Abstract: Apparatus for anaerobic digestion of organic waste material comprises a container divided into three interconnected chambers situated one below the other, the upper chamber being an inlet chamber, the lower chamber being a mixing chamber and the intermediate chamber being a settlement chamber, the inlet chamber having an inlet for material to be treated, a duct from the inlet chamber down into the mixing chamber, at least one transfer duct from the mixing chamber up into the settlement chamber, the duct or ducts extending below the upper extent of that mixing chamber, a gas collection zone at the upper extent of the mixing chamber, at least one recirculation duct from the base of the settlement chamber down into the mixing chamber, a further gas collection zone in the upper extent of the settlement chamber and an outlet from the settlement chamber situated below the further gas collection zone.