Limited　filamentous　bulking　caused　by　low　dissolved　oxygen　(DO)　was　proposed　to　establish　a　low　energy　consumption　wastewater　treatment　system.　This　method　for　energy　saving　was　derived　from　two　full-scale　field　observations,　which　showed　pollutants　removal　would　be　enhanced　and　energy　consumption　could　be　reduced　by　atleast　10%　using　limited　filamentous　bulking.　Furthermore,　preliminary　investigation　including　the　abundance　evaluation　and　the　identification　of　filamentous　bacteria　demonstrated　that　the　limited　filamentous　bulking　could　be　repeated　steadily　in　a　lab-scale　anoxic-oxic　reactor　fed　with　domestic　wastewater.　The　sludge　loss　did　not　occur　in　the　secondary　clarifier,　while　COD　and　total　nitrogen　removal　efficiencies　were　improved　by　controlling　DO　for　optimal　filamentous　bacterial　population.　Suspended　solids　in　effluent　were　negligible　and　turbidity　was　lower　than　2　NTU,　which　were　distinctly　lower　than　those　under　no　bulking.　Theoretical　and　experimental　results　indicated　the　aeration　consumption　could　be　saved　by　the　application　of　limited　filamentous　bulking.　(C)　2009　Published　by　Elsevier　Ltd.