The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate remova - Part 6: Modellingof simultaneous chemical-biological P removal - Review of existing models

This paper reviews three published models for simultaneous chemical phospho rus precipitation in activated sludge systems using metal salts. In the fir st, a chemical equilibrium approach is used. based on observations made fro m batch and continuous-flow tests, a theoretical formula for metal (e.g. fe rric) hydroxy-phosphate and a set of metal phosphate complexes or ion pairs for dissolved orthophosphate (orthoP) species. Apart from applying the pre cipitation stoichiometry observed in admixture with activated sludge, in th is model no interaction between the chemical and biological mechanisms is a ccounted for and no biological processes are modelled. In the second model, a combined equilibrium-kinetic approach is used to model the chemical and biological processes. The chemical and biological processes become kinetica lly linked through soluble orthoP as a variable. This model includes biolog ical processes for conventional activated sludge systems, but does not incl ude biological excess P removal processes (BEPR). Apart from this limitatio n, a potential problem in the combined equilibrium-kinetic approach was ide ntified. The precipitation reactions were modelled based on equilibrium che mistry and assumed to be complete at the start of simulation, precipitate, therefore, could not form dynamically during the ensuing kinetic simulation . Furthermore, the model predictions were very sensitive to the choice of c ertain key equilibrium (or solubility product) constants. The third approac h was to model the precipitation (and dissolution) reactions as kinetic pro cesses within a fully kinetic model for activated systems, including the pr ocesses for BEPR. This approach depends on the appropriate selection of rat e constants for the forward (precipitation) and reverse (dissolution) react ions. In effect, a number of reactions from equilibrium chemistry are combi ned and replaced with one "surrogate" reaction having its own apparent equi librium constant. The kinetic approach offers a number of advantages but is still subject to the limitation that it requires calibration against actua l data from activated sludge systems in which simultaneous precipitation is applied. Moreover, interaction between the chemical and biological P remov al mechanisms in the model is confined to "competition" for available solub le orthoP. This aspect requires further examination.