Regulate oxygen concentration using a co-culture of activated sludge bacteria and Chlorella vulgaris to maximize biophotolytic hydrogen production

The biophotolytic hydrogen (H₂) production is contingent on proper oxygen (O₂) concentration regulation. Co-culturing bacteria along with microalgae leads to the uptake of excess O₂ produced by algal cells. This uptake of O₂ by the bacteria triggers the hydrogenase enzyme activity present in microalgae which is responsible for H₂ production. In this study, indigenous bacteria present in activated sludge were co-cultured with Chlorella vulgaris at different ratios. The 1:1.5 v/v (algae to bacteria) co-culture ratio produced the most significant H₂ gas volume (1246 mL L⁻¹) with a minimum O₂ content (57 mL L⁻¹) during 6 days. Gas production stopped when the pH dropped to about 4. The gas composition at different ratios consisted mainly of H₂ (29–37%) and CO₂ (34–35%) along with a minor fraction of N₂ (10–21%) and O₂ (2–4%). No methane (CH₄) was observed in any co-culture under experimental conditions. Considering the exceptional inverse relation in H₂ production and O₂ consumption, algae-activated sludge correlation leads to substantial improvement in the H₂ gas production compared to previous studies using pure bacterial cultures.