The mechanically separated liquid fraction of organic waste from households was used as a substrate for anaerobic fermentation. A two-step system consisting of a 200 1 continuously stirred tank reactor (CSTR) and a 50 1 upflow anaerobic filter filled with glass foam pearls was constructed. The CSTR was operated for 5 months with a loading rate of 9.8 kg CSB m(-3) day(-1). At a resulting hydraulic retention time (HRT) of 24 days, 68% COD was degraded and a gas productivity of 4.0 m(3) m(-3) day(-1) was achieved. Further digestion of the CSTR output was separately optimised in a 20 1-UFAF and based on these results a 50 1-UFAF was connected to the CSTR. At a resulting hydraulic retention time (HRT) of 6 days 38% COD was degraded and a gas productivity of 1.8 m(3) m(-3) day(-1) was achieved with the 50 1-UFAF. Thus, the overall degradation efficiency of the two-phase system was 80%. The methane content (61%) of the biogas produced in the 50 1-UFAF was higher than that for the CSTR (47%). The effluent from the second stage of the two-phase system was odourless and could thus be re-used in the composting process.