GPS-X Technical Reference

137 Suspended Growth Models
Although there is no well accepted definition of the state of colloidal substrate in
wastewater, for the purpose of modelling, it is assumed that this fraction represents the
portion of the substrate COD which lies in the size range of 0.45 micrometers to 1.2
micrometers. These are typically macromolecules which require hydrolysis before
oxidation. This fraction of COD is assumed to behave differently in different unit
processes for example the colloidal COD will behave:
as soluble in solid liquid separation in settlers
as particulate in biological degradation
as particulate or soluble in membrane separation
Soluble Inorganic
In addition to the soluble organic states, the MANTIS2 model considers inorganic states
of soluble inorganic carbon (stic), soluble nitrite-N (snoi), soluble nitrate-N (snoa),
soluble Ammonia-N (snh), soluble Organic nitrogen (snd), soluble ortho-P (sp),
dissolved calcium (sca), dissolved (smg), dissolved potassium (spot), dissolved anion
(sana) and dissolved cation (scat). These inorganic states are chosen so as to
appropriately describe the N and P transformation, inorganic precipitation and pH
changes across the various unit processes in wastewater treatment plant.
The soluble inorganic carbon, stic, represents the sum of carbon in all the ionic species in
the carbonic acid system i.e. H 2CO 3, HCO 3- , CO 3
2-
. In previous models like ASM2D and
New General, the alkalinity (salk) is used to express the buffer capacity in the
wastewater. In MANTIS2, soluble inorganic carbon is used instead of alkalinity as the
state variable as it is a conserved quantity. The alkalinity in the model is estimated by
considering the soluble inorganic carbon and the estimated pH of the system.
Two oxidized form of soluble nitrogen, (i.e. soluble nitrite and nitrate) are considered in
the model. This choice is necessary to model the two step nitrification process.
The additional inorganic states like soluble calcium (sca), soluble magnesium (smg) are
introduced to model the key precipitation reaction involving these ionic species. The state
variable of soluble potassium (spot) is included to model the uptake and release of
potassium during polyphosphate formation and degradation. The dissolved anion (sana)
and cation (scat) represent all other strong anion and cation in the wastewater. These
states are used in formulating the charge balance equation for estimating the pH in the
wastewater.
GPS-X Technical Reference