Abstract: The specification discloses a method and installation for purifying waste. A tank (1) is fed with the waste and with a biological filter substrate, both of which are fermented within the tank. Pipes and ducts (17, 28) supply air to an upper aerobic fermentation region and to a lower aerobic fermentation region, and ducts (22) remove gaseous and liquid products from a middle anaerobic fermentation region.The fermentation products are removed by a sweeping worm (29) and sucked by a fan (43) through a separator (33) in which heavy particles fall out (through 40) but lighter ones are retained by the suction. The product is triturated (44) and may be dried (45, 46) and made into briquettes (51) for use as fuel.