An anaerobic biofilter was operated with 4500 mg COD/L food-processing waste water, fed at 10 d of hydraulic retention time (HRT). In 45 d after seeding, established biofilms were dominated by dense layers of Methanococcus-like and Mefhanothrix-like cells. The average COD and grease removal efficiencies were 85 and 53%, respectively, when the HRT was decreased stepwise to 1.5 d. Immobilized biofilms enabled the biofilter to be started up easily, to have stable treatment performance, and to recover rapidly from shock loadings and process failure.