<p>The invention relates to a reactor and to a method for anaerobically treating waste water or similar by means of an anaerobic biological reaction of organic compounds and/or contaminants using biomasses which are granulated and/or flocculated and/or fixed to fluidised support materials. Said reactor comprises a waster water line (18) and at least one inlet cyclone (4) in the lower area (1) into which the waste water can be introduced in a tangential manner and creates an upwardly directed eddy current within the cyclone (4).</p>
