The invention relates to a method for purifying effluents in an anaerobic reactor (1) in which the micro-organisms are held by supports, the supports forming a bed which is fixed in a part (4, 5) of the reactor. The inventive method is characterized in that it includes a step wherein the reactor is backwashed once it has been at least partially clogged, by temporarily suspending the supports. Advantageously, the method includes an initial step for starting the reactor, during which the load of the reactor is increased with a short and constant hydraulic residence time.