ASSESSMENT OF NITRITE DENITRIFICATION BEHAVIOUR BY DENITRIFYING HETEROTROPHIC ORGANISMS IN A NDBEPR ACTIVATED SLUDGE SYSTEM

Denitrification behaviour in nitrification denitrification biological excess phosphorus removal (NDBEPR) activated sludge systems is more complex than currently understood. Nitrite denitrification occurring in NDBEPR systems has been found to be limiting and only proceeds at the correct rate once nitrate denitrification is complete. From an engineering perspective this has remained unexplainable and, therefore, need exists for a better understanding of the micro-organisms involved in order to improve modelling of these systems. This study aimed at isolating and characterising denitrifying heterotrophic bacteria responsible for nitrite reduction occurring in a NDBEPR system in order to gain a better understanding of nitrite denitrification behaviour in such a system. Heterotrophic bacteria isolated from the pre and secondary anoxic zones of the Darvill NDBEPR process, situated on the outskirts of Pietermaritzburg in Kwazulu Natal, were screened and characterised according to their ability to reduce nitrates and/or nitrites under anoxic conditions. Nitrite denitrification behaviour of the isolates was further assessed over a range of nitrite concentrations to establish the effect varying concentrations of nitrite have on the reduction capacity of heterotrophic bacteria capable of nitrite reduction. Results showed that different heterotrophic bacteria have different capacities for nitrite reduction at varying nitrite concentrations. Nitrate was found to inhibit nitrite reduction in many of the isolates tested with only a few heterotrophic bacteria demonstrating concurrent nitrate and nitrite reduction under anoxic conditions. These findings confirm and offer explanation to nitrite denitrification behaviour observed in denitrification kinetics studies of NDBEPR systems.