Disclosed is a reactor for anaerobically treating waste water, especially treating waste water from the paper industry. Said reactor comprises a reactor vessel, at least one supply pipe which is arranged in the bottom area of the reactor vessel and is used for feeding waste water that is to be treated into the reactor, at least one supply distributor for mixing the waste water fed to the reactor with the medium located in the reactor, at least one overflow port which is disposed on the top reactor vessel and is used for discharging treated water, and at least one separator. The supply distributor is provided with at least one deflection means that is embodied such that waste water discharged from the at least one supply pipe is diverted into a circular current from the perspective of the cross-section of the reactor, one to 80 percent of the cross-section of the reactor being filled by the at least one deflection means. The invention further relates to a method for anaerobically treating waste water.