BIOKINETIC CHARACTERIZATION OF THE ACCELERATION PHASE IN AUTOTROPHIC AMMONIA OXIDATION

Batch ammonia oxidation is characterized by a preliminary acceleration phase. It was postulated that a critical mass of ammonium-nitrogen (NH4 +-N) must be processed for overcoming the preliminary acceleration. Based on a commonly accepted scheme for electron transport and energy transduction in Nitrosomonas. europeae, it was hypothesized that this critical NH4 +-N requirement is proportional to the nitrifying biomass concentration and independent of the initial NH4 +-N concentration. However, respirograms for batch NH4 +-N oxidation showed that the extent of the acceleration phase expressed as the oxygen consumption between the point of NH4 +-N injection and the point of maximum NH4 +-N oxidation activity (ΔDOc) decreased with an increase in the NH4 +-N oxidizing biomass concentration (Xns,o) and increased with an increasing initial NH4 +-N concentration (Snh,o). Further, prolonged endogenous respiration also resulted in an increase in ΔDOC. Based on the experimentally observed dependence of ΔDOc on Xns,o and Snh,o we inferred that the ratio of Snh,o to Xns,o is a principal parameter governing the extent of the preliminary acceleration phase in batch NH4 +-N oxidation. Failure to recognize the acceleration phase and attempts to fit batch ammonia oxidation profiles with standard Monodtype mathematical models can result in meaningless kinetic parameter estimates. Therefore, it is essential to determine the impact of process variables such as the initial substrate (NH4 +-N, NH2OH) and biomass concentrations on the duration of the acceleration phase and to develop a mathematical relation for its description. In this study, a leading attempt was made at mathematically characterizing the preliminary acceleration phase, based solely on batch respirometric data. The dynamics of the preliminary acceleration phase were described more accurately by a non-linear than by a linear relationship between oxygen uptake and the maximum specific growth rate (μmax). In all the experiments performed, oxygen was the sole measured analyte. Assays in which internal concentrations of reducing power are also determined could facilitate an even better understanding of the acceleration phenomenon.