GPS-X Technical Reference

Suspended Growth Models 124
ACTIVATED SLUDGE MODEL NO. 1 (ASM1)
Introduction
The International Association on Water Pollution Research and Control (IAWPRC) Task
Group realized that due to the long solids retention times and low growth rates of the
bacteria, the actual effluent substrate concentrations between different activated sludge
treatment plants did not vary greatly. What were significantly different were the levels of
MLSS and electron acceptor (oxygen or nitrate). Thus the focus of the Activated Sludge
Model No. 1 (called asm1 in GPS-X) is the prediction of the amount and change of the
solids and electron acceptor.
The Task Group considered the trade-off between model accuracy and practicality. They
identified the major biological processes occurring in the system and characterized these
processes with the simplest rate expressions that could be used, resembling the real
reactions.
The use of switching functions was made by the Task Group since some reactions
depended on the type of electron acceptor present. These functions were of the form:
Equation 6.34
At low concentrations of dissolved oxygen (SO), the parameter KOH dominates the
expression and approaches a value of zero. At high values of SO, the parameter
KOH would be negligible and the expression approaches unity. If the switching
function was inverted, then the limits when SO were high or low are reversed. A
consequence of using switching functions of this form is that they are continuous
functions unlike discontinuous on/off switches which are more difficult to
simulate.
Conceptual Model
In the development of activated sludge modelling, the manner in which the quantity of
organic matter is measured (BOD, COD or TOC) is inconsistent. The Task Group
decided to use COD since mass balances can be carried out and since it has links to the
electron equivalents in the organic substrate, biomass and electron acceptor.
GPS-X Technical Reference