Abstract: A process for the anaerobic treatment of organic substrates is disclosed. The substrate to be treated is mixed with biomass, then passed through a bed of filler bodies and sludge, and also, after separation of the gas formed, is introduced into a distribution chamber for the separation of any floating sludge particles from sludge particles that can deposit. The latter, if necessary, may be guided to a post-clarification device for the removal of sludge (biomass).The apparatus for carrying out this process consists of a reactor comprising a buffer zone (14) limited by an inner cylinder (7), which buffer zone is connected to the distribution chamber (15) over the distribution impellers (6).