Assessment of Selected Parameters in CO₂ and CH₄ Mass Transfer During Photosynthetic Biogas Upgrading Using Bubble Columns Filled with Wastewater-Derived Microalgae

This study investigated the simultaneous effects of four variables─column packing, diffuser pore size, biogas flow rate, and liquid height─on the mass transfer of CO2 and CH4 in bubble columns for photosynthetic biogas upgrading. Two bubble columns were used: an empty column and a column packed with granular media, both filled with wastewater-derived microalgae. The microalgal suspension was obtained from a high-rate algal pond that treated the anaerobic effluent from an upflow anaerobic sludge blanket reactor fed with urban wastewater. The results indicated a direct relationship between the methane–water volumetric mass transfer coefficient (KLa) and both biogas flow rate and liquid height in both columns. Although higher KLa values were observed at increased biogas flow rates, higher masses of methane were transferred to the liquid phase at lower flow rates due to longer contact times. Additionally, lower biogas flow rates enhanced CO2 transfer, driving it toward saturation in the liquid phase, whereas extended contact times led to oxygen enrichment of the biogas. The packed column achieved higher KLa, suggesting that the granular media fragmented biogas bubbles, preventing coalescence and improving gas–liquid contact. However, connecting a coarse bubble diffuser to the packed column proved disadvantageous due to high oxygen enrichment and methane loss. By contrast, the use of a fine bubble diffuser in the empty column improved biogas energy potential, reduced oxygen enrichment, and enhanced methane retention, resulting in a more efficient upgrading process. Overall, this study revealed a positive energy balance under the tested conditions and demonstrated that optimizing diffuser and column design is crucial for enhancing the energy efficiency of biogas upgrading.