Hydrodynamics　played　an　important　role　in　the　design　and　operation　of　bioreactors　for　wastewater　treatment.　In　this　work,　an　up-flow　anaerobic　hybrid　bioreactor　built-in　with　fixed　bio-carriers　was　designed　and　optimized　using　computational　fluid　dynamics　(CFD)　simulation.　The　results　indicated　that　the　flow　regime　involving　with　vortex　and　dead　zone　was　greatly　affected　by　the　positions　of　water　inlet　and　bio-carrier　modules.　The　ideal　hydraulic　features　were　obtained　when　the　water　inlet　and　bio-carrier　modules　located　9　cm　and　60　cm　above　the　bottom　of　reactor.　Using　the　optimum　hybrid　system　for　nitrogen　removal　from　wastewater　with　low　carbon-to-nitrogen　ratio　(C/N　=　3),　the　denitrification　efficiency　could　reach　80.9　±　0.4%.　Illumina　sequencing　of　16S　rRNA　gene　amplicons　revealed　that　the　microbial　community　divergence　occurred　among　the　biofilm　on　bio-carrier,　the　suspended　sludge　phase　and　the　inoculum.　Especially,　the　relative　abundance　of　denitrifying　genera　Denitratisoma　in　the　biofilm　of　bio-carrier　reaches　5.73%,　6.2　times　higher　than　that　in　the　suspended　sludge,　implying　the　imbedded　bio-carrier　was　conductive　to　enrich　the　specific　denitrifiers　to　polish　the　denitrification　performance　with　low　carbon　source.　This　work　provided　an　effective　method　for　the　design　optimization　of　bioreactor　based　on　CFD　simulation,　and　developed　a　hybrid　reactor　with　fixed　bio-carrier　for　nitrogen　removal　from　wastewater　with　low　C/N　ratio.　©　2023　Elsevier　Inc.