A B S T R A C T Nitrous oxide (N 2 O) generation and emission from No. 3 wastewater treatment plant (WWTP) in Xi'an (Orbal oxidation ditch) and No. 4 WWTP in Xi'an (reversed A/A/O) were measured throughout a year following the water environment research foundation protocol. The N 2 O generation and emission rates of No. 4 WWTP (generation rate of 14.43 kg (N 2 O)/ d and emission rate of 13.78 kg (N 2 O)/d) were about twice that of No. 3 WWTP (generation rate of 7.09 kg (N 2 O)/d and emission rate of 6.52 kg (N 2 O)/d). The N 2 O emission factor of No. 4 WWTP (54.64 mg (N 2 O)/m 3 INF) was about 1.7 times that of No. 3 WWTP (36.20 mg (N 2 O)/m 3 INF).The ammonia oxidation rate, nitrite oxidation rate, and nitrous oxide generation rate were also measured. Results showed that the key factors of N 2 O generation and emission were the microbial population and aeration strategy.

A B S T R A C TNitrogen removal was studied in a pilot-scale anaerobic-anoxic-oxic (AAO) system that was bioaugmented with nitrifiers cultivated from reject water after seasonal deterioration. The process of nitrogen removal was evaluated with increased temperature from 11 to 24˚C. Nitrification efficiency rapidly recovered with increased temperature from 12 to 15˚C, and the nitrification rates at 18˚C were 3.1 times that at 12˚C, higher than the report in ASM1. Bioaugmentation may shorten the recovery time of nitrification activity in WWTPs. The nitrification activity and microbial ecology of the full-scale system operating parallel with the pilot-scale were correspondingly studied, and similar community structures were observed. Despite the lower nitrifying bacteria count, the nitrification activity of the bioaugmented pilot-scale plant was still higher than that of the full-scale one.