Manjinder Singh, Rekha Shukla, and Keshav Das

Biorefining and Carbon Cycling Program College of Engineering The University of Georgia Athens, Georgia

CONTENTS

5.6 Introduction……………………………………………………………………………………………….. 77

5.7 Harvesting Processes………………………………………………………………………………….. 79

5.8 Gravity Sedimentation……………………………………………………………………………….. 79

5.9 Centrifugation…………………………………………………………………………………………….. 80

5.10 Filtration……………………………………………………………………………………………………… 80

5.11 Flotation……………………………………………………………………………………………………… 81

5.12 Flocculation………………………………………………………………………………………………… 81

5.13 Electrolytic Coagulation…………………………………………………………………………… 82

5.14 Energy Efficiencies of Harvesting Processes………………………………………………. 83

5.15 Conclusion…………………………………………………………………………………………………. 85

References…………………………………………………………………………………………………………… 86

6.1 INTRODUCTION

Microalgae have been identified as a potential alternative resource for biofuel production. Significant drawbacks to algaculture include dilute culture density and the small size of microalgae, which translates into the need to handle large volumes of culture during harvesting. This energy-intensive process is therefore considered a major challenge for the commercial-scale production of algal biofuels. Most of the currently used harvesting techniques have several drawbacks, such as high cost, flocculant toxicity, or nonfeasibility of scale-up, which impact the cost and quality of products. As harvesting cost may itself contribute up to one-third of the biomass pro­duction cost, substantial amounts of research and development initiatives are needed to develop a cost — and energy-effective process for the dewatering of algae. Several factors, such as algae species, ionic strength of culture media, recycling of filtrate, and final products, should be considered when selecting a suitable harvesting tech­nique. Harvesting cost and energy requirements must be reduced by a factor of at least 2 if algal biomass production is to be viable for very low-cost products such as biofuels. There could be considerable cost and energy savings in custom-designed, multi-stage harvesting techniques for algae farms. In such systems, a variety of har­vesting technologies are arranged in a sequence based on culture chemistry, specific characteristics of each technique, and its energy requirements to dewater pond water to either 5% or 10-20% solids.

Techniques and processes of microalgae cultivation, harvesting, and dewatering have been reviewed extensively in the literature (Lee et al., 1998; Spolaore et al., 2006; Khan et al., 2009; Harun et al., 2010; Uduman et al., 2010). Due to the very dilute culture (<1.0 g of solids L-1) and typically small size of microalgae with a diameter of 3 to 30 pm, large volumes must be handled to harvest algal biomass, which is an energy-intensive process. Therefore, harvesting microalgal biomass is considered a challenging issue for commercial-scale production of algal biofuels. Conventional processes used to harvest microalgae include concentration through centrifugation, foam fractionation, chemical flocculation, electro-flocculation, membrane filtra­tion, and ultrasonic separation. The resulting high cost of biofuel production is a major bottleneck to its commercial application. The cost of harvesting may itself contribute to approximately 20% to 30% of the total cost of algal biomass, and the above methods would be viable only if the biomass harvested is used for extract­ing high-value products such as nutraceuticals (Girma et al., 2003). Harvesting, in general, can be defined as a series of processes for removing water from the algal growth culture and increasing the solids content from <1.0% to a consistency of up to 20% solids, depending on the downstream processing requirements for conversion to fuel. Thermal drying is generally discouraged (except when sufficient waste heat is available), because the amount of thermal energy needed to dry the algae would be a major fraction, if not all, of the energy content of the algal biomass.

Industrial production of algal biofuel is still in its infancy and therefore uncer­tainty in all stages of production and unpredictability of economy has been highly debated. Some very optimistic estimates on algae biofuels propose that the cost of algal oil production must be reduced by 5 to 6 times, in addition to the tax and envi­ronmental subsidies, to make them competitive with petroleum fuels (Chisti, 2007). For economical production of algal biomass, the selection of harvesting technol­ogies is so crucial. Several factors, such as algal strain, ionic strength of culture media, recycling of filtrate, and final fate of harvested biomass, must be consid­ered when selecting the harvesting technique. For example, the filamentous alga Cladophora with very long thread-like filaments (several centimeters long) lends itself to relatively cost-effective harvesting using membrane filtration. In contrast, chemical flocculation is not recommended if the harvested biomass must be pro­cessed for nutraceutical and pharmaceutical products because of the residual con­tamination caused by the flocculants. In general, it is quite difficult to recommend a single technique as the best for harvesting and recovery without consideration of specific process conditions and downstream product use. Scientists all over the world have developed several techniques for harvesting and recovery processes that rely on facts to simplify this overall process. Judicious exploitation of the different harvesting technologies is therefore necessary to reduce the harvesting cost and energy requirements by the desired factor of 2 if algal biomass production is tar­geted for very low-cost products such as biofuel. Advances in different methods of algal harvesting and dewatering to resolve our energy crisis, along with energy

utilization by various techniques with respective constraints and drawbacks of each method, are discussed in this chapter. In addition, this chapter discusses the pros and cons of different algae harvesting techniques along with their energy requirements. The potential advantages of multi-stage hybrid harvesting systems involving more than one technique deployed in a specific sequence for efficient and energy-effective biomass recovery are discussed.