EVIDENCE OF CARBOXY ESTER MECHANISMS IN NITRIFICATION FROM THE DYNAMIC REACTION KINETICS AT NATURAL CONCENTRATIONS. Part III. The Oxidation of Ammonia in Nitrification Follows Pseudo Half Order Kinetics Compatible with a Carboxy Ester Mechanism where Hydroxamic Acid is an Intermediate not Hydroxylamine.

Ammonia pulse test experiments at different pHs were conducted on a submerged biofilter in fast recycle acting as a batch reactor. The steady state ammonia concentrations, in a organic carbon substrate free environment, were similar to natural water courses and the introduced pulses were under 1 mg/l total ammonia nitrogen. The ammonia concentrations returned to the steady state values within 30 minutes at pH's of 7.2 to 9.0, but at a pH of 6.2 a noticeable lag is noted with a slower reaction. Integrated half order plots have better correlations than the integrated Michaelis-Menten plots. Half order reaction kinetics differs in a subtle way, in which a second molecule leaves in the equilibrium binding step, from Michaelis-Menten type kinetics. At very low ammonia concentrations, as found in natural water courses, half-order kinetics results in faster reaction rates. This then allows ammonia oxidising bacteria to out compete organisms for ammonia at very low concentrations that use mechanisms resulting in Michaelis-Menten kinetics. Half-order kinetics is compatible with the hypothesis that the reactions proceed via a carboxylic ester route, using ATP, in a cyclic fashion with iron stabilized hydroxamic acid as the true intermediate in ammonia oxidation. The intermediate undergoes a hydrolysis reaction to nitrite and the ester is recycled. The mechanisms contain features allowing for efficient dynamic control and optimisation as demanded by evolution. Issues of toxic intermediates and products are thereby also avoided. Design equations for optimising nitrification filters at low and medium TAN concentrations, based upon half order kinetics and liquid film diffusion, are given.