A waste grinding apparatus (101) including a rotor having a central body (13) defining at least one radial vane (15a, 15b), provided, at its end (17a, 17b) distal relative to a rotation axis (“S”) of the rotor (11), with a hammerhead (19) in which a first, knocker impact surface (21) adapted to operate when the rotor (11) rotates in a first, clockwise or counter-clockwise direction, and a second, wedge-shaped impact surface (23) adapted to operate when the rotor (11) rotates in a second direction opposite to the first direction, are defined.