Critical review on end-of-pipe technologies for nitrous oxide removal as part of a novel comprehensive concept for greenhouse gas emission mitigation at wastewater treatment plants

The greenhouse gas nitrous oxide, a byproduct of the biological nitrogen removal in wastewater treatment plants (WWTPs), strongly impacts the carbon footprint of the wastewater sector due to its high global warming potential. Although plant design and operational process optimization can reduce N₂O generation and emission, complete avoidance is not possible according to the current knowledge. While physical-chemical end-of-pipe technologies are available for the removal of nitrous oxide in industrial off-gases, a feasible treatment for the exhaust air of WWTPs still needs to be established. This paper critically reviews the currently available treatment technologies, with particular focus on biological systems, as bioscrubbers. The review indicated that implementing N₂O removal technologies is more feasible in sidestream than in the mainstream wastewater treatment. This is primarily due to the smaller area required for exhaust air collection, lower air flowrates, and higher N₂O concentrations observed in the sidestream. Therefore, the proposed concept focuses on sidestream application and is based on the following two main pillars: (i) introducing sidestream deammonification of the sludge dewatering effluent to deplete the N₂O emission factor in the mainstream biological stage and (ii) treating the N₂O-rich exhaust air from the deammonification process using a denitrifying bioscrubber with wastewater as an organic carbon source.