Modeling of fixed-film, fluidezed-bed bioreactors for biological denitrification

Nitrate containing wastewaters are produced in many industries. These nitrates impact the environment by promoting eutrophication of lakes, streams, and coastal waters and may produce toxic effects in humans. Nitrates may be biologically degraded with nitrogen and carbon dioxide gases produced as by-products. The use of fixed-film, fluidized bed bioreactors (FBBRs) for biological denitrification has advantages over other continuous reactors which may result in larger volumetric reaction rates, smaller reactor volumes, and potentially lower capital costs. In order to utilize these FBBRs for denitrification, a model which predicts nitrate concentration as a function of axial position within the reactor is useful for sizing the reactor and for optimizing the FBBR process.

A steady state kinetic model was developed to predict the concentration of nitrate as a function of axial position for FBBRs. The model, which utilized Monod kinetics with pH inhibition, was compared with the operating data from several FBBRs which operated at Oak Ridge National Laboratory (ORNL) for periods during 1980-1986. These FBBRs ranged from 10 to 50 cm in diameter and from 6 to 12 meters in height. The model developed agrees with the experimental data from these reactors very well for inlet nitrate concentrations up to -10,000 mg/L and for pHs within the FBBRs ranging from -6.5 to 9.