Limited　filamentous　bulking　(LFB)　was　proposed　to　save　aeration　energy　consumption　and　enhance　the　capacity　of　filaments　to　degrade　substrates　with　low　concentrations　in　activated　sludge　systems.　Operational　parameters　favorable　for　maintaining　the　LFB　state　were　investigated　in　an　anoxic-oxic　reactor　treating　domestic　wastewater.　The　experiments　showed　that　the　LFB　state　would　deteriorate　with　sharply　decreasing　temperature,　reducing　substrate　gradients　or　removing　anoxic　zones.　The　balance　between　filaments　and　floc-formers　could　be　achieved　by　controlling　dissolved　oxygen　and　sludge　loading　rates　to　be　in　optimal　ranges.　Eikelboom　Type　0041　and　Candidatus　Microthrix　parvicella　were　the　filamentous　bacteria　responsible　for　the　LFB　state.　However,　the　excess　growth　of　Eikelboom　Type　021N　and　Sphaerotilus　natans　were　observed　when　serious　bulking　occurred　under　low　substrate　gradients.　It　was　demonstrated　that　stable　maintenance　of　LFB　for　energy　saving　was　feasible　by　process　control　and　optimization.　(C)　2011　Elsevier　Ltd.　All　rights　reserved.