Effect of pH on phosphorus, copper, and zinc elution from swine wastewater activated sludge

With the goal of reducing the amounts of phosphorus (P), copper (Cu), and zinc (Zn) discharged from swine wastewater activated sludge treatment facilities, we studied the elution of these elements from activated sludge at various pH values. Sludge samples with neutral pH collected from three farms were incubated at pH values ranging from 3 to 10. The soluble concentrations of these elements changed dramatically with pH and were highest at pH 3. We assumed that P present in the sludge under neutral and alkaline conditions was in insoluble form bound up with magnesium (Mg) and calcium (Ca), because Ca and Mg also eluted from the sludge at low pH. To clarify forms of Zn and Cu in the sludge, we performed a sequential extraction analysis. Zinc in adsorbed, organically bound, and sulfide fractions made up a large proportion of the total Zn. Copper in organically bound, carbonate, and sulfide fractions made up a large proportion of the total Cu. The soluble P concentrations were lowest at pH 9 or 10 (11–36 mg/L), the soluble Zn concentrations were lowest at pH 8 or 9 (0.07–0.15 mg/L), and the soluble Cu concentrations were lowest at pH 6–9 (0.2 mg/L, the detection limit).