Early membrane contactors were developed from microporous propylene fibre, which includes polypropylene (PP), polytetrafluoroethylene (PTFE), and polyvi — nylidene fluoride (PVDF). These materials have been used dominantly to make membrane contactors for the removal of gases in gas-liquid phase separation processes and absorption processes. While most membrane contactors have a porous structure, some have a more dense structure.

The use of PP made membrane contactors suitable for CO2 removal from the gas-liquid phase (Dindore et al. 2004). The main reason that most membrane con­tactors were made from hollow-fibre PP is that the contactor has a fixed and well — defined gas-liquid interfacial area, which resulted in easy evaluation of the hydrodynamic situation on the liquid side of the membrane contactor (Dindore et al. 2005). According to Khaisri et al. (2009), the effectiveness of membrane con­tactors for removing CO2 from the liquid phase through absorption can be ranked as PTFE > PVDF > PP.

The productivity and selectivity of PP also was reported to be better than poly­phenylene oxide (Simons et al. 2009). However, PP has the lowest absorption rate compared to PTFE and PVDF, but PP continues to be the most applied membrane contactor in the industry due to its efficiency for capturing and removing CO2 the liquid phase (Zhang et al. 2008; Agrahari et al. 2011; Lv et al. 2012).

The application of PP in gas-liquid separation processes required extra care due to its extreme sensitivity to small variations in the feed pressure that could cause severe performance losses. As shown in Simons et al. (2009), high liquid losses are observed for the PP fibres, especially at elevated temperatures. This issue is not a hindrance in CO2 supply to microalgae culture because it is a low-temperature operation.

The PTFE membrane has been reported to be effective for use in the ozonation of wastewater. In comparison with PDVF, PTFE gave more a stable and higher flux (Bamperng et al. 2010). The PTFE also was reported to be effective for use in gas-liquid separations in microgravity conditions. The O2 produced from the bio­logical life and the required CO2 capable to evolve and exchange within the PTFE membrane (Farges et al. 2012). PVDF has been reported to be less effective, and the CO2 flux decreases as the operation period increases. Among the reasons that were identified was the formation of liquid droplets on the gas side of the mem­brane (Zhang et al. 2008).