Effect of Membrane Hydrophobicity and Thickness on Energy-Efficient Dissolved Oxygen Removal From Algal Culture

Removal of dissolved oxygen from algal photobioreactors is essential for high productivity in mass cultivation. Gas-permeating photobioreactor that uses hydrophobic membranes to permeate dissolved oxygen (pervaporation) from its body itself is an energy-efficient option for oxygen removal. This study comparably evaluated the characteristics of various commercial membranes and determined the criteria for the selection of suitable ones for the gas-permeating photobioreactors. It was found that oxygen permeability is limited not by that in the membrane but in the liquid boundary layer. Membrane thickness had a negative effect on membrane oxygen permeability, but the effect was as minor as less than 3% compared with the liquid boundary layer. Due to this characteristic, the lamination of non-woven fabric with the microporous film did not significantly decrease the overall oxygen transfer coefficient. The permeability in the liquid boundary layer had a significantly positive relationship with the hydrophobicity. The highest overall oxygen transfer coefficients in the water-to-air and water-to-water oxygen removal tests were 2.1 ± 0.03 × 10^-5 and 1.39 ± 0.09 × 10^-5 m s^-1, respectively. These values were considered effective in the dissolved oxygen removal from high-density algal culture to prevent oxygen inhibition. Furthermore, hydrophobicity was found to have a significant relationship also with water entry pressure, which needs to be high to avoid culture liquid leakage. Therefore, these results suggested that a microporous membrane with strong hydrophobicity laminated with non-woven fabric would be suitable characteristics for gas-permeating photobioreactor.