ALTHOUGH THE FUNCTION OF THE AUTO-trophic nitrifier is well known, little is known about the function, if any, of the heterotrophic nitrifier. It has long been accepted that in natural ecosystems and wastewater treatment processes using biological nutrient removal (BNR), nitrification is performed by the autotrophic and denitrification by the heterotrophic bacteria. This study assessed the nitrification potential of heterotrophic bacteria isolated from a BNR installation. Nitrification enrichment media were used to isolate potential heterotrophic nitrifiers from the aerobic zones of the Darvill wastewater treatment works, after which the isolates were screened individually in batch experiments for nitrifying ability. We identified numerous, predominantly Gram-positive, nitrifying heterotrophic bacteria, of which Staphylococcus, Micrococcus, Streptococcus, Pseudomonas and Bacillus species were prominent. Unlike autotrophs, however, the heterotrophic nitrifiers directly oxidized ammonium to nitrate with little or no nitrite accumulation. Furthermore, different patterns of nitrification were observed among the heterotrophic nitrifiers, with some even displaying potential nitrification-denitrification activity under aerobic conditions.
