A sulphide-rich anaerobic sludge acclimated with a molasses wastewater was used to carry out studies on nitrate and nitrite reductions in continuously stirred batch reactors. It was shown that a ratio as high as COD/N-NO x 65É6 mg mg~1 did not promote dissimilatory reduction of nitrogen oxides to ammonia. DenitriÐcation was characterized by a probable accumulation of gaseous intermediates, nitric oxide (NO) and nitrous oxide by sulphide (N 2 O), consumption with concomitant elemental sulphur production and by an increase of the redox potential. In addition, sulphate reducers were completely inhibited by nitrogenous oxides. Cultures performed without any carbon source proved that denitriÐers were able to use sulphides as electron donors. Furthermore, while a lag phase preceded nitrate denitriÐcation, nitrite was consumed immediately. Chemical reduction of nitrite by ferrous iron (Fe2) was considered to be responsible for this di †erence. Evidence of such a chemodenitriÐcation has been presented by using a sterilized sludge which kept its ability to reduce nitrite while it lost its capacity to use nitrate. Moreover, this chemical activity was favoured by Fe2addition. Finally, it has been suggested that during the cultures performed with non-sterilized sludge, a biological reduction of the ferric ions (Fe3) would be coupled to nitrite chemodenitriÐcation and would allow a regeneration of Fe2.
1998 SCI (