Hollow fibre membrane arrays for CO2 delivery in microalgae photobioreactors

Microalgae can serve as a carbon sink for CO2 sequestration and as a feedstock for liquid biofuel production. Methods for microalgal biomass and biofuel cultivation are progressing, but are still limited in the efficiency of light delivery and gas exchange within cultures. Specifically, current gas exchange methods are very energy intensive since they rely on mixing algal cultures at high flow rates. One method that can improve gas exchange within photobioreactors without excessive mixing is the use of hollow fibre membranes, which enable simultaneous transport of gases deep into the reactor and rapid exchange with the culture media. Here we demonstrate the optimal geometric and operational conditions for CO2 transport to planar cultures of Synechococcus elongatus via hollow fibre membrane arrays. Specifically, we investigated the effects of inter-fibre spacing and active/passive aeration on the growth rate, planar surface density, and total biomass accumulation. We show that spacing in excess of 3 times the fibre diameter lead to significant variations in the uniformity of the surface density and spatially resolved growth rate, whereas spacing of 3 times the fibre diameter supported culture surface densities nearing 90%, which were maintained for 17 days without decreasing. Active aeration with the fibres showed an increase in the specific growth rate and the average surface density with respect to passive aeration by approximately 15% and 35%, respectively, while also eliminating gradients in localized growth rates along the length of the fibres.