Mitigating nitrous oxide emission by a down-flow hanging sponge bioprocess enabling the removal of high concentration N2O

• Novel N 2 O removal process using down-flow hanging sponge was developed. • Rapid and efficient removal was achieved for high concentrations of N 2 O. • N 2 O dissolution rate from gas to liquid was the rate-limiting factor. • Various N 2 O reducers belonging to both clade I and II were detected. • Azonexus thought to play a key role in N 2 O removal. Nitrous oxide (N 2 O) is known as a greenhouse gas as well as an ozone-depleting substance. Wastewater treatment process is one of the sources of N 2 O emission, and the N 2 O in off-gas were reported from anoxic tanks, for example, two-stage partial nitrification-anammox processes. To mitigate the environmental impact associated with such greenhouse gas emissions, a process to remove N 2 O generated is urgently needed. In this study, a novel N 2 O removal process using a down-flow hanging sponge (DHS) reactor for low and high concentrations of N 2 O was developed. Different concentrations of N 2 O (5, 40, 300, and 2,000 ppm) in nitrogen gas were subjected to the DHS reactors fed with the supernatant of anaerobic sludge digestate. More than 96 % and 99 % removal efficiencies were achieved for up to 300 ppm and 2,000 ppm N 2 O with 3 and 18 min gas retention time, respectively. A maximum removal rate of 166 ± 24 mg-N/L-reactor/day was achieved, which was significantly faster than that of the earlier studies. Kinetic analysis indicated that the N 2 O dissolution rate is a crucial factor in determining the N 2 O removal rate in the reactor. Various N 2 O reducers were detected in the reactors, and clade II nosZ was more predominant and diverse than those of clade I nosZ . The most abundant nosZ genes retrieved from all reactors were similar to those in Azonexus , harboring clade II nosZ . Thus, Azonexus was thought to play a key role here.