Protein recovery from fish canning wastewater using PEG/sodium citrate aqueous two-phase systems

Polyethylene glycol (PEG) - sodium citrate aqueous two-phase systems (ATPS) were evaluated for the concentration and recovery of proteins from fish canning cooking wastewater. Tuna fishmeal was used as a model protein source, and partition coefficients were experimentally determined at 298.2 K. The effects of key operational parameters, including PEG molecular weight (1500–20,000 Da), sodium chloride concentration (1–6%wt), and pH, were systematically investigated. Particular attention was paid to sodium chloride concentration, a parameter often overlooked in ATPS design despite its relevance for the treatment of complex saline effluents. Proteins preferentially partitioned into the PEG-rich top phase. Partitioning was enhanced by lower PEG molecular weight and higher sodium chloride content, consistent with conditions representative of fish processing. Protein partitioning showed a strong dependence on pH, correlating with protein solubility and isoelectric behavior, with optimal performance observed under alkaline conditions. Under optimized conditions, partition coefficients up to 7.5 were achieved. The applicability of the system was validated using real cooking wastewater from industrial tuna and sardine processing. ATPS formation via direct addition of phase-forming components induced partial protein precipitation and concentration in the PEG-rich phase, enabling up to 94% total protein recovery. Overall, PEG‑sodium citrate ATPS demonstrates strong potential as an efficient and scalable strategy for protein recovery and valorization of fish canning wastewater within a circular economy framework.