The　mainstream　anaerobic　ammonium　oxidation　(anammox)　has　attracted　extensive　attention　recently,　particularly　due　to　its　potential　of　transforming　current　wastewater　treatment　plants　from　energy　consuming　to　energy　neutral　or　positive.　However,　the　presence　of　biodegradable　chemical　oxygen　demanding　(COD,　20-80　mg　COD　L　(1))　in　the　mainstream　anammox　reactor　stimulates　the　growth　of　heterotrophic　bacteria,　which　would　compete　for　oxygen　with　ammonia-oxidizing　bacteria　(AOB)　and　for　nitrite　with　anammox　bacteria,　thus　interfering　with　the　autotrophic　nitrogen　removal　process.　In　the　present　work,　with　consideration　of　granule　size　distribution,　a　one-dimensional　model　describing　the　mainstream　simultaneous　partial　nitrification,　anammox　and　denitrification　(SNAD)　in　a　granule-based　reactor　was　established,　calibrated　and　validated,　based　on　the　long-term　experimental　results.　Through　applying　the　verified　model,　simulation　studies　were　conducted　and　the　results　showed　that　the　effluent　total　nitrogen　concentration　of　＜5　mg　N　L　(1)　could　be　achieved　at　C/N　ratio　of　0.2-0.6,　DO　concentration　of　0.2-0.4　mg　L　(1)　and　granule　radius　of　300-600　mu　m.　The　combined　effects　indicated　that　the　SNAD　process　with　TN　removal　efficiency　＞90%　was　obtained　at　C/N　ratio　and　DO　concentration　of　0.2-1.0　and　0.2-0.4　mg　O-2　L　(1)　respectively.　Finally,　the　various　granule　size　distribution　patterns　were　simulated,　which　confirmed　that　the　size　distribution　needed　to　be　incorporated　in　the　model　to　accurately　describe　the　granular　anammox　system,　considering　a　model　based　on　a　uniform　granule　size　does　not　reflect　the　real　situations.　These　results　provide　guides　to　optimize　the　operation　of　mainstream　granular　autotrophic　nitrogen　removal　process.　(C)　2017　Elsevier　B.V.　All　rights　reserved.