Melinda J. Griffiths

Centre for Bioprocess Engineering Research University of Cape Town, South Africa

CONTENTS

5.1 Introduction………………………………………………………………………………………………… 51

5.2 Growth Requirements and Design Parameters……………………………………………. 52

5.2.1 Light……………………………………………………………………………………………….. 52

5.2.2 Temperature…………………………………………………………………………………… 55

5.2.3 Nutrient Provision…………………………………………………………………………… 56

5.2.4 Mixing…………………………………………………………………………………………….. 57

5.3 Cultivation Systems……………………………………………………………………………………. 58

5.3.1 Open Systems…………………………………………………………………………………. 58

5.3.1.1 Natural Waters…………………………………………………………………. 59

5.3.1.2 Circular Ponds………………………………………………………………….. 59

5.3.1.3 Raceway Ponds……………………………………………………………… 59

5.3.2 Closed Systems…………………………………………………………………………….. 61

5.3.2.1 Tubular Reactors……………………………………………………………. 62

5.3.2.2 Flat-Plate Reactors…………………………………………………………. 65

5.3.3 Alternative Designs……………………………………………………………………….. 66

5.3.3.1 Stirred Tank Fermenter…………………………………………………….. 66

5.3.3.2 Wave/Oscillatory Flow Reactors……………………………………. 66

5.3.3.3 Hybrid Production Systems…………………………………………….. 68

5.4 Comparison of Reactor Types……………………………………………………………………. 68

5.4.1 The Open versus Closed System Debate………………………………………… 68

5.5 Conclusion………………………………………………………………………………………………….. 72

References…………………………………………………………………………………………………………… 72

5.1 INTRODUCTION

One of the most important factors in achieving economically and environmentally feasible commercial-scale production of microalgae is the development of cost — effective, sustainable culture systems (Borowitzka, 1999; Richmond, 2000). The design of the cultivation system influences the environmental conditions experienced by the cells, which in turn determine the productivity (Greenwell et al., 2010).

Improving productivity is key to achieving economic viability in large-scale, outdoor cultures (Lee, 2001).

Microalgal cultivation has been carried out in a variety of vessels, ranging from natural open lakes and ponds to highly complex and controlled photobioreactors (PBRs). Typically, the term photobioreactor has been used to refer to closed systems exclusively; however, by definition, open systems are also PBRs. A bioreactor is a container in which living organisms are cultivated and carry out biological conver­sions (e. g., biomass or product formation) (Grobbelaar, 2009). A PBR is a reactor in which organisms that obtain energy from light, such as algae, plants, and certain microbial cells (phototrophs), are used to carry out reactions (Mata et al., 2010). Each type has advantages and disadvantages, but the overall goals are similar. In the design of commercial algae cultivation systems, the aim is to achieve:

• Optimal volumetric and/or areal productivity

• Efficient conversion of light energy to product

• Consistency and reliability of production

• Cost effectiveness

Effective reactor design requires knowledge of both algal physiology and reac­tor engineering, such as aspects of hydrodynamics and mass transfer (Ugwu et al., 2008). Section 5.2 outlines the key requirements for algal growth and how these relate to design considerations of the cultivation system. Section 5.3 describes the range of open and closed systems that have been used for microalgal cultivation. These are compared with respect to a range of attributes in Section 5.4.