Engineering an Automated Microbubble-assisted Hybrid Photobioreactor for CO₂ Capture and Valorization to Polyhydroxybutyrate in Indigenous Algal Biomass

Saptarshi Dey¹Vivek Dalvi¹Biswajit Samir De²Abhishek Sahu¹Anushree Malik^1*

• ¹Centre for Rural Development and Technology, indian institue of technology delhi, Delhi, India
• ²Department of Chemical Engineering, indian institue of technology delhi, Delhi, India

Integration of microalgal systems into carbon capture technologies offers a dual advantage; mitigation of anthropogenic CO₂ emissions and sustainable production of high-value macromolecules. The study presents the engineering and pilot scale operation of a 200 L microbubble-assisted hybrid photobioreactor for CO₂ bio-fixation. Subsequent valorization into lipids and biopolymers was obtained using an indigenous alga, Poterioochromonas malhamensis. A novel 3D-printed microbubble generator assembly (MBG) was retrofitted to a 1.2 m carbonation column (CC) and integrated with a 200 L high-rate algal pond (HRAP). Sequential high speed bubble imaging at different column heights 'H', under different liquid flow (QL) and gas flow (QG) regimes were processed and interpreted using a MATLAB based bubble analyser. Gaussian-distribution function was used to established the most probable bubble diameter (DM) in the range 400 – 800 µm while achieving microbubble density in the range 61 % - 90 % in the CC supporting efficient gas–liquid exchange. The hybrid reactor was further automated using a real-time pH-feedback loop for CO₂ dosing, under photoautotrophic conditions with 5 % (v/v) CO₂ supplementation. The system maintained the culture media bicarbonate buffer in the optimal range (pH 7.2–8.5). The hybrid reactor yielded 0.423 gL⁻¹ biomass having a carbon content of 43.06 % DCW, a CO2-biofixation rate of 44.05 mgL⁻¹d⁻¹, and Polyhydroxybutyrate (PHB) content of 5.79 % DCW. Our findings demonstrate the scalability, automation potential, and bioproduct yield enhancements of the hybrid system, making it a viable model for CCUS (carbon capture, utilization, and storage) through algal valorization. The approach offers a technically sound, energy-efficient route for transforming inorganic carbon into commercially relevant algal macromolecules.