Abstract: A biofilter for treatment of waste waters and gases, comprising a bed of coniferous tree barking residue containing microorganisms and where the waste water to be treated can be uniformly conducted upon the barking residue bed and from below the barking residue bed oxygen containing gas can be blown against the waste water flow. The barking residue bed has been partly placed in a depression provided liquid and gas tightly in the ground, and that on the bottom of the depression there is a pipe system for draining the waste water that has traversed the barking residue bed and for conducting oxygen containing gas into said bed.