Metabolic　products　play　a　significant　role　in　membrane　fouling　during　biological　wastewater　treatment.　However,　the　differences　in　metabolic　product　composition　and　membrane　fouling　potentials　between　autotrophic　(anammox,　AN)　and　heterotrophic　(denitrification,　DN)　nitrogen　removal　systems　are　not　well　understood.　This　study　cultivated　autotrophic　and　heterotrophic　nitrogen　removal　systems　and　analyzed　the　compositions　of　their　metabolic　products.　The　analysis　methods　included　fluorescence　excitation-emission　matrix-parallel　factor　analysis　(EEM-PARAFAC),　Fourier　transform　infrared　spectroscopy　(FTIR),　size　exclusion　chromatography　coupled　with　organic　carbon　and　nitrogen　detection　(SEC-OCD-OND),　and　Fourier-transform　ion　cyclotron　resonance　mass　spectrometry　(FT-ICR　MS).　Subsequently,　the　membrane　fouling　potential　of　these　metabolic　products　was　evaluated　by　dead-end　membrane　filtration　experiments　with　microfiltration　(MF)　and　ultrafiltration　(UF)　membranes.　Significant　differences　in　metabolic　product　composition　existed　between　AN　and　DN　nitrogen　removal　systems,　which　led　to　a　distinct　impact　on　membrane　fouling.　In　the　AN　nitrogen　removal　system,　membrane　fouling　was　relatively　mild　despite　the　high　abundance　of　polysaccharides　(with　C-O-C　ring　vibrations)　and　tannin-like　substances　in　metabolic　products.　Its　fouling　was　primarily　caused　by　the　rejection　of　humic　substances.　Conversely,　in　the　DN　nitrogen　removal　system,　the　high　rejection　of　proteins　and　polysaccharides　in　SMP　(52　%　and　62　%)　and　EPS　(18　%　and　59　%)　caused　a　pronounced　flux　decline,　greatly　increasing　membrane　fouling　potential.　These　findings　enhance　understanding　of　membrane　fouling　mechanisms　in　autotrophic　and　heterotrophic　nitrogen　removal　systems,　providing　actionable　strategies　to　mitigate　membrane　fouling　in　biological　nitrogen　removal　coupled　with　MBR　process.　©　2025