Abstract: The water to be treated is sent in a co-current with oxygenated gas into an upflow reactor with a fixed bed of particles, and the bacteria support medium used comprises particles with a bulk density of 35 to 65 kg/m.sup.3. The reactor or biological filter (or reactors or biological filters in series) comprises or comprise: a zone for the densification of sludges; at least one oxygenated gas injection device; a zone of filtering material, a retention ceiling and a washing water storage zone with removal of the treated effluent. Application to the treatment of wastewater and water to be made fit for drinking purposes.

Abstract: A process and apparatus for biological purification of waste water wherein waste water and oxygenated gas are introduced into a reactor using ascending cocurrents. The reactor is equipped with a lower fluidized bed and an upper fixed bed for filtration. The particles in the beds are composed of expanded materials having a density less than 1. The particles in the fixed bed are both smaller and lighter than those of the fluidized bed.