The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal - Part 7: Application of the IAWQ model

The IAWQ Activated Sludge Model (ASM) No. 2 is a kinetic-based model and in corporates two simple processes for chemical precipitation and redissolutio n that are readily integrated with biological processes for carbon. nitroge n and phosphorus removal. This model was applied to experimental data colle cted as part of this study from parallel pilot-scale 3-stage Phoredox syste ms with and without simultaneous dosing of chemical precipitant. The precip itants tested were alum, ferric chloride and ferrous-ferric chloride. The m odel was calibrated to the control unit (without precipitant addition) in o rder to match effluent phosphate (P) predictions (and hence P removal) as c losely as possible. The same calibration was then applied to modelling the test unit (with precipitant addition). It was found that the default model input stoichiometry for the precipitation reaction (ideal 1:1 molar ratio o f metal ion (Me) to P) was suitable for ferric chloride addition at a 20 d sludge age, but did not accurately reflect the test system behaviour for al l experimental periods. A lower stoichiometry (0.60 to 0.75) was required f or alum at a 20 d sludge age, and for a blend of predominantly ferrous chlo ride at a 10 d sludge age. The input stoichiometry was further decreased un der P-limiting conditions. A simple approach to. and possible reasons for, the manipulation of the model stoichiometry are discussed in the light of o bserved stoichiometry from system P removal and fractionation data collecte d as part of this study. Furthermore, an alternative approach based on mani pulation of the precipitation (and hence redissolution) kinetic constant is suggested and evaluated using available experimental data. Model predictio ns and observed data in respect of polyphosphate (polyP) and suspended soli ds are also compared and discussed. It is concluded that the ASM No. 2 mode l provides a useful basis for modelling simultaneous P precipitation, provi ded certain minor modifications are made. Further investigation into the ki netics of the precipitation process(es) is recommended, particularly in rel ation to the effect of system sludge age. The model lends itself to further enhancement by incorporating additional physico-chemical processes.