A method and apparatus for the anaerobic treatment of wastewater containing organic matter. The apparatus includes a closed tank to which the wastewater is delivered through liquid inlet means below a reaction hood. The reaction hood defines a mixing zone in which influent water mixes with liquid and biological solids already in the zone. A portion of the liquid being mixed is withdrawn and passed through a gas separator which removes entrained gas and returns the liquid to the tank. The tank may include fixed film media for improving treatment efficiency. Other features of the invention are the use of a sludge thickener for concentrating sludge removed from the tank and the use of a second gas separator in a treated effluent conduit from the tank. That separator has an adjustable baffle or weir which can be raised to flush scum into surface launders in the tank.