Volatile fatty acid impacts on nitrite oxidation and carbon dioxide fixation in activated sludge.

Batch test were performed to assess nitrite removal, nitrate formation, CO2 fixation, gaseous nitrogen production and microbial density in activated sludge exposed to volatile fatty acid (VFA) mixtures. Nitrite removal and nitrate formation were both affected by the presence of VFAs, but to different degrees. Nitrate formation rates were reduced to a greater extent (79%) than nitrite removal rates (36%) resulting in an apparent unbalanced nitrite oxidation reaction. Since the total bacterial density and the nitrite oxidizing bacteria (NOB, Nitrospira) concentration remained essentially constant under all test conditions, the reduction in rates was not due to heterotrophic uptake of nitrogen or to a decrease in the NOB population. In contrast to the nitrogen results, VFAs were not found to impact CO2 fixation efficiency. It appeared that nitrite oxidation occurred when VFAs were present since the oxidation of nitrite provides energy for CO2 fixation. However, nitrate produced from the oxidation of nitrite was reduced to gaseous nitrogen products. N2O gas was detected in the presence of VFAs which was a clear indication that VFAs stimulated an alternative pathway, such as aerobic denitrification, during biotransformation of nitrogen in activated sludge.