GPS-X Technical Reference

Influent Models 70
The stoichiometric fraction fss (soluble substrate:BODultimate), is used to determine
what fraction of the total BODultimate is soluble substrate (sf for asm2d or ss for all
other biological models). In terms of sampling measurements, the fraction fss can be
estimated from the ratio of filtered BOD 20 :BOD 20. The particulate substrate state variable
(xs) is then calculated by difference (i.e. in cnlib, xs = BODultimate * (1-fss) -xba-xbhxsto)).
The particulate inert organics (xi) state variable is calculated by subtracting all the
other particulate carbonaceous organic states (xsto, xbh, xba, xu (all input by user) and xs
(calculated above)) from the particulate COD (XCOD).
Calculation of the three unknown nitrogen state variables depends on the calculation of
TKN which is biological model-dependent. For instance, with the asm1 or mantis
models the amount of free and ionized ammonia (snh) is the fraction of ammonia (fnh)
multiplied by the TKN. The remainder is the biodegradable organic nitrogen, which is
partitioned into three areas including nitrogen associated with particulate state variables
(i.e. xbh, xba, xi and xu), and soluble and particulate organic nitrogen (snd and xnd) using
the fxn fraction and the nitrogen fractions of the particulate state variables (Figure 5-8).
Figure 5-8 - CN Library Nutrient Fractions for the Influent Models
This differs from the calculations made when asm3 has been chosen as the local
biological model because snd and xnd are not state variables in asm3. In this case,
soluble TKN is calculated as the difference between the total TKN and the particulate
TKN. The nitrogen associated with the soluble state variables (ss and si) is subtracted
from the soluble TKN and the remaining TKN is equated to ammonia (snh). Hence, in
the asm3 BOD based influent model, fnh and fxn are not used. Influent Advisor will
help users determine what parameters are necessary for each mode.
GPS-X Technical Reference