In　order　to　improve　the　nitrogen　removal　efficiency　and　save　operational　cost,　the　feasibility　of　the　alternating　aerobic-anoxic　process　(AAA　process)　applied　in　a　sequencing　batch　reactor　(SBR)　system　for　nitrogen　removal　was　investigated.　Under　sufficient　influent　alkalinity,　the　AAA　process　did　not　have　an　advantage　over　one　aerobicanoxic　(OAA)　cycle　on　treatment　efficiency　because　microorganisms　had　an　adaptive　stage　at　the　alternating　aerobic-anoxic　transition,　which　would　prolong　the　total　cycling　time.　On　the　contrary,　the　AAA　process　made　the　system　control　more　complicated.　Under　deficient　influent　alkalinity,　when　compared　to　OAA,　the　AAA　process　improved　treatment　efficiency　and　effluent　quality　with　NH4　+-N　in　the　effluent　below　the　detection　limit.　In　the　nitrification,　the　average　stoichiometric　ratio　between　alkalinity　consumption　and　ammonia　oxidation　is　calculated　to　be　7.07　mg　CaCO3/mg　NH4　+-N.　In　the　denitrification,　the　average　stoichiometric　ratio　between　alkalinity　production　and　NO3　--N　reduction　is　about　3.57　mg　CaCO3/mg　NO3　--N.　As　a　result,　half　of　the　alkalinity　previously　consumed　during　the　aerobic　nitrification　was　recovered　during　the　subsequent　anoxic　denitrification　period.　That　was　why　the　higher　treatment　efficiency　in　the　AAA　process　was　achieved　without　the　supplement　of　bicarbonate　alkalinity.　If　the　lack　of　alkalinity　in　the　influent　was　less　than　1/3　of　that　needed,　there　is　no　need　for　external　alkalinity　addition　and　treatment　efficiency　was　the　same　as　that　under　sufficient　influent　alkalinity.　Even　if　the　lack　of　alkalinity　in　the　influent　was　more　than　1/3　of　that　needed,　the　AAA　process　was　an　optimal　strategy　because　it　reduced　the　external　alkalinity　addition　and　saved　on　operational　cost.　©　Higher　Education　Press　2007.