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

• Acid-treated concrete sludge (A-CS) was used for phosphate (Pi) recovery.
• The Pi removal efficiency of CS particles was improved by soaking them in HCl.
• A-CS slurry served as a bifunctional adsorption-aggregation agent.
• A-CS slurry effectively recovered Pi from anaerobic sludge digestion liquor.
• The recovered product could be used as a slow-release Pi fertilizer.

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 CaCO3, thereby releasing approximately 87% Ca2+ 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 (Pi) at the Ca/P molar ratio of 2.5, A-CS showed 20 times higher Pi removal efficiency than that of untreated CS particles. Although A-CS could precipitate 72% Pi in 5-min free sedimentation, the Ca2+-rich liquid fraction of A-CS alone led to the precipitation of only 48% Pi. This suggests that the solid fraction of A-CS can serve as an auxiliary aggregation agent. When Pi recovery was examined using anaerobic sludge digestion liquor from a full-scale wastewater treatment, A-CS could recover 96% Pi at the Ca/P molar ratio of 2.5. Citrate-soluble P2O5 accounted for 19% of the dry weight of the recovered Pi product. The levels of heavy metals such as As, Cd, Pb, Ni, and Cr in the recovered Pi product were much below their regulatory standards for fertilizers.

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