The　feasibility　of　in　situ　start-up　of　mainstream　anammox　process　was　investigated　in　three　parallel　sequencing　batch　biofilm　reactors　(SBBRs)　inoculated　with　nitrification　sludge,　partial　nitrification　sludge,　and　denitrifying　phosphorus　removal　sludge,　respectively.　The　SBBRs　were　operated　under　alternate　anaerobic/aerobic/anoxic　pattern　at　ambient　temperature　(16.5-26.8　degrees　C).　The　influent　organic　and　nitrogen　loading　rates　were　increased　stepwise.　Anammox　bacteria　grew　exponentially　with　relative　abundance　and　overall　bacterial　activity　increasing　from　0　to　0.004%　to　0.29-0.40%　and　＇not　detected＇　to　6-7　mg　N/L/h,　respectively.　Desirable　nitrogen　removal　efficiency　of　about　86%　was　obtained　in　3-4　months　for　the　influent　nitrogen　of　40.5-73.6　mg　N/L.　Anammox　was　the　primary　nitrogen　transformation　pathway.　For　the　anammox　bacterial　enrichment,　biofilm,　alternate　anaerobic/aerobic/anoxic　pattern,　and　limited　aeration　played　important　roles.　Seed　sludge　with　high　ammonium　oxidizing　bacterial　activity　further　promoted　the　start-up　of　anammox　process.　The　in　situ　start-up　strategy　could　promote　the　full-scale　application　of　mainstream　anammox.