Abstract: A biological purification process, a secondary treatment of partially cleaned waste water for example, is carried out in a porous bed of loose granules which float in the liquid. The granules are continuously washed to remove coagulated impurities by means of a small quantity thereof being entrained from the top of the bed in a small volumn flow of the effluent of the process, mixed with the inflowing impure liquid and then returned to the bottom of the bed. Excess reaction gas may also be delivered to the bed via the inflowing liquid and unreacted gas collected for reuse.