Improved CO₂ utilization efficiency using membrane carbonation in outdoor raceways

Membrane carbonation, the delivery of CO₂ by diffusion through non-porous hollow fiber membranes, was successfully integrated into outdoor raceways (5.6 m², 900 L) and operated continuously for up to 45 days. Biomass productivity associated with membrane carbonation was similar to using traditional sparging, but the average carbon utilization efficiencies for membrane carbonation was 3-fold higher than for sparging when 100% CO₂ was delivered to both systems. When the pH setpoint was 8.5, growth on ammonium bicarbonate using membrane carbonation achieved a carbon utilization efficiency of 106 ± 45% due to uptake of some carbon from bicarbonate. Using nitrate as the N source had a 78 ± 55% carbon utilization efficiency, due to dissolved inorganic carbon accumulation as the medium alkalinity increased. Lowering the pH setpoint to 8.0 created a carbon-rich environment that decreased the carbon utilization efficiency to 51 ± 27% due to CO₂ off-gassing, but it still was 3-fold higher than with sparging.