GPS-X Technical Reference

Other Models 302
Chemeq Precipitation Model
The P-removal model is available only in the CNP library. The model is based on the
stoichiometry of the chemical precipitation of soluble phosphorus. The model is set up to
simulate the addition of one of four possible chemicals:
1. alum;
2. ferric compounds;
3. ferrous compounds; and
4. other user defined metal compounds.
In each case, the dosage is expressed in terms of mass of metal ion. For example, for
dosing alum, the dosage is expressed in terms of mass of aluminum ion.
As an example, the basic reaction of phosphorus with aluminum is shown in Equation
11.1.
Equation 11.1
Therefore, one mole of aluminum ion reacts with one mole of phosphate or, on a mass
basis; one gram of aluminum ion (Al 3+ ) reacts with 1.148 grams of phosphorus (P). The
dosage that is required is the mass of aluminum ion that is added rather than the mass of
alum, which may be in different forms (hydrated, etc.). This method of calculating the
stoichiometric requirements for alum and the other chemicals avoids the issue of different
compounds and their molecular weights. The phosphorus stoichiometric coefficients for
alum and the other chemicals are already defined, but could be modified for modelling
another chemical that is not predefined.
Although the stoichiometric ratios are used as the basis of the precipitation model, the
complexities of wastewater chemistry, including the effects of pH, alkalinity and other
elements, means that the stoichiometric ratio is only the maximum achievable phosphorus
removal in presence of a large overdose of chemical. In practice, a significantly lower
removal is achieved because all of the chemicals added are not available for phosphorus
addition. However, the available fraction, and therefore the required dosage, must be
estimated or calibrated from site to site and specified in the model.
GPS-X Technical Reference