Name

Denitrifying bioscrubbers to reduce nitrous oxide emissions in the exhaust air of wastewater treatment plants

Speakers

Ali Valipoor

Authors

Ali Valipoor, Ernis Saracevic, Norbert Kreuzinger, Jörg Krampe and Vanessa Parravicini, Institute for Water Quality and Resource Management, TU Wien, Austria

Description

Nitrous oxide, a potent greenhouse gas and ozonedepleting substance, can be emitted from wastewater treatment plants (WWTPs) during biological nitrogen removal processes. Especially the sidestream treatment of the process water of anaerobic sludge dewatering (e.g. performing deammonification) exhibits high N₂O emission factors. Although nitrous oxide generation and emission can be reduced through operational process optimization, complete avoidance is not possible according to the current knowledge. Whereas physical-chemical end-of-pipe technologies (e.g. catalytic and thermal processes) are available for the removal of nitrous oxide rich industrial off-gas streams, a feasible treatment for exhaust air of WWTPs is still lacking. In this context, biological based treatment technologies offer several advantages over physical-chemical methods, including operation at ambient pressure and temperature, lower energy demand, lower capital and operational expenses, and resilience to high oxygen and water-vapor levels that often impair physical-chemical systems. Among these technologies, denitrifying bioscrubbers, whether in one stage configuration, can be a feasible option to first transport nitrous oxide from the exhaust air stream into a liquid phase, where it is then denitrified to nitrogen gas (N₂) by using activated sludge and sewage as organic carbon source. Implementing denitrifying bioscrubbers in sidestream deammonification is a feasible option, because here exhaust air flowrates are much lower and stable, can be collected over a smaller tank area, and exhibit higher N₂O concentrations compared to the mainstream activated sludge tank. Within a research project at TU Wien investigations at lab-scale were performed focusing on one-stage bioscrubber designed as a bubble column and on two-stage bioscrubber designed as an absorption column integrated with a denitrifying bioreactor. Based on these investigations both technologies achieved under optimized conditions an average N₂O removal efficiencies in the exhaust air of 85%. For an Austrian model WWTP of 100,000 P.E. with sidestream deammonification it was estimated, that deploying denitrifying bioscrubber systems can reduce carbon footprint of the plant by roughly 1 kg CO₂-e/P.E./year, considering N₂O reduction, and implementation and operation related CO₂-e emissions. In this model, the activated sludge tank was used as bioreactor and a very small portion of incoming COD as a carbon source. At plants without a sidestream treatment unit, combining sidestream deammonification with denitrifying bioscrubbers for N₂O emission control provides an attractive solution to improve nitrogen removal and reduce N₂O emissions in one step. The N₂O reduction potential of the combined approach is much greater, considering that the lower nitrogen load to be oxidized in the mainstream process creates advantageous operational conditions to deplete the N₂O emission factor in this stage.

 

Time

-

Location Name

Ludlow Concourse

Track

Posters