Integration of CO₂ capture and microalgae cultivation: demonstration assessment of outdoor Scenedesmus acutus cultivation using gaseous ammonia as a nitrogen source

Background: Towards the goal of integrating CO₂ capture using aqueous ammonia with its utilization for algae cultivation, Scenedesmus acutus (UTEX B72) was grown in 1100 L open raceway ponds using CO₂ and NH₃ supplied from gas cylinders. CO₂/NH₃ mole ratios of 7 and 10 were employed, the gas mixture acting as a surrogate for the output from a CO₂ scrubbing system using aqueous ammonia. Results: Compared to Scenedesmus acutus grown in open ponds using gaseous CO₂ and NaNO₃ as the N-source, the ponds supplied with gaseous CO₂ and NH₃ displayed higher productivity at both CO₂/NH₃ ratios, with the higher ratio providing the best growth. Depending on the culturing conditions and CO₂/NH₃ ratio, CO₂ utilization ranged up to 15.8% and NH₃ utilization to 23.0%. These rather low values reflect the fact the high CO₂/NH₃ feed rate used, resulting in a substantial release of NH₃ from the ORPs (~45%). Conclusions: These findings demonstrate the suitability of gaseous NH₃ as a N-source for microalgae cultivation, while highlighting the need for a control strategy that closely balances the CO₂/NH₃ supply with the algae growth rate. The produced algae biomass possessed a high protein and low ash content, rendering it particularly suitable for use as a bioplastic feedstock.