Phosphorus removal using ferric–calcium complex as precipitant: Parameters optimization and phosphorus-recycling potential

• A novel technology based on the complex of ferric and calcium salts was developed.
• The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1.
• The optimized parameters were: Fe/Ca = 2.4:1, A/P = 1.5:1, pH = 7, FMS = 100 rpm.
• The novel technology cut down the dosage by 36% compared with the traditional one.
• The novel technology increased the phosphorus (P2O5) content of precipitate to 30%.

Pollution control and phosphorus recycling have become one of focuses in the field of wastewater treatment. Both the ferric and calcium salts are effective chemical reagents for the phosphorus removal from wastewater. Based on the complex of these two chemical reagents, a novel technology was developed. The results showed that ferric and calcium had an antagonistic effect at Fe/Ca molar ratio of 1:10–1:1, but a synergistic effect at Fe/Ca molar ratio of 1:1–10:1. The strongest synergistic effect was observed at Fe/Ca molar ratio of 7:3–4:1. When the wastewater with concentration of 100 mg P/L was treated using the ferric–calcium complex as precipitant, the optimized parameters were: Fe/Ca = 2.4:1, ferric–calcium complex/phosphorous (A/P) = 1.5:1, pH = 7, fast mixing speed (FMS) = 100 rpm. Compared with the traditional technology using ferric salt as sole precipitant, the phosphorus removal technology using ferric–calcium complex cut down the dosage by 35.53%, leading to a low cost of $ 3.69/kg P; and it increased the phosphorus (P2O5) content of precipitate up to 29.77%, indicating a good potential for recycling. The phosphorus removal technology using ferric–calcium complex as precipitant provides a prospective alternative for the removal and recycling of phosphorus in wastewaters.