The anaerobic filter (AF) technology offers an alternative method for the direct treatment of low-strength wastewater and the study was undertaken to access AF-biomass reactivation after prolonged nonfeeding periods, an important characteristic making the process suitable for handling variable or intermittent pollution loads. Four upflow AF (three 12.5-L and one 3.9-L, each with different packing), which had treated municipal-type wastewaters (natural, amended or synthetic) for 34 months at 25 or 16 degrees C and varying hydraulic loads and had remained inactive for 24 months, were used. All units were fed synthetic wastewater [mean chemical oxygen demand (COD) 323 mg/L, total suspended solids (TSS) 47 mg/L] and operated at 27 degrees C for 2.5 months (phase 1); and following a 6-month idle period, the smaller filter treated municipal wastewater (mean COD and TSS 820 and 448 mg/L) at 16 degrees C for an additional 2.5 months (phase 2). The larger units operated at a 2.0-d hydraulic retention time and the smaller at 1.0-0.33-d in phase 1 and 2.0 or 1.0-d in phase 2. Reactivation was quick and yielded efficient treatment. Restart was affected by the AF history and packing morphology, the types of wastewater previously handled, and the duration of the nonfeeding period.