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Chemical modelling for the precipitation of metal ammonium phosphate salts from wastewater solutions

Webb, Kelvin March (1996) Chemical modelling for the precipitation of metal ammonium phosphate salts from wastewater solutions. PhD thesis, Murdoch University.

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The precipitation of struvite (MgNH4PO46H2O) in a variety of wastewaters and other environmental sources has been described by a number of authors. This technique offers the opportunity of simultaneous removal of soluble nitrogen (in the form of ammonia) and phosphorus (as ortho-phosphate) from wastewaters, thus moderating their impact in natural receiving waters, as well as producing a slow release fertilizer of considerable agronomic value.

The solution chemistry of struvite precipitation is not well described, despite a number of pilot scale investigations having been mounted.

A model of struvite solution chemistry was developed for pure component (Mg-NH3-PO4-H2O) solutions in order to elucidate the precipitation.

So that this model might be applied more generally to appropriate wastewater solutions, the significance of (struvite) non-reacting components was also considered, particularly solution ionic equilibrium.

Analysis of literature data for a range of wastewater solutions indicated a list of components of greatest significance to the wastewater ionic equilibrium.

A series of ionic equilibrium simulations were performed, based on the components and concentrations determined above. Directed by the results of these simulations, a number of field site wastewaters (suitable for struvite precipitation) were selected and analysed.

Field wastewater sample analysis extensively characterised the solutions, both by component chemical analysis and acid-base titration. The acid-base titrimetric analysis has proven useful on a number of levels. In the first instance, complete pH scans from 2 to 12 furnish data as to possible struvite precipitation process pH operating conditions. At a further level, the nature of the experimental (automated) titration apparatus and the data collected meant a novel solution ’fingerprinting’ type technique was developed which clearly identifies a range of the major acid-base components (and concentrations). This work was conducted in the context of its application to struvite precipitation from the analysed samples, however the technique may find wider application in a variety of water and wastewater technologies.

The titrimetric analysis of composite solutions (based on the field and literature solution samples) identified these equilibria and precipitation independent of the non-ionic organic and particulate constituents of the field samples. The combined data of composite and field solution samples (and further equilibrium calculations) has indicated significant component interactions and how they relate to struvite precipitation and the development of a comprehensive struvite solution chemistry model for wastewaters.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Supervisor(s): Ho, Goen and Scott, Bill
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