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. 2020 Aug 21:8:978.
doi: 10.3389/fbioe.2020.00978. eCollection 2020.

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

Masatoshi Kishi  1   2 , Kenta Nagatsuka  1 , Tatsuki Toda  2   3
Affiliations

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

Masatoshi Kishi et al. Front Bioeng Biotechnol. .

Abstract

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.

Keywords: dissolved oxygen; gas-permeating; hydrophobicity; microporous membrane; pervaporation; thickness.

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Figures

FIGURE 1
FIGURE 1
Gas-permeating algal photobioreactor (A) photo of the reactor in Arthrospira platensis culture; (B) Side view of a schematic diagram of the reactor [modified from Kishi (2018)]. Oxygen is removed from the microporous film side by diffusion. Light enters from the transparent film and reflects at the microporous film.
FIGURE 2
FIGURE 2
Design of oxygen permeability tests. Overall oxygen transfer coefficient (K) of each condition was used to deduce the transfer coefficient within the membrane (kM), upper liquid boundary (kA), and lower liquid boundary (kB). Transfer resistant between air and membrane was assumed to be negligible.
FIGURE 3
FIGURE 3
The relationship between oxygen transfer coefficient at the membrane (kM) and membrane thickness in the Air-Air permeation test. N = 3 for all the 11 films measured in this study. The error bar shows the standard deviation. MP: microporous; WP: woven porous; NW: microporous with a non-woven fabric; and NP-S: silicone films.
FIGURE 4
FIGURE 4
Breakdown of mass transfer resistance in the Water–Water oxygen permeation test. N = 3 for all membranes. Error bars show the standard deviation of each breakdown.
FIGURE 5
FIGURE 5
Tensile strength of the films. The error bar shows the standard deviation of N = 3. The values with asterisks (*) were derived from the product datasheet. MP: microporous; WP: woven porous; NW: microporous with a non-woven fabric; and NP-P: non-porous polypropylene film.
FIGURE 6
FIGURE 6
The effect of hydrophobicity (contact angle) on the oxygen transfer coefficients in (A) Room B and (B) Room A. N = 3 for all the 10 microporous membranes measured in this study. The error bar shows the standard deviation. MP: microporous; WP: woven porous; and NW: microporous with a non-woven fabric.
FIGURE 7
FIGURE 7
The positive relation between water entry pressure and hydrophobicity (contact angle). N = 1 for all the 10 microporous membranes measured in this study. MP: microporous; WP: woven porous; and NW: microporous with a non-woven fabric.
FIGURE 8
FIGURE 8
Relationship between optical reflectance and membrane thickness. MP: microporous; WP: woven porous; and NW: microporous with non-woven fabric support. The thickness of the membrane only was adopted for NW in the regression line. WP was removed from the regression analysis due to its black dye.
FIGURE 9
FIGURE 9
Proposed schematic structure of a gas-permeating photobioreactor composed of three film layers for dissolved oxygen removal.
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