NITBED - Nitrification in a Fluidised Bed Reactor

Nitrification is the sequential oxidation of NH4 to N02 and N03 which proceeds according to the following reaction sequence 

Both steps are influenced by dissolved oxygen and the corresponding substrate concentration. The nitrification as a wastewater treatment process benefits greatly from biomass retention, owing to the relatively slow growth rates of the nitrifiers. 

In this example, a fluidised biofilm sand bed reactor for nitrification, as investigated by Tanaka et al. (1981), is modelled as three tanks-in-series with a recycle loop (Fig. 1). With continuous operation, ammonium ion is fed to the reactor, and the products nitrite and nitrate exit in the effluent. The bed expands in volume because of the constant circulation flow of liquid upwards through the bed. Oxygen is supplied external to the bed in a well-mixed gas-liquid absorber. 

5 Simulation Tools and Examples of Chemical Engineering Processes 

The model balance equations are developed by considering both the individual tank stages and the absorber. Component balances are required for all components in each section of the reactor column and in the absorber, where the feed and effluent streams are located. Although the reaction actually proceeds in the biofilm phase, a homogeneous model apparent kinetics model is employed, which is justified by its simplicity. 

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