Implementation　of　biological　ammonia-to-nitrite　conversion　(nitritation)　is　crucial　to　more　sustainable　municipal　wastewater　treatment.　However,　achieving　stable　nitritation,　especially　in　low-temperature　mainstream　municipal　wastewater,　remains　a　global　challenge.　Here　we　develop　a　loading　rate　switch　strategy　to　enable　ultrastable　nitritation　in　a　pilot-scale　(6.5　m³)　reactor　treating　real　mainstream　municipal　wastewater.　This　strategy　consists　of　a　stage　with　high　organic　loading　rate　to　decrease　both　ammonia-oxidizing　bacteria　and　nitrite-oxidizing　bacteria　amounts　and　a　stage　in　which　the　organic　loading　rate　sharply　decreases　to　enhance　ammonia-oxidizing　bacteria　and　promote　their　bioactivity　while　containing　nitrite-oxidizing　bacteria.　By　implementing　this　strategy,　nitritation　initiated　within　7　days　and　a　high　nitrite　accumulation　ratio　of　~98.1%　was　maintained　for　＞450　days,　even　at　temperature　as　low　as　5.9　°C.　During　the　long-term　operation,　nitrite-oxidizing　bacteria　were　below　the　detection　level,　whereas　ammonia-oxidizing　bacteria　remained　abundant.　Moreover,　the　average　concentrations　of　effluent　ammonium　and　nitrite　were　kept　at　8.3　and　13.8　mg　N　l−1,　respectively,　favouring　further　nitrogen　removal　by　anammox　process,　an　inherently　low-carbon　and　low-energy　process.　Overall,　this　work　presents　a　viable　and　environmentally　friendly　strategy　for　nitrogen　removal,　paving　the　way　to　sustainable　sewage　management.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Limited　2024.