The　anaerobic　ammonium　oxidation　(anammox)　process　is　an　autotrophic　nitrogen　removal　process　with　great　potential　as　a　cost-effective　and　highly　efficient　technology　in　the　wastewater　treatment　field.　The　main　challenges　yet　to　be　overcome　in　this　new　frontier　technology　are　operating　at　lower　temperatures　and　achieving　a　high　and　stable　nitrogen　removal　efficiency.　In　this　study,　an　up-flow　expanded　bed　reactor　with　hydroxyapatite　(HAP)-anammox　granules　was　operated　for　more　than　200　days　at　7　degrees　C.　The　nitrogen　loading　rate　(NLR)　was　improved　from　1.0　g-N/L/d　to　3.6　g-N/L/d,　together　with　a　high-level　nitrogen　removal　efficiency　of　84-92%,　which　is　the　highest　to　date　at　extremely　low　temperatures　in　a　continuous　experiment.　Candidatus　Kuenenia　was　revealed　to　be　the　only　dominant　anammox　genus,　with　a　relative　abundance　of　35.3-37.5%.　The　optimal　operational　temperature　was　around　35　degrees　C　and　the　apparent　activation　energy　(Ea)　was　calculated　as　78.37　kJ/mol.　The　three-layers　architecture　and　architectural　evolution　of　HAP-anammox　granules　into　HAP-cores　and　peeling　biofilms　with　outstanding　settling　performance　were　characterized.　Under　7　degrees　C,　the　high　capacity　of　nitrogen　removal　with　robust　removal　efficiency　using　HAP-anammox　granules　was　achieved.