Sequential Evaluation of Liquid-to-Gas Ratio, Photoperiod, and Light Intensity for Chlorella vulgaris-Based Biogas Upgrading in a PBR–Absorption Column System

Abstract

Biological biogas upgrading using microalgae offers a sustainable route for simultaneous CO2 removal and biomass production. This study sequentially evaluated liquid-to-gas ratios, L/G, of 0.6–4.0, photoperiods of 0:24–16:8 h, and light intensities of 150–400 µmol m−2 s−1 in a semi-continuous photobioreactor–absorption column (PBR-AC) with Chlorella vulgaris under moderate alkalinity conditions of 1053–1350 mg L−1 CaCO3. The system operated at D = 0.1 d−1, a gas flow of 0.05 L min−1, and GRT of 1.30 h. Increasing L/G from 0.6 to 4.0 improved CO2-RE from 67.9% to 81.6% and CH4 from 77.0% to 82.9%, showing that intensified recirculation partly compensated for the moderate carbonate-buffering capacity. Among illuminated photoperiods, 16:8 h performed best, reaching 81.4% CO2-RE and 81.7% CH4. At L/G = 4.0 and 16:8 h, increasing photosynthetic photon flux density (PPFD) from 200 to 300 µmol m−2·s−1 further improved CO2-RE from 81.4% to 82.86%, CH4 from 81.7% to 84.4%, and biomass productivity from 0.230 to 0.250 g L−1 d−1. The dark control achieved 57.06% CO2-RE, indicating substantial physicochemical CO2 absorption, while illumination added up to 24.35 percentage points. Overall, the system showed strong upgrading potential under moderate alkalinity, although O2 contamination, which was 1.5–2.5%, remains a key limitation.