Improving paper mill effluent treatment: a hybrid approach using electrocoagulation and electrooxidation with oxone

Abstract

Paper mill effluent is considered to be one of the most polluting industries due to high chemical oxygen demand (COD), lignin, phenols, wood residues and recalcitrant compounds, and treatment of this challenging effluent often requires a combined treatment process using physical, chemical and biological treatment processes, which significantly treatment costs. The hybrid process of chemical coagulation (CC) with electrooxidation (EO) or electrocoagulation (EC) with EO can be a promising alternative for the treatment of these high strength effluents. The objective of this study was to investigate a hybrid treatment strategy using CC and EC as pre-treatment and EO as post-treatment for paper mill effluent. In the pre-treatment, CC with alum and FeCl3, and EC with different Fe and Al electrode combinations are investigated. In particular, iron-based CC or EC showed superior performance compared to Al-based, and a maximum COD removal efficiency of 33.5% was achieved under optimum conditions (1500 mg/L FeCl3 dosage, natural pH (7.5) and 30 min time) in the CC process, while in the EC process, Fe-Fe electrodes outperformed, achieving 49.9% COD removal under optimum conditions (2 A, pH 6, 70 min, 0.12 M NaCl). After selecting as EC pretreatment, different anodes (Ti/IrO2-Ta2O5, Ti/IrO2, Ti/Ir-Ru-SnO2, Ti/RuO2 and Ti/Pt) and cathodes (graphite and TiO2) were evaluated for the EO process, with the Ti/Pt anode and graphite cathode combination emerging as the most efficient, achieving 71.4% COD, 82.6% colour and 97.9% tannin/lignin removal at 3 A and 70 min. In order to meet the local discharge criteria, oxone was dosed at 15 mM in the EO process. This increased the COD removal by 97.4%. The hybrid process, which combined the EO-oxone treatment after the EC pre-treatment, showed excellent overall removal rates (> 98.4%) for all selected parameters. The specific energy consumption under optimum conditions was 17.75 kWh, 129.94 kWh and 105.83 kWh per kg COD removed for EC, EO and EO-oxone respectively, demonstrating the applicability of the hybrid approach in terms of energy efficiency.