Tandem “heterogeneous catalysis/biological degradation” optimization for the treatment of recalcitrant compounds in industrial streams

Abstract

This study developed a combined Advanced Oxidation Process (AOP)/biological treatment to reduce the recalcitrant organic load in an industrial black liquor from alkaline sulfite pulping. The methodology involved a Fenton-type reaction using a 2.5 % CuO/γ-Al₂O₃ catalyst, optimized to minimize Cu²⁺ leaching (reduced Cu²⁺ leaching from 8.2 mg/L to 1.0 mg/L) and maximize catalyst reuse. The catalyst demonstrated high stability, maintaining effectiveness for up to five cycles. The optimized Fenton pre-treatment enhanced biodegradability, increasing the Biological Oxygen Demand/COD ratio to 0.23 (initial ratio of 0.05), and exhibited minimal toxicity to the microbial community. Subsequent biological treatment was carried out using activated sludge cultured in an aerobic laboratory-scale reactor with partial biomass recycling. Operating conditions were 80 °C, 65.1 mg/L of supported copper (0.32 g of catalyst) and 3.23 g/L of H₂O₂. The combined AOP/biological system achieved a global 86 % TOC reduction and 93 % COD reduction. With the combination of processes, the current Argentine regulations regarding COD levels in industrial discharges are complied with, adhering to the maximum allowed limit of 250 mg/L in most provinces. Although some Cu leaching occurred, the dissolved copper retained catalytic activity, likely due to complexation with oxidation intermediates. The biological stage effectively treated the pre-oxidized liquor without requiring biomass acclimatization, maintaining microbial activity. These findings confirm the long-term viability of this integrated treatment for industrial applications. Future work should focus on scaling up the process, particularly addressing agitation requirements during the AOP stage.