GPS-X Technical Reference

ROTATING BIOLOGICAL CONTACTOR (RBC) MODEL
Introduction
Attached Growth Models 228
The rotating biological contractor (RBC) model is available in all the libraries, using the
same biological reactions found in the suspended-growth models in the appropriate
library. The model can predict the extent of carbon and nitrogen removal (by uptake or
oxidation) and denitrification, as well as phosphorus uptake and release (in the CNP
library). This model incorporates the growth kinetics and transport processes for the
corresponding state variables. The profiles of the various components through the biofilm
are modelled so that different environments (aerobic, anoxic and anaerobic) can exist
within the biofilm.
To reduce the complexity of the model, some assumptions are necessary. The limitations
of this model concern the hydraulics of the rotating biological contactor and the biofilm
itself. The model assumes that the flow rate and solids loading to the RBC can always be
processed; that is, clogging and head loss are not modelled. Also the maximum thickness
of the biofilm is not calculated; rather it is specified by the user. This assumption was
primarily made because there are little or no data available for calibration/verification of
the maximum film thickness calculations. The effect of the rotation speed and direction
of the RBC and its impact on media sloughing and aeration requirements is neglected.
Similar to the trickling filter model, the RBC is more complex than the suspended-growth
models since the state variables are modelled through the biofilm as well as through
various RBC stages.
Conceptual Model
The rotating biological contactor is divided into ‘n’ stages (default is 1 stage) each
representing a baffled RBC system. The transfer of the state variables between each of
these stages is through the liquid flow. The biofilm in each stage is modelled as a number
of layers (default is one layer as the liquid film on top of five layers as the biofilm). The
transfer of soluble state variables between each of these layers is by diffusion only.
Particulate variables have a certain physical volume associated with them and can be
displaced into the neighbouring layer by growth processes. Each layer of the biofilm is
modelled as a CSTR with the same biological reactions as the suspended-growth
biological model (See Appendix A for the mantis model). Attachment and detachment
coefficients are used to provide for a means of transfer of particulate components
between the biofilm surface and the liquid film.
GPS-X Technical Reference