Sustainable treatment of methylene blue containing wastewater in a biochar and sludge integrated constructed wetland - microbial fuel cell system

Constructed wetland-microbial fuel cell (CW-MFC) is a novel technology for wastewater treatment coupled with electrical generation. This study evaluated rice husk biochar and sludge amended CW and CW-MFC systems for the removal and degradation of methylene blue (MB) from simulated wastewater. The open circuit system exhibited significantly enhanced removal efficiency (>99%) at higher MB loads (100–150 mg L −1 ), and stable voltage output of ~660–700 mV compared with CW. UV–Vis and FTIR analyses demonstrated the cleavage of thiazine ring, demethylation of dimethylamino groups, while GC–MS analyses confirmed the formation of biodegradable intermediates. CW-MFC effluents lacked persistent toxic intermediates such as Azure C and hydroquinone, indicating more complete mineralization. Microbial community analysis showed enrichment of electrogenic and fermentative taxa ( Clostridia , Citrifermentans , and Desulfovibrio ) in CW-MFC supporting coupled dye degradation and electrochemical activity. Functional predictions indicated the presence of xenobiotic degradation and energy metabolism pathways, with genes encoding laccase, catechol 1,2-dioxygenase, and cytochrome c in both systems. Phytotoxicity assays using Vigna radiata demonstrated detoxification, with germination increasing from 50 to 60% in untreated dye to 100% in both CW and CW-MFC effluents, while CW-MFC showed significantly enhanced plant growth, with 6–7% and 9–13% increases in root and shoot length, respectively, compared to CW. Collectively, the integration of biochar, sludge, and MFC in CW demonstrated potential for MB removal, detoxification, and enhanced electrochemical activity aligning with multiple sustainable development goals (SDGs) related to wastewater treatment (SDG 6), cleaner energy generation (SDG 7) and waste valorization (SDG 12). • Single chambered CW-MFC integrated with rice husk biochar (RHB) and sludge was developed. • Integrated open circuit CW-MFC achieved >99% removal of methylene blue (MB) dye. • UV–Vis, FTIR, GC–MS confirmed MB breakdown and detoxification in non-toxic products. • Microbes enriched by RHB-sludge integration supported degradation and electron transfer. • Energy metabolism and other functional genes ( LACC1, catA, NQO1) were stimulated.