In　full-scale　wastewater　treatment　systems,　phosphorus　removal　typically　occurs　together　with　nitrogen　removal.　Nitrite,　an　intermediate　of　both　the　nitrification　and　denitrification　processes,　can　accumulate　in　the　reactor.　The　inhibitory　effect　of　nitrite/free　nitrous　acid　(FNA)　on　the　aerobic　metabolism　of　poly-phosphate　accumulating　organisms　(PAOs)　is　investigated.　A　culture　highly　enriched　(90　+/-　5%)　in　Candidatus　＂Accummulibacter　phosphatis＂,　a　well-known　PAO,　was　used　to　perform　a　series　of　batch　experiments　at　various　nitrite　and　pH　levels.　FNA　was　found　to　inhibit　all　key　aerobic　metabolic　processes　performed　by　PAOs,　namely　PHA　oxidation,　phosphate　uptake,　glycogen　replenishment　and　growth.　The　inhibitory　effect　on　the　anabolic　processes　(growth,　phosphate　uptake　and　glycogen　production)　was　much　stronger　than　that　on　the　catabolic　processes　(PHA　oxidation).　50%　inhibition　on　all　anabolic　processes　occurred　at　FNA　concentrations　of　approximately　0.5　x　10(-3)　mg　HNO2-N/L　(equivalent　to　2.0　mg　NO2--N/L　at　pH　7.0),　while　full　inhibition　occurred　at　FNA　concentrations　of　approximately　6.0　x　10(-3)　mg　HNO2-N.　These　concentrations　could　be　found　in　full-scale　wastewater　treatment　systems　that　achieve　nitrogen　removal　via　the　nitrite　pathway.　In　comparison,　PHA　oxidation　remained　at　40-50%　of　the　highest　rate　at　FNA　concentrations　in　the　range　2.0　x　10(-3)-10.0　x　10(-3)　mg　HNO2-N/L.　Interestingly,　PAOs　were　able　to　reduce　nitrite　under　aerobic　conditions　(DO　approximate　to　3　mg/L),　with　the　rate　increasing　substantially　with　the　FNA　concentration.　The　inhibition　on　phosphate　uptake　was　found　to　be　reversible.　(C)　2010　Elsevier　Ltd.　All　rights　reserved.