A Mathematical Model for Chemical P-Removal

Dudley, J – WRc plc


The common view of chemical phosphorus removal is an equilibrium reaction between phosphate and iron or aluminium, resulting in the precipitation of a metal phosphate complex. A comprehensive model based on this was developed by Jenkins and Hermanowicz (1993). This view has been replaced by an alternative interpretation (surface complexation modelling), where the dominant mechanism at typical wastewater pH values is the precipitation of iron or aluminium hydroxide, with subsequent phosphate adsorption onto the hydroxide surface (Takács et al., 2006, 2011; Smith et al., 2008a, 2008b, 2011; Szabó et al. 2008) and has been accorded the status of the current best understanding of chemical phosphorus removal (WERF, 2008; WEF, 2008).
The Smith model has been extended to include the effects of alkalinity and sulphide on precipitation, competition for hydroxide surface by COD, the inclusion of temperature sensitivity, and the inclusion of pH as a calculated output.
WRc, supported by UK water companies, has evaluated this model at five UK sites. The model has one site-specific calibration parameter and the paper will describe the range found for the five sites, which is larger than that proposed in the original papers. The model explains why there can be an order of magnitude difference in performance between different sites for the same chemical dose.

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