Over the past decade, the concept of anaerobic processes for the treatment of low temperature domestic wastewater has been introduced. This paper uses a developed wastewater flowsheet model and experimental data from several pilot scale studies to establish the impact of integrating anaerobic process into the wastewater flowsheet. The results demonstrate that, by integrating an expanded granular sludge blanket reactor to treat settled wastewater upstream of the activated sludge process, an immediate reduction in imported electricity of 62.5% may be achieved for a treated flow of c. 10,000 m(3) d(-1). This proposed modification to the flowsheet offers potential synergies with novel unit processes including physico-chemical ammonia removal and dissolved methane recovery. Incorporating either of these unit operations can potentially further improve the flowsheet net energy balance to between +0.037 and +0.078 kWh m(-3) of produced water. The impact of these secondary unit operations is significant as it is this contribution to the net energy balance that facilitates the shift from energy negative to energy positive wastewater treatment.

BACKGROUND There is a need to examine the impact of anaerobic reactor type and wastewater strength on anaerobic–aerobic two‐stage biological systems for temperate wastewater treatment An expanded granular sludge blanket reactor and an anaerobic membrane bioreactor for crude wastewater treatment with downstream aerobic biological treatment were studied together with increasing the organic concentration by fortifying the crude wastewater with primary sludge. RESULTS A chemical oxygen demand and ammonia compliant effluent was produced from the anaerobic–aerobic two‐stage process. Due to the enhanced organics removal achieved by the membrane, a lower denitrification rate, kd, was recorded for the anaerobic membrane bioreactor effluent treatment. However, the residual organic carbon in the anaerobic effluents from both the expanded granular sludge blanket reactor and the anaerobic membrane bioreactor treating crude wastewater was not of sufficient quality to support denitrification. Complete nitrification was achieved during downstream treatment of the fortified effluent. In addition, fortification increased kd to values analogous to exogenous carbon substrates. CONCLUSIONS It is postulated that fortification currently presents the most sustainable strategy for anaerobic–aerobic two‐stage biological systems due to a combination of enhanced methane production and denitrification. © 2013 Society of Chemical Industry