In this study, the effects of the combination of acid hydrolysis and Donnan dialysis on the high-quality struvite production from digested sewage sludge were examined. The Box-Behnken design was applied in the hydrolysis using oxalic acid for the optimization of the conditions that affect the nutrients and metal release from digested sludge. An optimal condition was obtained at oxalic acid 0.5 M, acid/sludge ratio (mL/g) 10/1, and reaction time 60 min. The separation of the metals from hydrolyzed sludge liquid was carried out with the Donnan dialysis using a Nafion 117 cation exchange membrane. At the end of the 4 hours of operating time, the recovery values for Zn, K, Na, Mg, Fe, and Al were obtained as 67.9%, 62.1%, 57.6%, 39.4%, 5.3%, and 2.5%, respectively. The produced struvite meets the legal limits for fertilizer use in terms of Cd, Cu, Ni, Pb, Zn, Hg, and Cr content specified by Turkish regulations. A high Ca/Mg molar ratio and the presence of K, Na, and Al affected the efficiency of struvite crystallization. The use of oxalic acid resulted in low Ca release, and the metal ions that affect struvite purity were eliminated using Donnan dialysis.

Hydrolysis of phosphorus and metals from anaerobically digested sewage sludge was tested using inorganic acids (H 2 SO 4 , HCl, and HNO 3) and organic acids (citric, oxalic, and acetic). Then, the optimize conditions for release of high phosphorus and low metals from digested sludge using H 2 SO 4 by Box-Behnken design was investigated. Optimum PO 4-P and metals (Ca, Mg, Na, K, Al, Fe and Zn) release was obtained at H 2 SO 4 0.3 M, acid/sludge ratio (mL/g) 10/1 and mixing time 90 min, respectively. Donnan dialysis having a Nafion 117 cation exchange membrane was employed the selective separation of released PO 4-P and metals in the hydrolysed sewage sludge liquid obtained at optimum conditions. HCl at different concentrations (0.1 and 1.0 M) were used in receiver side. High levels of metals (Ca 78.73%, Mg 38.11%, Na 49.43%, K 64.62%, Al 97.59% and Zn 34.73%) were passed the receiver side using 1 M HCl for 24 h. Hence, it was observed that selective separation of phosphate and metals from digested sludge using Donnan dialysis process was achieved for phosphorus recovery.

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