There　is　limited　research　on　the　relationship　between　membrane　fouling　and　microbial　metabolites　in　the　nitrogen　removal　process　coupled　with　membrane　bioreactors　(MBRs).　In　this　study,　we　compared　anoxic-oxic　(AO)　and　partial　nitritation-anammox　(PNA),　which　were　selected　as　representative　heterotrophic　and　autotrophic　biological　nitrogen　removal-coupled　MBR　processes　for　their　fouling　behavior.　At　the　same　nitrogen　loading　rate　of　100　mg/L　and　mixed　liquor　suspended　solids　(MLSS)　concentration　of　4000　mg/L,　PNA-MBR　exhibited　more　severe　membrane　fouling　compared　to　AO-MBR,　as　evidenced　by　monitoring　changes　in　transmembrane　pressure　(TMP).　In　the　autotrophic　nitrogen　removal　process,　without　added　organic　carbon,　the　supernatant　of　PNA-MBR　had　higher　concentrations　of　protein,　polysaccharides,　and　low-molecular-weight　humic　substances,　leading　to　a　rapid　flux　decline.　Extracellular　polymeric　substances　(EPS)　extracted　from　suspended　sludge　and　cake　sludge　in　PNA-MBR　also　contributed　to　more　severe　membrane　fouling　than　in　AO-MBR.　The　EPS　subfractions　of　PNA-MBR　exhibited　looser　secondary　structures　in　protein　and　stronger　surface　hydrophobicity,　particularly　in　the　cake　sludge,　which　contained　higher　contents　of　humic　substances　with　lower　molecular　weights.　The　higher　abundances　of　Candidatus　Brocadia　and　Chloroflexi　in　PNA-MBR　could　lead　to　the　production　of　more　hydrophobic　organics　and　humic　substances.　Hydrophobic　metabolism　products　as　well　as　anammox　bacteria　were　deposited　on　the　hydrophobic　membrane　surface　and　formed　serious　fouling.　Therefore,　hydrophilic　membrane　modification　is　more　urgently　needed　to　mitigate　membrane　fouling　when　running　PNA-MBR　than　AO-MBR.