Poly-beta-hydroxyalkanoates　(PHAs)　and　free　nitrous　acid　(FNA)　have　been　revealed　as　significant　factors　causing　nitrous　oxide　(N2O)　production　in　denitrifying　phosphorus　removal　systems.　In　this　study,　the　effect　of　PHA　degradation　rate　on　N2O　production　was　studied　at　low　FNA　levels.　N2O　production　always　maintained　at　approximately　40%　of　the　amount　of　nitrite　reduced　independent　of　the　PHA　degradation　rate.　The　electrons　distributed　to　nitrite　reduction　were　1.6　times　that　to　N2O　reduction.　This　indicated　that　electron　competition　between　these　two　steps　was　not　affected　by　the　PHA　degradation　rate.　Continuous　feed　of　nitrate　was　proposed,　and　demonstrated　to　reduce　N2O　accumulation　by　75%.　While　being　kept　low,　a　possible　compounding　effect　of　a　low-level　FNA　could　not　be　ruled　out.　The　sludge　used　likely　contained　both　polyphosphate-　and　glycogen-accumulating　organisms,　and　the　results　could　not　be　simply　attributed　to　either　group　of　organisms.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.