Assessment of denitrification by the ordinary heterotrophic organisms in an NDBEPR activated sludge sytem

The purpose of this study was to isolate and characterise the ordinary hete rotrophic organisms (OHOs) present in a NDBEPR system in order to gain a be tter understanding of the organisms involved in denitrification as well as a more holistic and accurate evaluation of the OHO fraction attributable to denitrification in such a system. Heterotrophic bacteria were isolated fro m the pre- and secondary anoxic zones of the Darvill NDBEPR process and cha racterised according to their ability to reduce nitrates and/or nitrites un der anoxic conditions. Results showed that the OHO fraction is more complex than currently accepted and, with respect to denitrification, can be more accurately subdivided into five functional groups, four of which interactiv ely contribute to denitrification occurring in the system and one group tha t are nondenitrifying. These groups were defined as true denitrifiers (bact eria capable of both nitrate and nitrite reduction), incomplete denitrifier s (bacteria that reduced nitrates to nitrites with no further reduction of the nitrites produced), incomplete-nitrite reducers (bacteria capable of bo th nitrate and nitrite reduction, however, exhibiting severe inhibition of nitrite reduction by nitrates),exclusive nitrite reducers (bacteria only ca pable of reducing nitrites) and non-denitrifiers (bacteria not capable of n itrate or nitrite reduction).