GPS-X Technical Reference

GPS-X Composite Variable Calculations 42
Table 4-2 – CNLIB BOD, COD, and TSS Composite Variables (All Models)
SBODU XBODU BODU SBOD XBOD BOD SCOD XCOD COD VSS XISS X
si 1 1
ss 1 1 fbod fbod 1 1
xi 1 1 icv -1 icv -1
xs 1 1 fbod fbod 1 1 icv -1 icv -1
xbh 1 1 fbod fbod 1 1 icv -1 icv -1
xba 1 1 fbod fbod 1 1 icv -1 icv -1
xu 1 1 icv -1 icv -1
xsto 1 1 fbod fbod 1 1 icv -1 icv -1
xii 1 1
The COD composite variables are a sum of state variables (where units are gCOD/m 3 ). Soluble
COD (SCOD) is the sum of the soluble inert organics (si) and readily biodegradable substrate
(ss), while particulate COD (XCOD) is the sum of the slowly biodegradable substrate (xs), active
heterotrophic biomass (xbh), active autotrophic biomass (xba), cell internal storage product (xsto),
un-biodegradable particulates from cell decay (xu), and particulate inert organics (xi). The total
COD (COD) is the sum of the soluble and particulate COD.
The suspended solids composite variable (X) is calculated from the particulate COD (XCOD) by
dividing it by the XCOD: VSS ratio (icv), which changes the units of the XCOD to mgVSS/L,
resulting in the volatile suspended solids (VSS) composite variable. To calculate the suspended
solids composite variable (X), VSS is added to particulate inert inorganic material (XII). By
default in the CN library, particulate inert suspended solids (XISS) is equal to xii.
The biochemical oxygen demand (BOD) composite variables are calculated from the state
variables. First, the biodegradable state variables (the state variables that exert BOD, which are in
units of gO/m 3 ) are summed to provide both a particulate and a soluble ultimate BOD (XBODU,
SBODU). The sum of these components is the total ultimate BOD measurement (BODU). To
determine BOD 5, the calculated BODU is multiplied by a stoichiometric fraction, fbod, which is
the ratio of BOD 5:BOD U.
In terms of variables containing nitrogen, the calculation of the composite variables involves
adding up various state variables and multiplying other state variables by fractions as appropriate
(Figure 4-3). The soluble total Kjeldahl nitrogen (STKN) is the sum of the free and ionized
ammonia (snh), soluble biodegradable organic nitrogen (snd), and (in asm3 only) the nitrogen
components of soluble substrate (ss) and soluble inerts (si). The particulate total Kjeldahl nitrogen
(XTKN) is the particulate biodegradable organic nitrogen (xnd), plus the nitrogen component of
biomass (xbh and xba), unbiodegradable cell products (xu), particulate substrate (xs) and
particulate inerts (xi). Total Kjeldahl nitrogen (TKN) is the sum of soluble (STKN) and
particulate TKN (XTKN). The total nitrogen (TN) is the sum of the TKN and the nitrate nitrogen
(sno).
GPS-X Technical Reference