GPS-X Technical Reference

Digestion Models 274
CHAPTER 10
Digestion Models
This chapter describes the aerobic and anaerobic digestion models used in GPS-X.
BASIC ANAEROBIC DIGESTION MODEL
This section describes the basic anaerobic digestion model associated with the anaerobic digester
object. It is a modified version of the model developed by Andrews (1969), and
Andrews et al. (1971). The modifications to the original model are:
The addition of temperature sensitivity for the hydrolysis of volatile suspended solids
(VSS) and the growth of methanogenic organisms. The Arrhenius equation is used with a
base temperature of 35 degrees Celsius.
The chemical equilibria were modified by introducing Hydromantis' pH model (See PH
Tool section in Chapter 12).
The introduction of particulate inert inorganic material (xii). This component remains
unchanged within the digester and is introduced for the sole purpose of assessing its
impact on other processes downstream of the digester.
The addition of a rate for toxic substance degradation.
Conceptual Model
The basic anaerobic digestion model consists of two reactors: one for the liquid phase
and the other for the gaseous phase. Both are modelled as completely mixed reactors.
Transfer of gaseous products between the liquid phase and the gaseous phase is modelled
using a standard two-film mass transfer equation. Gaseous carbon dioxide (CO 2) and
methane (CH 4) are assumed to follow the ideal gas law. No further reactions take place in
the gaseous phase, which has a total gas pressure of 760 mm of Hg (i.e., atmospheric
pressure).
GPS-X Technical Reference