Partial　nitritation　(PN)-anaerobic　ammonium　oxidation　(anammox)　is　a　promising　pathway　for　the　biological　treatment　of　wastewater.　However,　the　destruction　of　the　system　caused　by　excessive　accumulation　of　nitrate　in　long-term　operation　remains　a　challenge.　In　this　study,　PN-anammox　was　initialized　with　low　inoculation　quantity　in　an　air-lift　reactor.　The　nitrogen　removal　rate　of　0.71　kgN/(m(3).　d)　was　obtained,　which　was　far　higher　than　the　seed　sludge　(0.3　kgN/(m(3).d)).　Thereafter,　excess　nitrate　build-up　was　observed　under　low-loading　conditions,　and　recovery　strategies　for　the　PN-anammox　system　were　investigated.　Experimental　results　suggest　that　increasing　the　nitrogen　loading　rate　as　well　as　the　concentration　of　free　ammonium　failed　to　effectively　suppress　the　nitrite　oxidation　bacteria　(NOB)　after　the　PN-anammox　system　was　disrupted.　Afterwards,　effluent　back-flow　was　added　into　the　reactor　to　control　the　up-flow　velocity.　As　a　result,　an　aggressive　discharge　of　sludge　that　promoted　the　synergetic　growth　of　functional　bacteria　was　achieved,　leading　to　the　successful　restoration　of　the　PN-anammox　system.　The　partial　nitritation　and　anammox　activity　were　in　balance,　and　an　increase　in　nitrogen　removal　rate　up　to　1.07　kgN/(m(3).　d)　was　obtained　with　a　nitrogen　removal　efficiency　of　82.4%　after　recovery.　Besides,　the　proportion　of　granular　sludge　(over　200　mu　m)　increased　from　33.67%　to　82.82%.　Ammonium　oxidation　bacteria　(AOB)　along　with　anammox　bacteria　were　enriched　in　the　granular　sludge　during　the　recovery　period,　which　was　crucial　for　the　recovery　and　stable　operation　of　the　PN-anammox　system.　(c)　Higher　Education　Press　2020