GPS-X Technical Reference

Attached Growth Models 218
TRICKLING FILTER MODEL
Introduction
The trickling filter 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, and 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 trickling filter and the biofilm itself. The
model assumes that the flow rate and solids loading to the filter can always be processed;
that is, clogging and head losses through the filter are not modelled. Also the maximum
thickness of the biofilm is not calculated, rather the user specifies it. 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 trickling filter and the media inside the trickling filter.
The effect of the rotation speed of the rotary distributor is neglected.
The dimensions of the trickling filter model are larger than the suspended-growth models
since the state variables are now modelled through the film as well as down through the
trickling filter. The suspended-growth models only considered the state variables along
the reactor (1-dimensional). The additional dimension in the biofilm has some impact on
the simulation speed; therefore, more time is required when using this model.
Improvements to the speed of this model have been made by integrating the state
variables with different frequencies. For example, the particulate components were found
to change more slowly than the soluble components since they are diffusing through the
biofilm. Therefore, the integration of the soluble components was handled differently
from the particulate components (see Figure 7-3).
Conceptual Model
The trickling filter is divided into ‘n’ horizontal sections (default is six sections) each
representing a cross-section of the trickling filter at a different depth. The transfer of the
state variables between each of these horizontal sections through the liquid film is
through liquid flow. The biofilm in each of these horizontal sections is modelled as a
number of layers (default is one layer for the liquid film on top of five layers for 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.
GPS-X Technical Reference