A　sludge　population　optimization　strategy　aims　to　select　the　ammonia　oxidation　bacteria　(AOB)　by　using　sodium　chloride　(NaCl)　as　a　selective　inhibitor　to　nitrite　oxidation　bacteria　(NOB)　is　applied　in　biological　nitrogen　removal　process　during　municipal　wastewater　treatment　using　a　sequencing　batch　reactor(SBR).　Different　salinity　tests　contributed　to　the　best　inhibition　salinity　chosen　were　applied.　In　order　to　optimize　nitrifying　microbial　communities　to　get　steady　nitrite　pathway,　7.6　g/L　salinity　and　4　month　salt　inhibition　period　were　applied.　FISH　analysis　indicated　that　AOB　(Nitrosospira)　became　the　dominant　nitrifying　bacteria　and　NOB　(Nitrobacter)　had　been　washed　out　of　the　activated　sludge.　An　understanding　of　salt　inhibition　mechanism　on　NOB　is　also　discussed.　©　2010　Peking　University.