Sewage sludge management for phosphorus recovery as struvite in EBPR wastewater treatment plants.

The influence of separate and mixed thickening of primary and secondary sludge on struvite recovery was studied. Phosphorus precipitation in the digester was reduced from 13.7 g of phosphorus per kg of treated sludge in the separate thickening experiment to 5.9 in the mixed thickening experiment. This lessening of the uncontrolled precipitation means a reduction of the operational problems and enhances the phosphorus availability for its later crystallization. High phosphorus precipitation and recovery efficiencies were achieved in both crystallization experiments. However, mixed thickening configuration showed a lower percentage of phosphorus precipitated as struvite due to the presence of high calcium concentrations. In spite of this low percentage, the global phosphorus mass balance showed that mixed thickening experiment produces a higher phosphorus recovery as struvite per kg of treated sludge (i.e., 3.6 gP/kg sludge vs. 2.5 gP/kg sludge in separate thickening).