Nitrite　has　been　commonly　thought　to　have　a　broad　inhibitory　effect　on　bacterial　metabolism.　Little　is　known　about　the　impact　of　nitrite　on　nitrate　reduction　with　pH　considered　as　an　important　factor.　This　study　investigates　the　nitrite　inhibition　on　nitrate　reduction　during　denitrification　under　various　pH　conditions　by　using　a　biological　nutrient　removal　(BNR)　sludge.　The　results　showed　that　nitrate　reduction　performance　had　a　much　stronger　relationship　with　the　free　nitrous　acid　(FNA)　than　that　of　nitrite　concentration,　implying　that　FNA,　rather　than　nitrite,　is　likely　the　real　inhibitor　on　nitrate　reduction.　The　nitrate　reduction　activity　of　the　biomass　was　observed　to　be　inhibited　about　60%　in　the　range　of　0.01-0.025　mg　HNO2-N/L　and　was　totally　inhibited　when　FNA　level　was　greater　than　the　threshold　concentration　(0.2　mg　HNO2-N/L).　Moreover,　the　recovery　rate　from　inhibitory　effect　was　found　to　be　dependent　much　more　strongly　on　the　concentration　of　FNA,　of　which　the　biomass　was　exposed　to　during　the　inhibition　period,　than　on　the　duration　of　the　inhibition　and　the　feeding　mode　of　inhibitor.　It　was　also　found　that　nitrite　reduction　was　significantly　inhibited　by　FNA　and　the　nitrite　reduction　rate　was　linear　to　nitrate　reduction　rate　due　to　the　inhibitory　mechanism　under　which　FNA　may　react　with　the　enzymes　involved　in　the　denitrification　process.　(C)　2009　Elsevier　B.V.　All　rights　reserved.