Denitrification by heterotrophic bacteria during activated sludge treatment

It is generally accepted that Pseudomonas spp. are the predominant heterotr ophic bacteria involved in denitrification during activated sludge treatmen t. However, uncertainty still exists regarding other bacteria involved. Thi s study therefore aimed to determine which heterotrophic bacteria present i n mixed liquor samples from a biological nutrient removal process are respo nsible for denitrification as well as to establish the extent to which thes e bacteria contribute to nitrate and nitrite reduction under anoxic conditi ons. Heterotrophic bacteria were isolated, using plating techniques, from t he anoxic zone of the Darvill activated sludge process and assessed for nit rate and nitrite reduction under anoxic conditions. Results show a signific ant involvement of Pseudomonas spp. in nitrate and nitrite reduction. It wa s also found that many other heterotrophic bacteria are involved to some ex tent in denitrification, most of which were found to be incomplete denitrif iers only capable of reducing nitrates to nitrites with no further reductio n of the nitrites produced. Furthermore, results demonstrated varying stren gths of nitrate and nitrite reduction amongst the isolated heterotrophic ba cteria, possible simultaneous oxygen and nitrate respiration by many incomp lete denitrifiers as well as involvement of gram-positive rods and gram-neg ative cocci.