The　long-term　impact　of　fulvic　acid　(FA)　on　partial　nitritation　(PN)　system　was　initially　examined　in　this　study.　The　obtained　results　revealed　that　the　FA　lower　than　50mg/L　had　negligible　effect　on　the　nitrite　accumulation　rate　(NAR　nearly　100%)　and　ammonium　removal　rate　(ARR　56.85%),　while　FA　over　50　mg/L　decreased　ARR　from　56.85%　to　0.7%.　Sludge　characteristics　analysis　found　that　appropriate　FA　(　＜　50mg/L)　exposure　promoted　the　settling　performance　and　granulation　of　PN　sludge　by　removing　Bacteroidetes　and　accumulating　Chloroflexi　.　The　analysis　of　metagenomics　suggested　that　the　presence　of　limited　FA　(0-50mg/L)　stimulated　the　generation　of　NADH,　which　favors　the　denitrification　and　nitrite　reduction.　The　negative　impact　of　FA　on　the　PN　system　could　be　divided　into　two　stages.　Initially,　limited　FA　(50-120　mg/L)　was　decomposed　by　Anaerolineae　to　stimulate　the　growth　and　propagation　of　heterotrophic　bacteria　(　Thauera　).　Increasing　heterotrophs　competed　with　AOB　(　Nitrosomonas　)　for　dissolved　oxygen,　causing　AOB　to　be　eliminated　and　ARR　to　declined.　Subsequently,　when　FA　dosage　was　over　120　mg/L,　Anaerolineae　were　inhibited　and　heterotrophic　bacteria　reduced,　resulting　in　the　abundance　of　AOB　recovered.　Nevertheless,　the　ammonium　transformation　pathway　was　suppressed　because　genes　amoABC　and　hao　were　obviously　reduced,　leading　to　the　deterioration　of　reactor　performance.　Overall,　these　results　provide　theoretical　guidance　for　the　practical　application　of　PN　for　the　treatment　of　FA-containing　sewage.　(c)　2022　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.