GPS-X Technical Reference

Attached Growth Models 244
Physical
The physical parameters are found under the Parameters sub-menu item Physical. It
contains physical dimensions to describe the actual BAF being modelled and model
dimensions, which allow the user to specify how the physical system will be modelled.
There are three items under the heading Speed, which allow the user to optimize the
simulation speed of this model.
As seen in Figure 7-17, the unit dimensions of the BAF require inputs such as the single
filter bed surface area, the total filter bed depth from support, the media fill (empty
bed depth) (the difference between the last 2 inputs giving the water height above the
media), and the water height below support. The number of units makes the simplified
multiple units operation possible.
The next section of this form, media, includes inputs to characterize the media being
used in the filter: Specific surface of media together with the filter bed depth and surface
area provides an estimation of the total biofilm surface area in the model; Equivalent
particle diameter, where multi-media filters can be described; Clean bed porosity (void
space); and ultimate bulk biofilm volume, the maximum space the biofilm can take up
before completely clogging the filter.
The next two items under the Biofilm heading, density of biofilm and dry material
content of biofilm, are used to convert the concentration of each state variable to a
volume measurement. The Model Dimensions section includes the number of sections
in filter; the model assumes each horizontal section is of equal size.
Figure 7-17 shows physical components associated with oxygen solubility and model
speed. The oxygen mass transfer coefficient is calculated from the physical conditions
within the filter (thickness of biofilm and diffusion rate of oxygen) and is affected by the
liquid and air temperatures, and the oxygen fraction in air (See Chapter 6). These values
are input in the general data entry area or specifically set for each object as shown in this
screen. The first two items under the heading Speed concern the integration of the
soluble components, which is described in the Trickling Filter Model section of this
chapter. The third item, calculate DO in liquid, is used to improve the speed of
simulation if the BAF installation maintains a relatively high level of DO in the liquid
(close to saturation). By setting this parameter to OFF, a constant DO level is maintained
in the liquid and integration of the DO state variable is bypassed, thereby speeding up the
simulation. The constant DO level can be adjusted in the initial conditions form
(Process Data > Initialization > initial concentrations).
GPS-X Technical Reference