GPS-X Technical Reference

241 Attached-Growth Models
ADVANCED BIOLOGICAL AERATED FILTER (BAF) MODEL
The advanced biological aerated filter (BAF model is a robust, mechanistically-based model. It
uses the same biological reactions found in the suspended-growth models discussed in Chapter 6.
Conceptual Model
The advanced BAF model consists of four major components: Hydraulics and Filter
Operation, Filtration, Biological Reactions, and a Biofilm. They are described in the
following sections.
Hydraulics and Filter Operation
The model needs to be able to describe the complex operation of the bio filter stages each
consisting of one or more units in different operating modes.
In the simplest implementation (number of units is 1); the model simulates the behavior
of a single BAF unit. This unit can be in filtration, standby, backwash, or flush mode. In
filtration, standby and flush mode, the filter is represented hydraulically as a tank
consisting of a certain number of horizontal sections (6 in the default case). The actual
filtration bed is preceded by a mixed tank without filter material to describe the dilution
effects of the volume of water under the filter bed. A similar mixed tank is added to
account for the liquid on top of the filter media. In backwash mode the horizontal
sections of the filter are combined and converted to one mixed tank. The model assumes
that during backwash the filter media is ideally mixed.
The influent loading to the filter determines if the filter is in filtration, standby,
backwash, or flush mode. In filtration and flushed modes, flow is entering the filter
through the input stream and coming out through the output stream. In standby mode,
there is no flow through the filter. In backwash mode, flow is entering the filter through
the backwash input stream and coming out through the backwash output stream.
A complex bio filter plant could be represented by the proper number of individual filter
units placed on the drawing board to describe the changing conditions in the plant and
forecast effluent quality. Due to the level of complexity within one filter unit and the
typical number of units in the plant this approach is not feasible even with substantial
computing capacity. Since the difference between individual units operating in the same
mode is not drastic, a simplified operation mode provides a good approximation of
operating and effluent conditions with substantially less overhead.
GPS-X Technical Reference