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 incorporates two simple processes for chemical precipitation and redissolution that are readily integrated with biological processes for carbon, nitrogen and phosphorus removal. This model was applied to experimental data collected as part of this study from parallel pilot-scale 3-stage Phoredox systems with and without simultaneous dosing of chemical precipitant. The precipitants tested were alum, ferric chloride and ferrous-ferric chloride. The model was calibrated to the control unit (without precipitant addition) in order to match effluent phosphate (P) predictions (and hence P removal) as closely 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 of 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 all experimental periods. A lower stoichiometry (0.60 to 0.75) was required for alum at a 20 d sludge age, and for a blend of predominantly ferrous chloride at a 10 d sludge age. The input stoichiometry was further decreased under P-limiting conditions. A simple approach to, and possible reasons for, the manipulation of the model stoichiometry are discussed in the light of observed stoichiometry from system P removal and fractionation data collected as part of this study. Furthermore, an alternative approach based on manipulation of the precipitation (and hence redissolution) kinetic constant is suggested and evaluated using available experimental data. Model predictions and observed data in respect of polyphosphate (polyP) and suspended solids are also compared and discussed. It is concluded that the ASM No. 2 model provides a useful basis for modelling simultaneous P precipitation, provided certain minor modifications are made. Further investigation into the kinetics of the precipitation process(es) is recommended, particularly in relation to the effect of system sludge age. The model lends itself to further enhancement by incorporating additional physico-chemical processes. WaterSA Vol.27(2) 2001: 151-168