A method and apparatus for biological and/or chemical decomposition of waste containing organic matter. The method includes waste pretreatment in which 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 has either two rotors (62, 64) or a rotor and stator disposed inside a housing (26), which rotors and/or stator are equipped with blades having impact surfaces, and which form one or more rings (1,2,3, . . . ) coaxial with the rotor. The waste is fed into the housing through the hub of rings formed by the blades, and travels 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') at the outer ring.