The　nitritation　process　is　considered　to　be　one　of　the　most　energy-saving　and　efficient　methods　for　treating　polluted　water.　In　this　study,　active　sludge　was　immobilized　with　waterborne　polyurethane　(WPU),　and　a　heat-shock　method　was　employed　to　treat　the　immobilized　aggregates.　When　environmental　factors　that　adversely　affect　nitritation　(pH,　dissolved　oxygen,　temperature,　etc.)　were　controlled,　steady　nitrite　nitrogen　accumulation　was　also　successfully　achieved.　We　investigated　the　effect　of　temperature　and　timing　of　the　heat-shock　method　on　the　activity　of　ammonia-oxidizing　bacteria　(AOB)　and　nitrite-oxidizing　bacteria　(NOB)　using　polymerase　chain　reaction-denaturing　gradient　gel　electrophoresis　(PCR-DGGE).　Subsequently,　temperatures　of　60　and　70　degrees　C　were　selected　to　evaluate　the　nitritation　stability　of　the　heat-shocked　immobilized　aggregates,　and　continuous-flow　experiments　were　conducted.　The　results　of　preliminary　experiments　indicated　that　NOB　can　be　completely　deactivated　at　temperatures　above　60　degrees　C　after　10min,　whereas　AOB　continued　to　exhibit　some　activity.　In　addition,　the　effect　of　temperature　was　more　significant　than　the　effect　of　heating　time　for　NOB.　The　results　of　the　nitritation　stability　assay　indicated　that　the　heat-shocked　immobilized　aggregates　that　formed　at　two　temperatures　remained　in　stable　nitritation;　however,　the　performance　of　the　AOB　was　decreased　at　the　highest　temperature　tested.　A　high　temperature　and　long　duration　were　required　for　stable　nitritation.　In　the　continuous-flow　experiments,　we　discovered　that　nitrate　nitrogen　accumulation　occurred　after　65　d　of　operation.　Therefore,　the　immobilized　aggregates　were　heat　shocked　again,　and　nitrite　nitrogen　accumulation　recurred　in　the　reactor　(results　not　shown).　Heat　shock　is　an　effective　method　for　stabilizing　the　nitritation　of　immobilized　nitrifying　sludge;　this　method　also　provides　new　ideas　for　a　nitritation-based　nitrogen　removal　process.　Key　innovations　are:　(1)　a　new　nitration　method　based　on　heat　shock　has　been　proposed　to　generate　steady　nitritation　using　immobilized　bacterial　aggregates;　(2)　polymerase　chain　reaction　(PCR)　has　been　applied　to　analyze　variations　in　the　AOB　and　NOB　populations　of　immobilized　aggregates　before　and　after　heat　shock;　(3)　optimal　heat-shock　conditions　(including　duration　and　temperature)　have　been　identified.