A simple technology for phosphorus recovery using acid-treated concrete sludge

Abstract

Abstract A simple technology for phosphorus (P) recovery from aqueous solution has been developed using acid-treated concrete sludge (A-CS) as a bifunctional adsorption-aggregation agent. Dried particles of concrete sludge (CS), an alkaline waste containing hydrated cement and fine aggregates, were soaked in 1.3 M HCl at a concentration of 0.1 g/mL for 60 min. The HCl-soaking treatment solubilized alkaline substances such as Ca(OH) 2 and CaCO 3 , thereby releasing approximately 87% Ca 2+ from the CS particles to the acid solution. When A-CS was added to 500-mL synthetic anaerobic sludge digestion liquor containing 273 mg/L of phosphate (P i ) at the Ca/P molar ratio of 2.5, A-CS showed 20 times higher P i removal efficiency than that of untreated CS particles. Although A-CS could precipitate 72% P i in 5-min free sedimentation, the Ca 2+ -rich liquid fraction of A-CS alone led to the precipitation of only 48% P i . This suggests that the solid fraction of A-CS can serve as an auxiliary aggregation agent. When P i recovery was examined using anaerobic sludge digestion liquor from a full-scale wastewater treatment, A-CS could recover 96% P i at the Ca/P molar ratio of 2.5. Citrate-soluble P 2 O 5 accounted for 19% of the dry weight of the recovered P i product. The levels of heavy metals such as As, Cd, Pb, Ni, and Cr in the recovered P i product were much below their regulatory standards for fertilizers.