Biochar application in anoxic–oxic membrane bioreactor for azo dye removal: Effects of in-reactor addition and post-treatment adsorption

This study investigated the use of biochar as an adsorbent in an anoxic–oxic membrane bioreactor treating azo dye-containing wastewater. Three strategies were evaluated: (1) conventional reactor operation, (2) biochar addition to the mixed liquor, and (3) post-treatment using a fixed-bed biochar adsorption column. COD removal remained high (>98%) in strategies 1 and 2, indicating that biochar did not affect organic matter removal. Respirometric assays showed higher heterotrophic activity during the biochar-amended period, with the specific oxygen uptake rate increasing from 8.6 ± 0.7 to 10 ± 1.1 mg O 2 g -1 MLVSS h -1 . Biochar incorporation improved dye removal (79.1% to 89.2%) and UVA254 reduction (38.1% to 45.5%). Batch assays showed that the dye removal rate increased from 0.64 to 1.79 mg RBV-5R gVSS⁻¹ h⁻¹ during the biochar-amended period. Concurrently, membrane fouling was significantly mitigated, with an 87% reduction in fouling rate, accompanied by improved sludge filterability, as reflected by lower sCOD, CST, and SRF values. The highest effluent quality was achieved with post-treatment, resulting in 98.1% dye removal and 73.5% UVA254 reduction. Overall, in-reactor biochar improved dye removal, enhanced heterotrophic activity, and mitigated membrane fouling, whereas post- treatment provided additional polishing of residual dye, color, and aromatic compounds.