Abstract: Apparatus for composting biological waste material, comprising a screen for separating the waste material into solids and liquids, a sludge outlet for adding sludge to the solids to form a mixture therewith and a compacting section for compacting the mixture. Further sludge is mixed with the compacted mixture to form a further mixture therewith, which is composted in a composter. The liquid separated from the biological waste is screened to remove fine solids and the fine solids are added to the first-mentioned mixture prior to the compacting thereof, the liquid being added to the further mixture in the composting section.