Nitrite/Free　Nitrous　Acid　(FNA)　has　previously　been　shown　to　inhibit　aerobic　and　anoxic　phosphate　uptake　by　polyphosphate　accumulating　organisms　(PAOs).　The　inhibitory　effect　of　FNA　on　the　aerobic　metabolism　of　Glycogen　Accumulating　Organisms　(GAOs)　is　investigated.　A　culture　highly　enriched　(92　+/-　3%)　in　Candidatus　Competibacter　phosphatis　(hereafter　called　Competibacter)　was　used.　The　experimental　data　strongly　suggest　that　FNA　likely　directly　inhibits　the　growth　of　Competibacter,　with　50%　inhibition　occurring　at　1.5　x　10(-3)　mgN-HNO2/L　(equivalent　to　approximately　6.3　mgN-NO2-/L　at　pH　7.0).　The　inhibition　is　well　described　by　an　exponential　function.　The　organisms　ceased　to　grow　at　an　FNA　concentration　of　7.1　x　10(-3)　mgN-HNO2/L.　At　this　FNA　level,　glycogen　production,　another　anabolic　process　performed　by　GAOs　in　parallel　to　growth,　decreased　by　40%,　while　the　consumption　of　polyhydroxyalkanoates　(PHAs),　the　intracellular　carbon　and　energy　sources　for　GAOs,　decreased　by　approximately　50%.　FNA　likely　inhibited　either　or　both　of　the　PHA　oxidation　and　glycogen　production　processes,　but　to　a　much　less　extent　in　comparison　to　the　inhibition　on　growth.　The　comparison　of　these　results　with　those　previously　reported　on　PAOs　suggest　that　FNA　has　much　stronger　inhibitory　effects　on　the　aerobic　metabolism　of　PAOs　than　on　GAOs,　and　may　thus　provide　a　competitive　advantage　to　GAOs　over　PAOs　in　enhanced　biological　phosphorus　removal　(EBPR)　systems.　(C)　2010　Elsevier　Ltd.　All　rights　reserved.