The　discovery　of　complete　ammonium　oxidation　(comammox)　has　redefined　the　perception　of　the　nitrification　process　which　plays　a　vital　part　in　biological　nitrogen　removal　(BNR)　from　wastewater.　Despite　the　reported　detection　or　cultivation　of　comammox　bacteria　in　biofilm　or　granular　sludge　reactors,　limited　attempts　have　been　made　to　enrich　or　assess　comammox　bacteria　in　floccular　sludge　reactors　with　suspended　growth　of　microbes,　which　are　most　extensively　applied　at　wastewater　treatment　plants.　Therefore,　through　making　use　of　a　comammox-inclusive　bioprocess　model　reliably　evaluated　using　batch　experimental　data　with　joint　contributions　of　different　nitrifying　guilds,　this　work　probed　into　the　proliferation　and　functioning　of　comammox　bacteria　in　two　commonly-used　floccular　sludge　reactor　configurations,　i.e.,　continuous　stirred　tank　reactor　(CSTR)　and　sequencing　batch　reactor　(SBR),　under　mainstream　conditions.　The　results　indicated　that　compared　with　the　studied　SBR,　the　CSTR　was　observed　to　favor　the　enrichment　of　comammox　bacteria　through　maintaining　a　sufficient　sludge　retention　time　(40–100　d)　while　avoiding　an　extremely　low　DO　level　(e.g.,　0.05　g-O2/m3),　irrespective　of　the　varied　influent　NH4+-N　of　10–100　g-N/m3.　Meanwhile,　the　inoculum　sludge　was　found　to　greatly　influence　the　start-up　process　of　the　studied　CSTR.　By　inoculating　the　CSTR　with　a　sufficient　amount　of　sludge,　finally　enriched　floccular　sludge　with　a　high　abundance　of　comammox　bacteria　(up　to　70.5　%)　could　be　rapidly　obtained.　These　results　not　only　benefitted　further　investigation　and　application　of　comammox-inclusive　sustainable　BNR　technologies　but　also　explained,　to　some　extent,　the　discrepancy　in　the　reported　presence　and　abundance　of　comammox　bacteria　at　wastewater　treatment　plants　adopting　floccular　sludge-based　BNR　technologies.　©　2023　Elsevier　B.V.