Lowering the current density as a strategy to improve biogas upgrading and economic feasibility of a hybrid electrocoagulation – up-flow anaerobic sludge blanket reactor

While anaerobic digestion (AD) is widely used for biogas production, the high carbon dioxide (CO₂) content in biogas limits its energy potential. Conventional biogas upgrading techniques are often costly, energy-intensive or difficult to scale-up. This study investigated integration of electrocoagulation (EC) with up-flow anaerobic sludge blanket (UASB) as a new approach for in-situ biogas upgrading under low current density condition. Due to this integration involving iron electrodes, a current density of 0.42 mA/cm 2 , and an intermittent current exposure mode of 10-min-ON/30-min-OFF, all the carbon dioxide content in the biogas was successfully removed. Biogas production rate of 0.30 ± 0.01 L/L reactor .day and methane concentration of 78.2 ± 2.9 % were achieved in the EC-UASB system representing an increase of 8.2 % and 13.2 %, respectively, over the UASB system, while the hydrogen concentration in the biogas remained at less than 2.5 %. Furthermore, compared to the control system, an increase in the removal efficiencies of effluent total dissolved solids, turbidity, and chemical oxygen demand by of 17.8 %, 17.6 %, and 3.2 %, respectively was achieved. This newly introduced integration system under low current density also resulted in positive changes in the anaerobic sludge bed characteristics in terms of sludge settling ability (13 % improvement), tightly bound extracellular polymeric substances (24 % increase in protein/carbohydrate ratio), and particle size distribution (4–300 μm). The economic analysis revealed that these favorable outcomes, in terms of biogas upgrading and wastewater pollutant removal, were achieved with low additional energy consumption (0.094 kWh/m 3 ) and operating costs (0.033 USD/m 3 ), making this approach both environmentally sustainable and cost-effective. • Lowering current density of EC-UASB used as a strategy for in-situ biogas upgrading. • Complete CO₂ removal and improved CH 4 content achieved at low current EC-UASB. • Improvement in pollutants and turbidity removals achieved at low current EC-UASB. • Sludge image, size and morphology showed larger and denser sludge flocs in EC- UASB. • Low energy consumption and operating costs make EC-UASB sustainable and cost-effective.