The　present　study　aims　to　investigate　the　impacts　of　a　typical　recalcitrant　organic　compound,　namely　oleic　acid,　on　the　floc　characteristics　of　activated　sludge,　including　extracellular　polymeric　substances　(EPS)　and　polyhydroxyalkanoates　(PHA)　under　different　operation　modes　of　a　sequencing　batch　reactor　(SBR).　In　addition,　the　morphological　structure,　sludge　settling　performance,　and　microbial　community　structure　were　further　compared　with　those　under　a　treatment　system　based　on　glucose　as　a　substrate.　The　results　showed　comparatively　lower　EPS　concentrations　under　the　oleic　acid-based　SBR　system　than　those　observed　under　the　glucose-based　treatment　system,　particularly　the　polysaccharides　(PS)　contents　of　EPS.　Furthermore,　the　oleic　acid　compound　resulted　in　comparatively　reduced　PHA　concentrations.　The　oleic　acid-based　SBR　system　revealed　a　better　settling　performance　than　that　of　the　glucose　control　group,　showing　maximum　sludge　volume　index　(SVI)　values　of　170　and　633　mL/g,　respectively.　Indeed,　the　oleic　acid-based　SBR　system　inhibited　the　formation　of　filamentous　sludge　bulking.　The　obtained　high-throughput　sequencing　results　demonstrated　the　reduction　effect　of　the　recalcitrant　oleic　acid　organic　compound　on　the　sludge　microbial　abundance　and　diversity.　However,　the　relative　abundance　showed　Mycobacterium　a　temporal　increase　over　the　experiment,　reaching　up　to　16.66　%　and,　consequently,　becoming　the　dominant　microbial　species.　These　findings　provide　a　theoretical　basis　for　the　addition　of　recalcitrant　biodegradable　organic　compounds　to　influents　of　actual　wastewater　treatment　plants,　outlining　the　impact　of　oleic　acid　on　sludge　floc　characteristics.　©　2024　Elsevier　Ltd