NITRATE AND NITRITE REDUCTION OF A SULFIDE-RICH ENVIRONMENT

A sulphide-rich anaerobic sludge acclimated with a molasses wastewater was used to carry out studies on nitrate and nitrite reductions in co ntinuously stirred batch reactors. It was shown that a COD/N-NOx ratio as high as 65.6 mg mg(-1) did not promote dissimilatory reduction of nitrogen oxides to ammonia. Denitrification was characterized by a pro bable accumulation of gaseous intermediates, nitric oxide (NO) and nit rous oxide (N2O), by sulphide consumption with concomitant elemental s ulphur production and by an increase of the redox potential. In additi on, sulphate reducers were completely inhibited by nitrogenous oxides. Cultures performed without any carbon source proved that denitrifiers were able to use sulphides as electron donors. Furthermore, while a l ag phase preceded nitrate denitrification, nitrite was consumed immedi ately. Chemical reduction of nitrite by ferrous iron (Fe2+) was consid ered to be responsible for this difference. Evidence of such a chemode nitrification has been presented by using a sterilized sludge which ke pt its ability to reduce nitrite while it lost its capacity to use nit rate. Moreover, this chemical activity was favoured by Fe2+ addition. 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 chemodenitrification and would allow a regeneration of Fe2+. (C) 1998 SCI.