GPS-X Technical Reference

127 Suspended Growth Models
Processes
Looking at the particulate states, the model includes three types of biomass:
1. heterotrophic organisms (xbh);
2. nitrifying organisms (xba); and
3. phosphate accumulating organisms (xbp)
The phosphate accumulating organisms’ state variables do not include the internal
storage products, which are separate state variables. Particulate non-biodegradable
organic material is also modelled (xi). Although it is not removed from the system, it may
be generated during cell decay. The two internal storage products of the phosphorus
accumulating organisms are:
1. internally stored COD (xbt); and
2. poly-phosphate (xpp).
Neither of these two components is included in the mass of the phosphorus accumulating
organism. The last state variable is the slowly biodegradable substrates (xs) which must
undergo hydrolysis before available as a substrate. Particulate inorganic inert solids (xii)
are present in the system, but do not interact with the biological model.
The processes described by this model are separated into four groups:
1. processes involving hydrolysis;
2. processes involving heterotrophs;
3. process involving autotrophs; and
4. processes involving phosphorus accumulators.
The hydrolysis processes include the aerobic, anoxic and anaerobic hydrolysis of slowly
biodegradable organic material into soluble substrate (processes 1-3). The rate equations
are similar for the three processes; however, the rate constants under these different
environmental conditions are not well known. The hydrolysis of organic nitrogen is not
explicitly included in this model. Rather, a fraction of the organic particulate material is
assumed to be organic nitrogen and therefore hydrolyzes at the same rate as the organic
particulate substrate.
GPS-X Technical Reference