DESCRIZIONE
A method and apparatus for biological and/or chemical decomposition of waste containing organic mattter. The method comprises waste pretreatment in which the waste is fragmented and homogenised in a pretreatment apparatus (10a, 10b) operating on the principle of a multi-ring double action impact mill. The apparatus consists of either two rotors (62, 64) or a rotor adn stator disposed inside a housing (26), which rotors adn/or stator are equipped with blades (1a,b,c,... 2a,b,c,... ) having impact surfaces (a', b', c', ...), and which form one or more, typically at least two rings (1, 2, 3, ...) coaxial with the rotor. The waste is fed into the housing through the hub of rings formed by the blades, from where the waste is arranged to travel by the effect of a rotor or rotors, to the outer ring of the ring (7) formed by the outermost blades, and further to the discharge opening (25, 27') in at the outer ring.