Airlift photo-bioreactors for Chlorella vulgaris cultivation in closed-loop zero waste biorefineries

Highlights: • Design of a closed loop biorefinery for biodiesel and syngas production at pilot-scale. • Chlorella vulgaris for CO₂ capture and reuse of wastewater containing glycerol. • Experimental campaign in semi-CSTRs and external-loop airlift photo-bioreactors. • Dimensionless numbers and operating ratios are kept fixed in changing reactor type. • Set-up of optimal operating conditions by overall plant mass balancing. This study aims to set up the operating conditions of a battery of airlift photo-bioreactors (AL-PBRs) for Chlorella vulgaris cultivation integrated into a pilot-scale Biowaste-to-Biofuels (BtB) production plant, producing both biodiesel by transesterification of waste frying oils and syngas by wood chips gasification. Microalgae are fed with both wastewater rich in glycerol and flue gas. Mixing, mass transport, kinetics, and light conditions inside the AL-PBRs are chosen by keeping some dimensionless numbers ($\text{Re}$, $\text{Sh},{\text{Da}}_{\text{II}}$ and A) fixed around the values found at lab-scale (11000, 1550, 23, 35, respectively). $\text{pH}$, T, nutrient-related ratios, and light intensity are adjusted inside the optimal ranges found at indoor conditions. The number of AL-PBRs, feeding/extraction time intervals, and flowrates are designed by mass balancing the entire pilot-plant in order to operate in continuous quasi-zero waste. Outdoor tests confirmed that 12–15 AL-PBRs with a total volume of 125–155 L reuse 28.8 L d⁻¹ of wastewater by-produced from the biodiesel units providing 228 L d⁻¹ of biodiesel and reuse the total of about 90 m³ h⁻¹ of flue gas in exit from the syngas combustion unit with power generation of 9–9.5 kW. The battery works at an initial concentration for each cycle of around 0.5 g L⁻¹ and concentration at extraction of around 1.05 g L⁻¹.