A　combined　system　of　anaerobic-aerobic　techniques　was　used　for　deep　co-treatment　of　mature　landfill　leachate　and　domestic　sewage,　mixed　at　a　ratio　of　1:　5,　through　partial　nitrification-anammox.　While　ensuring　synchronous　removal　of　organic　matter　and　nitrogen,　we　investigated　changes　in　several　factors　under　different　reflux　ratios　(0%,　100%　and　300%).　High　nitrification　efficiency　and　a　relatively　high　accumulation　rate　of　NO2--N　were　achieved,　the　latter　through　selective　inhibition　of　bacteria　by　free　ammonia.　The　results　indicated　that　maintaining　the　reflux　ratio　within　an　optimum　range　contributes　to　the　enrichment　of　anammox　bacteria.　In　this　study,　the　optimum　ratio,　which　permitted　the　highest　removal　rates　of　chemical　oxygen　demand　(COD),　NH4+-N,　and　total　nitrogen,　was　300%.　The　COD　level　in　the　anammox　reactor　(ANAOR)　decreased　to　lower　than　70　mg/L,　resulting　in　no　inhibitory　effect　on　anammox　and　maximum　enrichment　of　bacteria.　(C)　2015　Elsevier　Ltd.　All　rights　reserved.