Rapid　enrichment　of　anaerobic　ammonia　oxidation　bacteria　(AnAOB)　is　highly　associated　with　the　granulation　process;　however,　the　interactive　mechanism　remains　unclear,　especially　for　the　initial　granulation　process.　A　single-stage　partial　nitritation-anammox　(PN/A)　bioreactor　combined　with　granular/floc　sludge　was　operated　for　400　days.　During　the　experimental　period,　the　nitrogen　removal　rate　increased　from　0.60　to　1.21　kg　N　m(-3)　d(-1),　and　the　nitrogen　removal　capability　improved　primarily　during　a　transition　period　(days　200-250),　which　was　accompanied　by　a　particle　size　increase　and　AnAOB　proliferation　(4.9　+/-　1.7　days).　Moreover,　as　observed　by　the　biomass　physio-morphology,　the　size　distribution,　and　the　microbial　community　shift,　small　flocs　(＜　200　mu　m)　aggregated　due　to　the　addition　of　excess　sodium　acetate.　The　emerging　floc　aggregates　represented　an　early　form　of　granules,　providing　the　initial　biological　carrier　and　necessary　anaerobic　microenvironment　for　the　growth　of　attached　AnAOB,　resulting　in　a　high　AnAOB　growth-rate.　These　results　are　the　first　direct　evidence　that　floc　aggregates　are　essential　to　AnAOB　enrichment,　and　that　they　can　be　affected　by　operational　conditions.　This　study　provides　an　in-depth　understanding　of　the　link　between　floc　aggregations　and　AnAOB　enrichment　and　broadens　the　feasibility　of　optimizing　PN/A　applications.　(c)　2021　Elsevier　Ltd.　All　rights　reserved.