Phosphorus Fractionation of Activated Sludges from Modified Bardenpho Processes with and without Chemical Precipitant Supplementation

The natural occurrence of chemically precipitated phosphate in activated sludge (AS) has been hypothesized for over two decades and deserves fresh attention in view of the increasingly common practice of chemical addition to modified AS systems to achieve P removal. In this study, orthophosphate extracted from AS of modified Bardenpho plants could not be accounted for on the basis of polyphosphate interferences. In a plant not dosed with chemicals at least 14 mg P/g VSS were therefore formed by natural precipitation reactions while an identical plant dosed with ferric sulphate showed at least 37 mg P/g VSS to be of this fraction. A plant dosed with alum and receiving wastewater of mainly industrial origin contained up to 8 mg P/g VSS as chemical precipitate which was over half the total P content of that sludge. Restraint in dosing chemicals is suggested to prevent unnecessary suppression of the biological mechanism of P removal. Ferrous sulphate dosing may be superior to that with ferric sulphate Since the latter apparently causes poly P hydrolysis. Complex interactions between iron, ortho P and biomass are indicated. Furthermore, iron-ortho P complexes in mixed liquor supernatant may be unreactive in the colorimetric ortho P assay.