GPS-X Technical Reference

131 Suspended Growth Models
Nitrogen Source for Cell Synthesis
In reviewing the initial asm1 model version, Dold and Marais (1986) postulated that
under certain circumstances, nitrate, instead of ammonia nitrogen, may serve as the
nitrogen source for cell synthesis purposes. This postulate was confirmed from analysis
of data collected over an extensive period, particularly in multiple series reactor
configurations operated at long sludge ages and which exhibited high nitrification rates.
The use of nitrate as a nitrogen source for polyP organism synthesis, when the ammonia
concentration dropped to low levels, was also observed by Wentzel et al., (1989b). On
the basis of this information, an additional two processes have been incorporated into the
asm1 model version to give four growth processes: aerobic and anoxic growth of nonpolyP
heterotrophs with either ammonia or nitrate as the N source for synthesis.
Growth of non-polyP heterotrophs on SCFA
In the asm1 model, readily biodegradable soluble COD is utilized by the non-polyP
organisms (i.e. heterotrophs) in four possible growth modes - under aerobic or anoxic
conditions with either ammonia or nitrate as the nitrogen source for synthesis purposes.
For biologically-enhanced phosphorus removal (BEPR) systems it is necessary to
distinguish between "complex" and SCFA readily biodegradable COD; therefore, it is
necessary to duplicate the four growth processes in the asm1 model to account for
possible growth on the two components of the readily biodegradable COD for the mixed
culture system. With regard to growth on SCFA it is likely that only one of the four
processes would be of consequence - anoxic growth with ammonia as the N source. This
is because SCFAs are removed in the unaerated zones at the "front end" of the continuous
flow systems and do not enter the aerobic zones in appreciable concentrations. However,
for completeness all four growth processes in the asm1 model (for "complex" COD)
were duplicated in the general model for growth of non-polyP organisms with SCFA as
substrate. The same kinetic formulations and stoichiometry for growth on "complex"
readily biodegradable COD and SCFA have been used.
General Model Components (PolyP Organisms)
The general model component proposed for describing the kinetic response of the polyP
heterotrophic organism mass is based on the enhanced culture model of Wentzel et al.
(1989b), with one additional process for anoxic growth of polyP organisms described by
Dold (1990) (and the one additional stoichiometric constant, fup, for anoxic growth). It
should be noted that the phenomenon of accumulation of un-biodegradable soluble COD
from endogenous processes in the enhanced cultures also received attention. Wentzel et
al. (1989a) suggested that this material would be used as a substrate source by non-polyP
organisms in mixed culture systems.
GPS-X Technical Reference