GPS-X Technical Reference

Modelling Fundamentals 20
COD overcomes the above-mentioned problems. It can be automated and measures all
organic fractions of the wastewater. The sludge COD can also be easily determined. COD
measures all organics in oxygen equivalent; that is the electron donating capacity of the
organic matter. This way it provides a direct link between organic load and aeration
requirement. The yield constant is truly constant only if expressed in COD units. Mass
balance is easy to establish with COD in a non-nitrifying plant: in steady-state, the
influent COD must equal the effluent COD plus the COD of the wasted sludge, plus the
oxygen consumed in the degradation of organic matter.
It is for this reason that the International Association on Water Quality (IAWQ)
committee selected and endorses the use of COD as a measure of organic parameter in
simulation of activated sludge plants.
Data Requirements
For modelling purposes, each unit process/operation is represented by a process model
(mathematical model) that reflects the dynamic behaviour of that particular process. One
of the main features of GPS-X is that it is model-independent, meaning that GPS-X is not
limited to a specific process model. Accordingly, a variety of modelling approaches
(process models) are available within GPS-X to handle a specific unit operation or unit
process. For example, the activated sludge process can be modelled using any one of the
following GPS-X activated sludge process models:
IAWQ Task Group models of the activated sludge process (Henze et al.,1987a;
Henze et al., 1994; Henze et al., 1998)
The general (bio-P) model (Dold, 1990, Barker and Dold, 1997)
Extended IAWQ (Mantis), described in Mantis Model (MANTIS) section of
Chapter 6)
Comprehensive plant-wide model developed by Hydromantis (Mantis2)
Consequently, a general calibration/verification approach to GPS-X must be broadly
defined. The calibration requirements of individual process models are established based
on the nature of each model (i.e., its mechanistic basis). Alternatively, modellers may
need to refer to the original literature reference to assess the calibration requirements of a
particular model in more detail.
Each calibration/verification study follows the same general principles. Accordingly, the
purpose of this section is to provide some guidelines pertaining to the calibration of the
models to full-scale wastewater treatment plants. The most popular process models have
been selected for illustration purposes, including the IAWQ Task Group Activated
Sludge Model No. 1 (Henze et al., 1987a) and layered settler model developed by
Hydromantis (Takács et al., 1991).
GPS-X Technical Reference