In　this　study,　the　effect　of　nitrite/FNA　on　the　anaerobic　metabolism　of　polyphosphate　accumulating　organisms　(PAOs)　and　glycogen　accumulating　organisms　(GAOs)　is　investigated.　The　results　clearly　show　that　FNA　has　a　detrimental　effect　on　the　acetate　uptake　rate　by　both　PAOs　and　GAOs,　but　this　adverse　effect　is　much　stronger　on　PAOs　than　on　GAOs.　Also,　when　FNA　was　increased,　phosphate　release　to　acetate　uptake　ratio　by　PAOs　increased　substantially　(250-300%　compared　to　control),　which　was　accompanied　by　decreases　(40-60%)　in　glycogen　degradation　and　PHA　production　to　VFA　uptake.　In　contrast,　these　ratios　for　GAOs　remained　constant　or　increased　slightly　towards　the　highest　FNA　concentration　applied.　These　results　indicate　that　the　anaerobic　metabolism　of　PAOs　is　more　adversely　affected　than　that　of　GAOs　when　FNA　is　present.　This　might　provide　a　competitive　advantage　to　GAOs　over　PAOs　in　enhanced　biological　phosphorus　removal　systems　when　nitrite　is　present.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.