Phosphorus Recovery and VFA Generation from Waste Activated Sludge Alone and Co-thickened with Primary Sludge

Primary sludge (PS) co-thickened with WAS and separate WAS from two high purity oxygen activated sludge (HPOAS) treatment facilities were fermented to explore the potential for phosphorus recovery and VFA production. It was found that the co-thickened HPO sludge produced, on average, concentrations of 6200 mg VFA L-1 and 8300 mg VFA L-1 from the semi-continuous fermenters with 2 days and 3 days hydraulic retention time (HRT), respectively. Solubilization of P occurred during the semi-continuous fermentation of co-thickened sludge resulting in average soluble phosphorous concentrations of 113 mg P L-1 at 2 days HRT from the NEWPCC and 178 mg P L-1 from the SEWPCC. Batch fermentation-of WAS produced significantly less soluble P and VFA than the co-thickened sludge; 46 mg P L 1 and 645 mg VFA L-1 after 2 days of fermentation. Phosphorus was recovered from fermented WAS supernatant by struvite precipitation. Magnesium was dosed by electro-dissolution using magnesium sacrificial anode. Increasing applied current density resulted in elevated phosphorus removal rates. No upper limits were observed in the studied current range of 50 mA to 200 mA. Phosphorus removal rate was proportional to the current density. The highest P-removal rate achieved was 4.0 mg PO4-P cm-2 h-1 at electric current density of 45 A m-2. Ammonia and phosphorus removal rates suggest high purity of precipitated struvite.