<p>The treatment of textile wastewater sludge (TWS) and cattle manure (CM) is a serious environmental concern because of their high organic content, toxic compounds, and potential greenhouse gas emissions. This study explores the feasibility and optimization of co-digesting TWS and CM in an upflow anaerobic sludge blanket (UASB) reactor for improved biogas yield, process efficiency, and digestate quality. Two laboratory-scale UASB reactors with a working capacity of 5 L each were maintained at mesophilic temperatures (35 ± 1&#xa0;°C) with varying proportions of textile wastewater sludge to cattle manure (TWS:CM = 75:25, 50:50, and 25:75, based on volatile solids). The ratios (TWS:CM) represent the proportion of textile wastewater sludge to cattle manure on a volatile solids (VS) basis. The influence of rice straw biochar amendment (2&#xa0;g L⁻¹) on reactor performance and digestion kinetics was also investigated. The best proportion of co-digestion was found to be 25:75 (TWS), which resulted in a maximum chemical oxygen demand (COD) removal of 94.2% and biogas production rate of 1.62 L L⁻¹ reactor d⁻¹, with methane concentration varying between 68 and 72%. Biochar addition increased methane production by up to 80% compared to the least efficient substrate ratio and shortened the lag period of methanogenesis by 64%, as modeled by the modified Gompertz equation. Analysis of heavy metals showed efficient stabilization of Cr, Cu, Zn, Ni, and Pb in the digestate. The concentrations of these heavy metals were found to be below the international regulatory limits for agricultural use. The findings of this study demonstrate the synergistic effects of co-digestion and biochar addition in UASB reactors and provide a viable circular economy strategy for the production of renewable energy and bioresources from industrial and agricultural wastes.</p>