GPS-X Technical Reference

233 Attached-Growth Models
The SBC model discussed herein 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 carbon-nitrogenphosphorus
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 submerged biological contactor and the
biofilm itself. The model assumes that the flow rate and solids loading to the SBC can
always be processed; that is, clogging and head loss is not modelled. The maximum
thickness of the biofilm is not calculated; rather it is specified by the user. This
assumption was made because there are little or no data available for
calibration/verification of the maximum film thickness calculations. It is assumed that
there is equal flow distribution over the entire surface area of the SBC and the media
inside the SBC. The effect of the rotation speed or direction of the SBC and its impact on
media sloughing is neglected. The oxygen diffused into the biofilm when the media is in
the air is neglected, since the SBC media is generally submerged more than 80 percent of
the time; therefore, the SBC is modelled as completely submerged.
Similar to the trickling filter model and the RBC, the SBC is more complex than the
suspended-growth models since the state variables are now modelled through the film
and through various SBC stages (or shafts).
Conceptual Model
The submerged biological contactor is divided into a number of stages (default is 2
stages) each representing a baffled SBC shaft. 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. 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 suspendedgrowth
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