Mainstream partial nitritation/anammox: Balancing overall sustainability with energy savings

Implementation of one-stage nitritation/anammox (i.e. deammonification) for the treatment of sludge digestates allowed net energy positive operation in the wastewater treatment plant (WWTP) in Strass (Austria). To further optimize the overall energy efficiency a first trial to expand the deammonification process to the full-plant wastewater treatment system was performed. The effluent quality, greenhouse gas emissions, energy balance and overall CO2 footprint before and after implementation of mainstream DEMON® were evaluated in this study after one year of operation. In the effluent, a shift from nitrate over nitrite ratios of 31 to ratios of 2 was observed. The higher nitrite levels caused higher N2O emissions up to 2.3% of the N load in the B-stage, increasing the overall greenhouse gas emission of the plant significantly. For the present situation with mainstream DEMON, 11% of the electricity demand in the B-stage could be saved allowing higher net energy productions. These higher energy recoveries could not totally counteract the increased N2O emissions, resulting in a higher CO2 footprint of 36 kg CO2-eq PE−1 year−1 compared to 7 kg CO2-eq PE−1 year−1 before mainstream DEMON. Further optimization of the operational conditions in the mainstream will reveal if higher energy savings can be obtained at lower N2O emissions. First hints towards this can be given based on experimental trials carried out for this study.