Biological carbon dioxide upgrading in waste matrices: gas-liquid configuration and operational control in...

Biological methanation of carbon dioxide (CO₂) with hydrogen (H₂) can upgrade biogas to biomethane, but reactor-design rules linking geometry to conversion kinetics remain poorly defined. To address this gap, a 15-run sequential-batch campaign was conducted in a mechanically stirred reactor using vinasse digestate (10 runs across two operational phases) and municipal sewage sludge (5 runs), under standardized pre-H₂ pulse sampling, without pH control, nutrient dosing, or selective enrichment. All runs showed sustained apparent first-order CO₂ depletion (R² = 0.67-0.98), with model-predicted times to 95% CO₂ depletion ranging from 6.3 to 34.3 d. The gas-liquid ratio (GL) was associated with the apparent depletion rate constant (k_app), an operational metric reflecting biological CO₂ conversion together with gas-liquid transport effects, following k_app = 0.040 + 0.059·GL (R² = 0.77) within the tested range. Increasing GL from ≈2 in Phase I to ≈5.5 in Phase II more than doubled k_app and halved the median completion time. Henry's-Law estimates indicated a larger physically dissolved CO₂ reservoir at low GL, highlighting a source of bias in headspace-based kinetics. Sludge runs at comparable GL showed similar functional depletion kinetics despite lower pH. Finally, a proof-of-concept 3-day H₂-withholding step reduced residual H₂ to 4-5% in one run per matrix, improving final gas quality without additional equipment. Together, these findings support an operational design framework for sequential-batch biomethanation within the tested operating range and reactor configuration.