Anaerobic filter technology for municipal wastewater treatment has been studied in Knoxville, Tennessee with a 190-m exp 3 /d facility from August 1981 to October 1983. The ORNL project (described by the acronym ANFLOW for the anaerobic, upflow characteristics of the technology) received funding from both DOE and EPA. During treatment of low-strength municipal wastewater before primary sedimentation, the effluent met the EPA secondary treatment discharge limits of 30 mg/L for TSS and BOD the vast majority of the time, with only an occasional increase to the 30 to 40 mg/L range for periods of 1 to 2 d. Loading rates used in this study were approx.0.25 kg/m exp 3 .d for both TSS and BOD; the hydraulic retention time ranged from 9 to 10 h. TSS and BOD removal efficiencies were 80% and 70% respectively. This performance was maintained in ambient cold weather tests (approx.12 exp 0 C water temperature). Approximately 20% of the influent carbon was converted to methane. Gas production increased at higher loading rates with higher strength wastewaters. The average rate of accumulation of dry solids in the bioreactor was approx.150 kg/3800 m exp 3 (3800 m exp 3 = 1 million gal) of wastewater treated, which represents a 75 to 80% reduction in solids production compared to primary sedimentation and activated sludge or trickling filtration. The rate was slightly higher in cold weather and somewhat lower in warm weather. A simple material balance model was shown to predict the sludge accumulation using measurable influent and effluent parameters. 26 refs., 31 figs., 10 tabs. (ERA citation 10:024321)