The formation and emission of N2O in various nitrogen removal processes for treatment of domestic sewage were investigated using oxic-anoxic SBR systems and a continuum aerobic submerged biofilm reactor system SBBR

from which complete

shortcut and simultaneous nitrification and denitrification(SND) processes were realized

respectively.Meanwhile

the effect of DO on the SND efficiency and N2O yield in SBBR was also studied.The results showed that for both complete and shortcut N removal processes the formation of N2O was in nitrification process

while denitrification one was favorable for decrease of N2O emission.N2O yield was 4.67 mg·L-1，6.48 mg·L-1 and 0.35 mg·L-1 for complete

shortcut and simultaneous nitrification-denitrification processes

respectively.The main reason for the formation of N2O was the accumulation of NO2--N during N removal process.A part of AOB could be denitrified between NH4+-N as electron donor and NO2--N as electron acceptor under low DO level

which led to the formation of N2O finally.The formation of N2O in simultaneous nitrification and denitrification process at various DO conditions showed that in SBBR system

the minimun N2O yield could be achieved by the control of DO concentration and the efficiency balance of nitrogen removal in both nitrification and denitrification processes.