Biologically treating a contaminated aqueous waste stream

Biologically treating a contaminated aqueous waste stream (1) to obtain a treated aqueous stream (3), comprising passing the contaminated aqueous waste stream (1) through a biological treatment system (6) to obtain a liquid effluent containing suspended biosludge (9) which is passed to a separator (11), separating in the separator (11) from the liquid effluent a biosludge-containing stream (15) to obtain the treated aqueous stream (3), returning at least part of the biosludge-containing stream to the biological treatment system and collecting the remainder in a collecting vessel (17), supplying at least part of the biosludge-containing stream (20) from the collecting vessel (17) into a reactor vessel (25) and adding (31) to the stream a predetermined amount of an agent that hydrolyses biosludge, passing a hydrolysed biosludge-containing stream (34) from the reactor vessel (25) to the biological treatment system (6) treating the contaminated aqueous stream (1), and removing the remainder of the biosludge-containing stream (30) from the collecting vessel (17) as excess biosludge for deposition.