The "temperature-phased anaerobic biofilter", or TPAB process (U.S. Patent pending) is a new high-rate anaerobic treatment system that includes a thermophilic (56°C) biofilter connected in series to a mesophilic (35°C) biofilter providing for two-temperature, two-phase treatment;Three TPAB systems of different thermophilic: mesophilic reactor size ratios were operated at system HRTs of 24 hrs, 36 hrs, and 48 hrs to characterize performance and to determine if an optimum size ratio exists between the thermophilic and mesophilic phases. The three TPAB systems achieved SCOD reductions in excess of 97% and TCOD reductions in excess of 90% for a synthetic milk substrate over a range of system COD loadings from 2 g/L/day to 16 g/L/day. There was little difference in performance between the three TPAB systems based on COD reductions and methane production; the 1:7 volume ratio thermophilic:mesophilic TPAB system performed as well as the 1:3 and 1:1 volume ratio TPAB systems. In applications of the process, a relatively small thermophilic first-stage can be used without sacrificing overall two-stage system performance;An energy balance was performed for the three TPAB systems in which heat exchangers would be utilized to remove heat from the thermophilic and mesophilic effluents, and use this heat for the incoming raw wastewater. The energy balance took into consideration a matrix of potential incoming raw wastewater temperatures of 50 to 110°F. It was determined that there would be a positive energy balance in terms of excess potential methane production at influent wastewater temperatures at or exceeding 50°F.