The　control　strategies　of　energy　saving　for　filamentous　sludge　bulking　were　investigated　in　the　A/O　process　under　low　dissolved　oxygen　(DO)　with　low　carbon/nitrogen　(C/N)　ratio,　and　the　dominant　filamentous　bacteria　were　identified　by　using　fluorescent　in　situ　hybridization.　Initially,　the　sludge　volume　index　reached　nearly　500mL/g　and　serious　bulking　occurred　when　the　DO　value　was　0.5mg/L,　with　Haliscomenobacter　hydrossis　as　the　major　filamentous　bacteria　in　the　bulking　sludge.　Later　on,　the　compartment　number　increased　in　the　aerobic　zone,　increasing　by　this　way　DO,　to　control　serious　bulking.　Increasing　DO　to　1mg/L　based　on　the　increase　of　compartment　number　in　the　aerobic　zone　was　the　favorable　controlling　method,　which　solved　the　sludge　loss,　improved　the　effluent　quality　to　the　national　discharge　standard　and　allowed　for　energy　costs　saving.　As　a　result,　the　effective　control　method　for　H.　hydrossis　filamentous　sludge　bulking　provided　the　economical,　convenient　and　longstanding　method　for　most　municipal　wastewater　treatment　plants　treating　real　low　C/N　domestic　wastewater.