Thisarticle　revealed　the　significant　role　of　bacterialaggregation　and　spatiotemporal　immigration　in　a　novel　partial　denitrification/anammoxsystem　for　stable　nitrogen　removal.　The　novel　partial　denitrification-driven　anammox　(PD/A)is　an　energy-efficientmethod　for　nitrogen　removal　from　wastewater.　However,　its　stabilityand　efficiency　are　impeded　by　the　competition　between　heterotrophicdenitrifying　bacteria　and　relatively　slow-growing　anammox　bacteria.In　this　study,　a　PD/A　granular　sludge　system　was　developed,　whichachieved　a　nitrogen　removal　efficiency　of　94%　with　98%　anammox　contribution,even　as　the　temperature　dropped　to　9.6　degrees　C.　Analysis　of　bacterialactivity　in　aggregates　of　different　sizes　revealed　that　the　largestgranules　(＞2.0　mm)　exhibited　the　highest　anammox　activity,　2.8timesthat　of　flocs　(＜0.2　mm),　while　the　flocs　showed　significantly　highernitrite　production　rates　of　PD,　more　than　six　times　that　of　the　largestgranules.　Interestingly,　fluorescent　in　situ　hybridization　(FISH)combined　with　confocal　laser　scanning　microscopy　(CLSM)　revealed　anest-shaped　structure　of　PD/A　granules.　The　Thauera　genus,　a　key　contributor　to　PD,　was　highly　enriched　at　the　outeredge,　providing　substrate　nitrite　for　anammox　bacteria　inside　thegranules.　As　temperature　decreased,　the　flocs　transformed　into　smallgranules　to　efficiently　retain　anammox　bacteria.　This　study　providesmultidimensional　insights　into　the　spatiotemporal　assembly　and　immigrationof　heterotrophic　and　autotrophic　bacteria　for　stable　and　high-ratenitrogen　removal.