Rosalam Hj. Sarbatly and Emma Suali

Abstract The integration of a membrane contactor with a photobioreactor serves two major purposes for the mitigation of CO2 by microalgae, i. e., to enhance the mass transfer and interfacial contact between two different phases and to increase the exchange process of CO2-O2 by microalgae in the photobioreactor. The membrane integrated with a photobioreactor for CO2 mitigation by microalgae can be consid­ered as a relatively new field, and only four or five related research efforts have been published in the literature, suggesting that a significant amount of work remains to be done in this field. In addition, all of the authors agreed that a membrane contactor is capable of achieving better mass transfer than the conventional approach of using a separation column in the gas-liquid separation process. One significant problem associated with using a membrane as a CO2-O2 gas exchanger is its susceptibility to pore fouling due to the micron-size cells of the microalgae. However, pore fouling can be prevented by using a hydrophobic membrane contactor and appropriate oper­ating conditions, both of which are discussed in detail in this work.

Keywords CO2 sequestration • Microalgae • Membrane photobioreactor • Biomass

14.1 Introduction

I nterest in microalgae has been increasing in many fields of research, including biofuels and pharmaceutics, because of their high photosynthetic rate. With an aver­age growth rate that is estimated to be 40 times greater than that of a fast-growing plant (Suali et al. 2012), microalgae are capable of producing biomass, on a dry — weight basis, that contains 20-80% lipids, mainly triglycerides.

R. Hj. Sarbatly (*) • E. Suali

School of Engineering and Information Technology, Universiti Malaysia Sabah, Jalan UMS, 84000 Kota Kinabalu, Sabah, Malaysia e-mail: rslam@ums. edu. my; emma. suali@gmail. com

R. Pogaku and R. Hj. Sarbatly (eds.), Advances in Biofuels, 241

DOI 10.1007/978-1-4614-6249-1_14, © Springer Science+Business Media New York 2013

Microalgal biomass is being considered as a renewable and sustainable energy source; the algae require nutrients that are widely available for producing the biomass. In addition, microalgae have the ability to utilize and convert CO2 into biomass, which can be harvested and used as biofuel (Sarbatly and Suali 2012), thereby pro­viding an alternative source of energy.

The ability of microalgae to utilize greater quantities of CO2 compared to ter­restrial plants makes them one of the most promising biological approaches for CO2 mitigation. Microalgae can remove significant quantities of CO2 from the atmo­sphere because they optimize the photosynthesis process by maximizing CO2 utili­zation and oxygen production. The CO2 is usually supplied to a microalgae culture system via CO2-enriched air. Thus, the CO2 supply also alters the circulation of cells in the culture system through the formation of bubbles, thereby enhancing the mass transfer rate of gas in the culture system. The mass transfer of gaseous O2 away from the sites where photosynthesis is occurring is important because, if the concentra­tion of O2 is allowed to increase, the CO2 utilization by microalgae will be decreased.

The current conventional injection technique that is used to supply CO2 to the cul­ture system creates bubbles that have random sizes. This can impede the uniform dis­persion CO2 throughout the media culture, which will have an adverse effect on the productivity of the microalgae. In addition, the injection of non-uniform, excessive concentrations of CO2 can result in high hydrodynamic stress that can kill the microal­gae and increase the release of CO2 to the atmosphere. Thus, dispersion devices are essential for the success of the process, and membrane contactors have high potential for successful use as a dispersion device. A photobioreactor that uses a membrane contactor as a dispersion device is referred to as a membrane photobioreactor.

The efficiency of a membrane photobioreactor for sequestering CO2 depends strongly on the CO2 concentration (Cheng et al. 2006), because this is associated with the transfer of O2 from the liquid phase to the gas phase and the transfer of CO2 from the gas phase to the liquid phase. The equilibrium that initially existed between the algal cultures in the solution and the inlet gas flow rate and composition is altered when the membrane module is integrated into the system. The membrane allows the recirculation of unused CO2 and also allows the operation of the system at lower gas pressures. To optimize the photosynthesis process, the periods of CO2 availability and light availability must be in phase with each other (Ferreira et al. 1998; Carvalho and Malcata 2001). Thus, the use of a membrane photobioreactor for CO2 mitigation must be coordinated carefully with the culture technique in order for the microalgae to optimize the photosynthesis process.