Anaerobic ammonium oxidation (anammox),a lithoautotrophic biological conversion process to remove nitrogen by oxidizing ammonium to N2 using nitrite as the electron acceptor and use CO2 as carbon source under the anoxic condition.However,the presence of organic carbon source would negatively affect anammox bacteria.This could be attributed to the competition for nitrite between anammox bacteria and heterotrophic denitrifiers.The anaerobic baffled reactor (ABR) was initially developed with the partial separation of different bacteria and form a distinct biological phase in each compartment to reduce the ineffective of COD addition.Nitrogen removal with different C/N ratio effect was investigated using an ABR anammox reactor with five compartments.Results indicated that the ABR reactor could remove nitrogen excellently by anammox coupling denitrification,while the C/N (NO2--N) ratio exert an important effect on nitrogen removal.When the C/N ratio was 1.29,the ABR could achieve the highest total nitrogen (TN) removal efficiency of 99.9%.Most of TN was removed in the 1st and 2nd compartment,accounted for about 81.0%-97.6% of total TN removal.Based on the nitrogen and carbon balance,the nitrogen removal was result of interaction by anammox coupling denitrification process.And the nitrogen removal contribution of anammox was varied from 6.8% to 32.4%.High-throughput Miseq sequencing analyses revealed that the C/N ratio was the most important factor regulating the bacterial community structure,and the predominant phylum changed from Proteobacteria to Chloroflexi with the elevated C/N ratio.In addition,the Candidatus Brocadia was the major anammox bacteria and the percent was varied from 1.0-2.9% to 2.8-9.1%.