Effects　of　external　carbon　sources　and　COD/NO3-N　on　nitrite　accumulation　through　denitrification　were　studied　at　a　temperature　of　28　+/-　2.0　degrees　C　using　mixed　activated　sludge.　Nitrite　accumulation　was　observed　for　each　type　of　carbon　source　studied.　Glucose　resulted　in　the　greatest　nitrite　accumulation　and　production　rate,　which　were　14.51　+/-　2.41　mg/L　and　0.121　+/-　0.013　g　N/(g　VSS　d),　respectively.　Moreover,　a　higher　COD/NO3-N　ratio　ranging　from　1.0　to　15.0　increased　accumulation　to　the　maximum　value　of　0.34　+/-　0.03　g　N/(g　VSS　d).　It　was　assumed　that　the　competition　for　electrons　between　nitrite　reductase　and　nitrate　reductase　led　to　different　reduction　rates　and　finally　caused　the　accumulation.　In　addition,　it　was　reasonable　to　use　the　pH　and　ORP　as　proxies　for　monitoring　the　real　endpoint　of　the　denitrification　process　with　the　addition　of　carbon　sources.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.