A2O　process　is　an　effective　nitrogen　and　phosphorus　removal　process.　A2O-Biological　Aerated　Filter　(BAF)　process　is　a　new　nitrogen　and　phosphorus　removal　process　basing　on　the　two-sludge　theory.　Separation　of　activated　sludge　is　one　of　the　difficult　problems　encountered　in　the　activated　sludge　process.　Sludge　bulking　is　the　most　difficult　problem.　The　experiment　put　great　emphasis　on　the　sludge　settleability　of　the　two　processes,　when　they　are　used　to　treat　the　higher　C/N　ratio　domestic　sewage.　The　two　systems　were　operated　for　30　days　at　the　C/N　ratio　of　about　7.1.　The　experiment　result　showed:　the　sludge　settleability　of　A2O-BAF　process　is　far　better　than　A2O　process,　the　former　is　without　the　risk　of　sludge　bulking,　while　the　latter　is　more　serious.　The　main　reasons　are:　A　2O-BAF　process　has　long　anoxic　zone　and　short　aerobic　zone,　so　it　can　inhibit　sludge　bulking　effectively;　phosphorus　content　in　sludge　of　A　2O-BAF　process　is　twice　more　than　that　of　A2O　process,　the　former＇s　is　about　6%,　the　latter＇s　is　about　3%;　the　sludge　floating　problem　faced　by　A2O-BAF　process　is　far　less　serious　than　A2O　process,　the　floc　is　denser　and　more　cohesive.　©　2011　IEEE.