The　organic　compound　composition　of　wastewater,　serves　as　a　crucial　indicator　for　the　operational　performance　of　activated　sludge　processes　and　has　a　major　influence　on　the　development　of　filamentous　bulking　in　activated　sludge.　This　study　focused　on　the　impact　of　typical　soluble　and　slowly　-biodegradable　organic　compounds,　investigating　the　pathways　through　which　these　substrates　affect　the　occurrence　of　filamentous　bulking　in　systems　operated　under　both　high-　and　low　-oxygen　conditions.　Results　showed　that　slowly　-biodegradable　organic　compounds　lead　to　a　concentrated　distribution　of　microorganisms　within　flocs,　with　inward　growth　of　filamentous　bacteria.　Both　Tween-80　and　granular　starch　treated　systems　exhibited　a　significant　increase　in　protein　content.　The　glucose　system,　utilizing　soluble　substrates,　exhibited　a　markedly　higher　total　polysaccharide　content.　Microbial　communities　in　the　Tween-80　and　granular　starch　treated　systems　were　characterized　by　a　higher　abundance　of　bacteria　known　to　enhance　sludge　flocculation　and　settling,　such　as　Competibacter,　Xanthomonadaceae　and　Zoogloea　.　These　findings　are　of　high　significance　for　controlling　the　operational　performance　and　stability　of　activated　sludge　systems,　deepening　our　understanding　and　providing　a　novel　perspective　for　the　improvement　of　wastewater　treatment　processes.　(c)　2024　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.