Electro-Fenton based treatment for tannery wastewater and by-product valorization

The discharge of properly treated wastewater is essential to protect human health and the environment. Wastewater Treatment Plants (WWTPs) are responsible for treating sewage generated by urban and industrial sources. In many countries, the tannery sector plays a significant role in the industrial economy, contributing notably to manufacturing processes, export activities, and employment generation. Nevertheless, the processing of raw animal hides involves the significant use of chemical agents and substantial water consumption, resulting in the generation of complex and highly polluted tannery wastewater. However, conventional biological processes in WWTPs are unable to remove recalcitrant and non-biodegradable compounds. Moreover, high concentrations of low-biodegradability pollutants can compromise the performance of biological treatments, requiring the implementation of physicochemical processes. In this context, electrochemical advanced oxidation processes (EAOPs) have attracted considerable attention in recent years due to their high removal efficiency for poorly biodegradable compounds and their operational simplicity, while also demonstrating cost-effective advantages when applied as a polishing step or under low organic load conditions, as they enhance wastewater biodegradability and improve the efficiency of subsequent biological treatment processes. This study aimed to assess the feasibility of an electro-Fenton like process for the treatment of secondary tannery effluents, targeting the removal of the recalcitrant COD at neutral pH, thus avoiding the use of chemical reagents. The efficiency of the electrochemical peroxidation (ECP) process was first investigated at laboratory scale, focusing on the impact of different electric current on COD and color removal, as well as on the characteristics of the resulting sludge. The influence of current density, electrolysis time and H2O2 concentration were investigated through a central composite design combined with response surface methodology (CCD-RSM), to maximize COD and color removals, while improving the cost-effectiveness of the process. Following the laboratory-scale optimization, a continuous serpentine plug-flow electrochemical reactor was tested at pilot scale as a tertiary treatment for tannery effluent. Key findings highlighted a COD and color removal efficiency ranging from 68-74% to 84-96%, respectively, supporting feasibility for industrial-scale implementation. Concurrently, the valorization of ECP-derived by-products as additives in dark fermentation was investigated as a potential resource recovery strategy, highlighting a promising pathway for waste reuse within a circular economy framework.