Abstract: Process for bio-processing of waste water with bio-rotors submerged in the water, with the substrate growth surface constituting the rotor. The rotor is dimensioned relative to the water volume so that the amount of water is 1 to 10 liter of water per m.sup.2 substrate surface, preferably 2 to 8 liters per m.sup.2 substrate surface. The material of the rotor has a specific surface of 150-400 m.sup.2 per m.sup.3 material outer volume (preferably 200-350 m.sup.2 per m.sup.3). When this latter ratio results in water volumes in excess of the submerged rotor volume, the volume of the basin is increased but with one or more shields being arranged in the basin to prevent the water from bypassing the rotor.