The　granulation　of　activated　sludge　was　investigated　using　two　parallel　sequencing　batch　reactors　(SBRs)　operated　in　biological　nitrogen　and　phosphorus　removal　conditions　though　the　reactor　configuration　and　operating　parameters　did　not　favor　the　granulation.　Granules　were　not　observed　when　the　SBR　was　operated　in　biological　nitrogen　removal　period　for　30　d.　However,　aerobic　granules　were　formed　naturally　without　the　increase　of　aeration　intensity　when　enhanced　biological　phosphorus　removal　(EBPR)　was　achieved.　It　can　be　detected　that　plenty　of　positive　charged　particles　were　formed　with　the　release　of　phosphorus　during　the　anaerobic　period　of　EBPR.　The　size　of　the　particles　was　about　5-20　mu　m　and　their　highest　positive　zeta　potential　was　about　73　mV.　These　positive　charged　particles　can　stimulate　the　granulation.　Based　on　the　experimental　results,　a　hypothesis　was　proposed　to　interpret　the　granulation　process　of　activated　sludge　in　the　EBPR　process　in　SBR.　Dense　and　compact　subgranules　were　formed　stimulated　by　the　positive　charged　particles.　The　subgranules　grew　gradually　by　collision,　adhesion　and　attached　growth　of　bacteria.　Finally,　the　extrusion　and　shear　of　hydrodynamic　shear　force　would　help　the　maturation　of　granules.　Aerobic　granular　SBR　showed　excellent　biological　phosphorus　removal　ability.　The　average　phosphorus　removal　efficiency　was　over　95%　and　the　phosphorus　in　the　effluent　was　below　0.50　mg　L-1　during　the　operation.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.