Free　ammonia　(FA)　inhibition　on　nitrite-oxidized　bacteria　(NOB)　and　real-time　control　are　used　to　achieve　nitrogen　removal　from　landfill　leachate　via　nitrite　pathway　at　low　temperatures　in　sequencing　batch　reactor.　The　inhibition　of　FA　on　NOB　activity　during　the　aerobic　period　was　prolonged　using　real-time　control.　The　degree　of　nitrite　accumulation　was　monitored　along　with　variations　of　the　ammonia-oxidizing　bacteria　and　NOB　population　using　fluorescence　in　situ　hybridization　techniques.　It　is　demonstrated　that　the　end-point　of　ammonia　oxidization　is　detected　from　the　on-line　measured　dissolved　oxygen,　oxidization-reduction　potential,　and　pH　signals,　which　could　avoid　the　loss　the　FA　inhibition　on　NOB　caused　by　excess　aeration.　At　low　temperature　(13.0-17.6　degrees　C),　the　level　of　nitrite　pathway　rapidly　increased　from　19.8%　to　90%,　suggesting　that　nitritation　was　successfully　started　up　at　low　temperature　by　applying　syntrophic　association　of　the　FA　inhibition　and　real-time　control,　and　then　this　high　level　of　nitrite　pathway　was　stably　maintained　for　as　long　as　233　days.　Mechanism　analysis　shows　that　the　establishment　of　nitritation　was　primarily　the　result　of　predominant　ammonia-oxidizing　bacteria　developed　in　the　nitrifying　bacteria　population　compared　to　NOB.　This　was　mainly　due　to　a　gradual　reduction　of　nitrite　amount　that　is　available　to　provide　energy　for　the　growth　of　NOB,　eventually　leading　to　the　elimination　of　NOB　from　the　bacterial　clusters　in　sequencing　batch　reactor　sludge　system.　(C)　2015　The　Research　Center　for　Eco-Environmental　Sciences,　Chinese　Academy　of　Sciences.　Published　by　Elsevier　B.V.