Anammox　is　a　green,　economical　and　efficient　nitrogen　removal　process.　Most　successful　anammox　studies　are　based　on　biofilm-　or　granule-based　systems,　but　pure　floc　sludge　partial　nitrification　(PN)　and　anammox　(A)　systems　that　are　not　inoculated　with　anaerobic　ammonia　oxidizing　bacteria　(AnAOB)　are　rarely　reported.　If　the　anammox　process　occurs　in　floc-based　systems,　the　large　specific　surface　areas　provide　more　efficient　nitrogen　removal,　and　are　much　more　economical　in　terms　of　construction　and　investment.　This　study　investigated　the　establishment,　performance　and　sludge　characteristics　of　a　one-stage　PN/A　system　with　pure　floc　sludge　and　exhibited　a　short　sludge　retention　time　(SRT)　and　low　mixed　liquor　suspended　solids　(SS)　content.　The　experiment　was　run　for　approximately　1260　days　and　divided　into　five　stages　based　on　the　SRTs　and　influent　ammonia　concentrations　treating　synthetic　wastewater　with　no　organic　matter.　The　results　showed　that　the　AnAOB　were　successfully　cultivated　and　enriched　with　ordinary　nitrification　and　denitrification　sludge,　which　formed　a　pure　floc-based　anammox　system　with　a　short　SRT　(at　least　14　days)　and　a　low　SS　control.　The　maximum　nitrogen　removal　efficiency　and　sludge　removal　loading　rate　reached　87.1　%　and　3.16　kg　N/(kg　VSS·d)　with　ammonia　loading　rates　=　0.55　and　0.56　kg-N/(m3·d),　dissolved　oxygen　=　0.2　and　0　mg/L,　temperature　=　30　and　28　°C,　mixed　liquor　volatile　suspended　solid　(VSS)　=　800　and　130　mg/L,　free　ammonia　(FA)/VSS　=　3.5　and　47.5　mg　NH3-N/g　VSS　and　SRT　=　30　and　15　days,　respectively.　Moreover,　the　FA/VSS　ratio　was　used　to　determine　the　operating　performance　of　the　PN/A　system,　and　the　thresholds　for　inhibiting　nitrite-oxidizing　bacteria　and　ammonia-oxidizing　bacteria,　including　AnAOB,　were　0.5–50　and　above　50　mg　NH3-N/g　VSS,　respectively.　The　floc-based　one-stage　PN/A　systems　proposed　in　this　study　provide　reductions　in　the　volumes,　and　floor　areas　for　the　reactor　tanks,　and　in　the　cost　of　the　carrier.　©　2023　Elsevier　B.V.