Introducing　anammox　technology　into　low-ammonia　sewage　treatment　offers　sustainable　nitrogen　removal,　but　there　remain　challenges　to　its　future　applications.　Here,　anammox　bacteria　were　enriched　by　integrating　into　partial　denitrification　(NO3--N　to　NO2--N)　in　the　anaerobic/anoxic/oxic　(A(2)/O)　reactor　with　low　COD/N　ratios　(2.7　+/-　0.4)　sewage.　This　significantly　enhanced　nitrogen　removal　through　partial　anammox　process　over　stable　operation　of　300　days.　Microbial　community　analysis　showed　that　the　anammox　bacteria　were　enriched　in　anoxic-carrier　biofilms　(Ca.　Brocadia,　0.69%),　and　their　abundance　was　significantly　higher　than　the　flocculent　sludge　(16　s　rRNA　sequencing:　P　＜　0.001;　qPCR:　P　＜　0.001).　Further　activity　testing　via　15　N-isotope　tracing　revealed　that　partial　denitrification　is　a　prerequisite　for　in　situ　occurrence　of　anammox　reaction.　Anammox　contributed　to　68%　of　the　total　nitrogen　loss　via　anoxic-carrier　biofilms　in　batch　tests.　Combining　denitrification　and　anammox,　the　nitrogen　removal　efficiency　of　the　A(2)/O　process　improved　by　approximately　16.9%　for　low　COD/N　ratios　real　sewage.