Chemical fractionation of phosphorus in stabilized biosolids

Three chemicals - ferrous sulfate (FeSul), calcium oxide (CaO), and aluminum sulfate (alum) - were applied at different rates to stabilize P in fresh, anaerobically digested biosolids (FBS) obtained from an activated sewage treatment plant. A modified Hedley fractionation procedure was used to assess P forms in these sludge-borne materials and in a biosolids compost (BSC) prepared from the same FBS. Each biosolids material exhibited a unique pattern of P distribution among fractions. The most available P forms, namely: (i) water-soluble P (WSP); (ii) membrane-P; and (iii) NaHCO₃-P, were stabilized by small rates of each of the chemicals; but the P transformation into more stable forms depended on the type of chemical added. The stabilized P forms were enhanced by high rates of CaO and FeSul, but were reduced by high rates of alum. The organic P (P_o) in the first three fractions of the FeSul- and alum-stabilized biosolids was enhanced by the chemical addition, and P_o transformation from NaOH-P_o into NaHCO ₃-P_o was found in calcium-stabilized biosolids. A positive relationship was found between NaHCO₃-P_o and the NaHCO₃-extracted organic C in all chemically stabilized biosolids. One-step extraction by NaHCO₃ or NaOH underestimated P extraction compared to the stepwise extraction. The reported results are consistent with solid-state P speciation reported earlier and contribute important information for optimizing biosolids stabilization to reduce P loss after incorporation in soils and for maximizing soil capacity to safely store pre-stabilized biosolids.