Modelling NOB activity under different dissolved oxygen, ammonia and nitrite concentrations

Batch tests were conducted on sludge samples drawn from a nitrifying/denitrifying SBR fed on a synthetic wastewater to search for causes of the observed nitrification unbalancing, focusing on the effects of dissolved oxygen and ammoniacal and nitrous nitrogen concentrations on nitrite oxidising bacteria. Ammonium and nitrite oxidation rates were assessed by pH/DO-stat titration, i.e. by measuring the rate of addition of an alkaline solution and of an oxygen-reach solution required to maintain constant, at a selected set-point level, both pH and dissolved oxygen (DO) concentration of the sludge sample. According to experimental results, nitrite oxidising bacteria (NOB) were penalised under DO limitation (DO half saturation constant for ammonia and nitrite oxidation were 0.1 and 0.4 mg l-1, respectively). Several inhibition functions were found to describe the effect of increasing nitrite and ammonia concentrations on NOB allowing to assess the EC50 values for both chemical species.