Although　the　limitation　or　deficiency　of　nutrients,　such　as　nitrogen　(N)　and　phosphorus　(P),　has　been　one　of　the　frequently　reported　factors　causing　filamentous　or　non-filamentous　bulking　of　activated　sludge,　the　mechanisms　are　still　unclear.　In　this　work,　the　long-term　effects　of　N　and　P　limitation　or　deficiency　on　sludge　settleability　and　bioflocculation　characteristics　were　investigated　in　six　sequencing　batch　reactors　(SBRs)　fed　with　wastewater　with　different　nutrient　availability.　The　sludge　volume　index　(SVI),　microbial　community　structures,　intracellular　poly-beta-hydroxyalkanoates　(PHAs)　and　extracellular　polymeric　substances　(EPS)　were　characterised　over　time.　Bulking　was　not　observed　in　SBRs　with　N　limitation　or　deficiency,　in　which　SVI　remained　below　150　mL/g.　In　contrast,　bulking　was　encountered　in　those　reactors　with　P　deficiency.　The　occurrence　of　non-filamentous　bulking　was　associated　with　a　higher　carbohydrates　fraction　and　a　lower　proteins　fraction　in　EPS.　In　the　case　of　filamentous　bulking,　SVI　correlated　negatively　with　the　amount　of　PHAs.　Our　experimental　data　support　the　hypothesis　that　the　occurrence　and/or　the　type　of　bulking　in　activated　sludge　could　be　affected　by　the　combination　of　kinetic　selection,　microbial　storage,　as　well　as　the　EPS　composition.　(C)　2014　Elsevier　B.V.　All　rights　reserved.