Kinetic model of autotrophic denitrification in sulphur packed-bed reactors

Autotrophic denitrification of synthetic wastewater by Thiobacillus denitrificans in upflow sulphur packed-bed reactors was studied in order to establish the process kinetics for prediction of effluent concentration. Elemental sulphur particles of different size served as energy substrate as well as the physical support for the microbial biofilm. Experiments were performed under operating conditions of (i) different flow rates at constant influent nitrate concentration; and (ii) different influent nitrate concentrations at constant flow rate. The experimental results show that autotrophic denitrification rates in upflow sulphur packed-bed reactors can be described by a half-order kinetic model for biofilms. It was found that the half-order kinetic constants of upflow packed-bed reactors are 2.94-3.60, 1.47-2.04, and 1.12-1.29 mg1/2/L1/2 h for sulphur particle sizes of 2.8-5.6, 5.6-11.2, and 11.2-16 mm, respectively. The half-order kinetic constants could be related to the specific surface area of the reactor media by a simple equation. Successful application of the half-order reaction rate model was demonstrated for an actual wastewater (nitrified leachate). A comparison with the literature showed that the half-order reaction rate constants for autotrophic denitrification using elemental sulphur are approximately one order of magnitude lower than those of heterotrophic denitrification. An improved stoichiometric equation for autotrophic denitrification using elemental sulphur as electronic donor is also proposed.