Optimization of electro-Fenton process applied to the treatment of codfish brines in a context of industrial symbiosis

The ancient method of preserving fish by salting is still widely practiced but generates two challenging waste streams: contaminated salt (solid) and brine (liquid). Conventional treatment methods are ineffective in reducing the organic content of brine due to its high salt content (≅ 25-30 % wt. NaCl). Although advanced oxidation processes are extensively used for treating certain wastewaters, their application to real saline effluents near saturation, such as food industry brines, remains underexplored. This study optimized the electro-Fenton (EF) process for treating real contaminated brines from the codfish industry, aiming to reuse the treated brines in the pickling stage of the tannery industry, thereby diverting waste streams from environment disposal. A central composite experimental design and response surface methodology were used to evaluate the effects of three EF process operating variables: (i) current density (76-429 A m^-2), (ii) electrolysis time (1.0-10.0 min), and (iii) hydrogen peroxide concentration ([H₂O₂], 50-201 mM), using iron electrodes. The primary goal was to maximize total organic carbon (TOC) removal from codfish brine. Additionally, the specific roles of reactive oxygen and chlorine species responsible for TOC removal (such as HO^•, Cl^•, ClO, O₂^•-/HO₂^• and HClO/OCl^-) were investigated using scavengers. The results revealed that O₂^•-, HO₂^• were the main active species. The optimal EF operating conditions were determined to be a current density of 275 A m^-2, electrolysis time of 5.2 min, and [H₂O₂] of 91 mM, resulting in a 70 % removal of TOC. The treated brines, diluted to ≅ 7.5-8.0 % wt. NaCl, were tested in hide pickling trials to assess their impact on the quality of the wet-blue leathers. Results showed that the treated brines did not affect leather quality; instead, they enhanced shrinkage temperature from 103 °C to 112 °C. This increase of shrinkage temperature broadens the potential applications of the leather, making it suitable for a wider range of markets and products. Furthermore, the chromium oxide content fixed in the leather increased from 4.1 % to 5.3 %, reducing chromium in the industrial wastewater generated at the end of the process. This valorisation of codfish brines presents a promising opportunity for industrial symbiosis between the codfish and the tannery industries.