It is generally accepted that Pseudomonas spp. are the predominant heterotrophic bacteria involved in denitrification during
activated sludge treatment. However, uncertainty still exists regarding other bacteria involved. This study therefore aimed to
determine which heterotrophic bacteria present in mixed liquor samples from a biological nutrient removal process are responsible
for denitrification as well as to establish the extent to which these bacteria contribute to nitrate and nitrite reduction under anoxic
conditions. Heterotrophic bacteria were isolated, using plating techniques, from the anoxic zone of the Darvill activated sludge
process and assessed for nitrate and nitrite reduction under anoxic conditions. Results show a significant involvement of
Pseudomonas spp. in nitrate and nitrite reduction. It was also found that many other heterotrophic bacteria are involved to some
extent in denitrification, most of which were found to be incomplete denitrifiers only capable of reducing nitrates to nitrites with
no further reduction of the nitrites produced. Furthermore, results demonstrated varying strengths of nitrate and nitrite reduction
amongst the isolated heterotrophic bacteria, possible simultaneous oxygen and nitrate respiration by many incomplete denitrifiers
as well as involvement of gram-positive rods and gram-negative cocci.