Nitrate　is　a　byproduct　of　the　anaerobic　ammonium　oxidation　(anammox)　process　and　is　related　to　its　electron　transfer.　However,　little　is　known　about　the　influence　of　nitrate　on　the　anammox　process.　In　this　work,　the　biphasic　effect　of　exogenous　nitrate　on　the　anammox　process　was　investigated　in　an　upflow　biofilter　(UBF)　reactor　with　ammonium　as　the　sole　electron　donor.　The　responses　of　anammox　to　increased　nitrate　were　analyzed　by　one-way　ANOVA　test　and　found　to　be　significantly　different　under　a　constant　and　decreased　nitrite　condition　(p　＜　0.01).　With　a　single　increase　in　nitrate　and　constant　ammonium　and　nitrite　in　the　influent,　the　total　nitrogen　removal　rate　(TNRR)　of　anammox　was　uninhibited,　but　stoichiometry　deviated　and　nitrate　production　always　showed　a　linear　decrease.　In　contrast,　anammox　exhibited　a　range　of　activity　with　constant　ammonium　and　simultaneously　increased　nitrate　and　decreased　nitrite　in　the　influent,　including　a　continuous　reduction　of　TNRR,　a　nonpersistent　ammonium　overconsumption　and　a　pronounced　nonlinear　response　of　nitrate　production.　Correlation　analysis　shows　that　the　lack　of　ammonium　overconsumption　was　accompanied　by　the　disappearance　of　nitrate　underproduction.　Kinetic　models　of　product　formation　were　effectively　used　to　explore　the　nitrate　production　behavior　of　anammox　subjected　to　increased　nitrate,　and　the　metabolite　of　nitrate　was　divided　into　a　growth　negative　coupling　type　and　growth　(partial)　coupling　type　under　a　constant　and　decreased　nitrite　condition,　respectively.　These　findings　collectively　suggest　that　nitrate　has　a　biphasic　effect　on　the　anammox　process　and　is　correlated　with　the　availability　of　nitrite.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.