It is proposed that anaerobic treatment of municipal wastewater in temperate climates is attainable through the fortification of wastewater with primary sewage sludge to enhance the chemical oxygen demand (COD). Three bench-scale up-flow anaerobic sludge blanket (UASB) bioreactors (0.5 L) were operated to provide a preliminary verification of this approach. Mass and energy balances were modelled using the GPS-X (COST model, population equivalent of 118,500) to determine the energy balance of the primary sludge wastewater fortification process flowsheet in comparison with conventional aerobic secondary treatment with activated sludge. The addition of 5% primary sludge by volume to crude wastewater pretreated with ultrasound resulted in an increase in total COD from 536 mg L(-1) to 2300 mg L(-1). A COD removal of 86% +/- 8% and methane production of 400 mL L(-1) d(-1) were achieved in the reactor that was supplied with this primary sludge fortified wastewater. In comparison a COD reduction of 77% +/- 8% and 40 mL L(-1) d(-1) of methane were observed in the reactor which was supplied with crude wastewater only, and a 79% +/- 8% COD removal and 156 mL L(-1) d(-1) of methane were observed in UASB2, which was supplied with crude wastewater pretreated with ultrasound. The modelled energy balance for the fortified wastewater treatment process flowsheet was -305 kWh d(-1) compared with -937 kWh d(-1) in the aerobic flowsheet, corresponding to an energy saving of 67%.