An upflow anaerobic reactor includes an effluent outlet configured to direct effluent out of the reactor and a fluid-filled gas trap configured to prevent loss of biogas from the vessel. An auger assembly is operably coupled to the effluent outlet to prevent clogging of the effluent outlet by solid matter that tends to collect in the effluent outlet. The auger assembly includes at least one helical screw conveyor that rotates so as to remove solid material that collects in the effluent outlet. auger assembly can be operated on a continuous or semi-continuous basis so as to allow for continuous operation of the reactor. The auger assembly can be operated on a continuous or semi-continuous basis so as to allow for continuous operation of the reactor.