Population dynamics of denitrifying bacteria in a model biocommunity

Accumulation of toxic nitrite is frequently observed during waste water treatment. In this study, a model biocommunity was established to investigate alterations in the population of nitrate reducing bacteria affecting the reduction of nitrate and nitrite. The model biocommunity contained a Pseudomonas pseudoalcaligenes strain, which reduced nitrate to N2O or N2 without any nitrite accumulation, Bacillus niacini which reduced nitrate with nitrite as an intermediate and a Staphylococcus sp. strain which reduced nitrate only to nitrite. The influence of substrate and culture conditions on the composition of the denitrifying biocommunity is described. An almost complete denitrification without accumulation of nitrite was found in a bacterial community containing balanced ratios of nitrite producing and nitrite reducing bacteria. Such populations were obtained at low C/N ratios (C/N ratio of 2.5 or below) and at an optimal hydraulic retention time of 3 days. Longer hydraulic retention times resulted in increasing portions of the nitrite accumulating Staphylococcus sp. and subsequently in incomplete denitrification. Hydraulic retention times of 1.5 days or less required a large excess of the carbon source to obtain complete denitrification. In populations dominated by Pseudomonas pseudoalcaligenes, high denitrification rates were obtained even at oxygen concentrations as high as 5 mg l−1. Complete denitrification with N2O or N2 as the only products was also found in biocommunities dominated by B. niacini. These populations showed however a limited efficiency with respect to nitrite reduction.