This　study　attempted　to　compare　the　enrichment　of　complete　ammonium　oxidation　(comammox)　bacteria,　which　are　affiliated　with　Nitrospira　and　not　able　to　generate　nitrous　oxide　(N2O,　a　potent　greenhouse　gas)　through　biological　pathways,　in　two　commonly-utilized　configurations　of　floccular　sludge　reactors,　i.e.,　sequencing　batch　reactor　(SBR)　and　continuous　stirred　tank　reactor　(CSTR),　under　the　ammonium　condition　of　mainstream　wastewater　(i.e.,　40.0　g-N/m3).　The　results　in　terms　of　nitrification　performance　and　microbial　analyses　during　216-d　operation　showed　that　compared　with　SBR　offering　a　fluctuating　but　generally　higher　in-situ　ammonium　concentration　(i.e.,　1.0–6.0　g-N/m3)　which　was　favorable　for　the　growth　of　ammonium-oxidizing　bacteria　(AOB,　belonging　to　Nitrosomonas　in　this　study),　CSTR　managed　to　lower　the　in-situ　ammonium　level　to　＜　2.0　g-N/m3,　thus　creating　a　competitive　advantage　for　comammox　bacteria　with　a　highly　oligotrophic　lifestyle.　Such　an　argument　was　further　supported　by　dedicated　batch　tests　which　revealed　that　Nitrospira-dominant　sludge　had　a　lower　maximum　ammonium　oxidation　rate　and　lower　apparent　ammonium　and　oxygen　affinity　constants　than　Nitrosomonas-dominant　sludge　(i.e.,　33.5　±　2.1　mg-N/h/g-MLVSS　vs.　139.9　±　26.7　mg-N/h/g-MLVSS,　1.1　±　0.1　g-N/m3　vs.　17.6　±　4.6　g-N/m3,　and　0.017　±　0.002　g-O2/m3　vs.　0.037　±　0.013　g-O2/m3,　respectively),　proving　the　nature　of　comammox　bacteria　as　a　K-strategist.　Overall,　this　study　not　only　provided　useful　insights　into　the　effective　enrichment　of　comammox　bacteria　in　floccular　sludge　but　also　further　revealed　the　interactions　between　comammox　bacteria　and　AOB,　thereby　contributing　to　the　future　development　of　comammox-inclusive　biological　nitrogen　removal　technologies　for　sustainable　wastewater　treatment.　©　2025　The　Author(s)