GPS-X Technical Reference

179 Suspended Growth Models
The methodology for the determination of the state variables xba, xbh, xs, xu, xi, xsto, si,
salk, snn, ss, snd and xnd are identical to that for anaerobic ponds.
Oxygen is determined by assuming that the DO is saturated in the aerobic zone, and zero
in the anaerobic zone. The concentration of so is determined from the following equation:
Equation 6.54
where:
SOST = saturated oxygen concentration (gO 2m 3 )
AERDEPTH
= fraction of depth that is aerobic (unitless)
For the nitrogen composite variables, nitrate is assumed to be converted to nitrogen gas in
the anaerobic zone; therefore:
Equation 6.55
As anaerobic conversion of biomass and biodegradable nitrogen only happens in a
fraction of the pond, the increase in ammonia (snh) as shown in the anaerobic model is
multiplied by AERDEPTH.
Aerated Ponds
The aerated pond uses the same empirical BOD and TSS reduction models as the
facultative pond model. However, the default value for the BOD removal rate coefficient
has been changed to reflect aerated conditions (Eckenfelder, 1980).
The methodology for the determination of the state variables xba, xbh, xs, xu, xi, xsto, si,
salk, snn, ss, snd and xnd is identical to that for facultative ponds.
Oxygen is assumed to be saturated at the effluent point of the aerated pond. Due to this
completely oxic environment, nitrate is assumed to remain unchanged, and is mapped
through the pond object.
Ammonia is assumed not to change, due to the assumption that most of the biomass will
be settled out of the water column. Therefore, snh is mapped through the pond object.
GPS-X Technical Reference