The biofilter is used for the purification of a waste liquid using layers of filtering material, wherein the waste liquid moves downwardly by gravity while an O.sub.2-containing gas moves upwardly therein. The biofilter comprises a gas collector to capture at least a portion of the gas moving upwardly therein to mitigate the problem of biofilter clogging due to a microbial seal at the surface of the uppermost filtering layer.