Includes bibliographical references. ; 2016 Fall ; Domestic wastewater contains chemical energy that is wasted by aerobic biological treatment processes, which require energy and maximize solids production. Mainstream anaerobic treatment of domestic wastewater has the potential be to the sustainable treatment scheme of the future because of the energy generation potential, lower energy requirements and reduced solids generation. A concern with direct anaerobic treatment of raw municipal wastewater in temperate climates is the effect of the low water temperatures on the organic removal rates and process stability. Anaerobic baffled reactors (ABRs) have a simple design, decouple hydraulic and solid retention time and provide excellent retention of solids in response to load variations. The ABR configuration allows for a naturally occurring spatial separation of the microorganisms that perform the sequential steps of hydrolysis, acidogenesis, and methanogenesis, in the conversion of complex organics to methane. Long-term performance of psychrophilic ABR systems for raw wastewater is lacking and it is critical to develop the data needed for technology adoption by wastewater utilities. A four-cell ABR was operated for two years treating raw domestic wastewater at ambient water and air temperatures down to 12 ˚C and -10 ˚C, respectively. The 1000-liter pilot reactor operated at a 12-hour hydraulic residence time and was located in the Headworks building of the Plum Creek Water Reclamation Authority, Castle Rock, Colorado. The two-year pilot study was conducted to determine the treatment efficiency of suspended solids and organic matter removal and methane production. To improve understanding of microbial dynamics for model development and potential diagnostic tools, the relative abundances of bacteria, archaea and methanogens were measured in each of the ABR cells and the relationship to conventional performance parameters was examined. The total suspended solid and organic removals were approximately 80% and 50%, respectively; methane production was stoichiometric and no settled solids were wasted over two-years. The full 12 hours of hydraulic residence time was required to achieve stoichiometric methane production from the organic matter removed. The estimated energy content of the biogas produced per unit volume of wastewater treated averaged 0.45 kWh/m3 and no energy input was required. Acetoclastic species were the dominant methanogens in the latter two cells, which also had the highest methane production and acetate utilization. The anaerobic baffled reactor was able to biologically achieve enhanced primary treatment of raw municipal wastewater under winter conditions with methane production. Anaerobic primary treatment may be incorporated at the front end of an existing facility to reduce organic loading to downstream processes, reduce aeration demand, reduce biomass production, and increase energy generation. The implementation of anaerobic primary treatment in conjunction with anaerobic secondary treatment will significantly reduce solids production and maximize methane production from the wastewater organics and has the potential to be energy positive.