F. C. Thomas Allnutt and Ben A. Kessler

Abstract Harvesting of dilute cultures of algae from large volumes of culture needed for production of biofuels and bioproducts is a substantial hurdle to the economic viability of algal biofuels. While centrifugation and sedimentation are already scaled to volumes that would allow direct application to algal biofuel production, their economics to the production of biofuel are not favorable. The industry has reevaluated the existing technologies and continues to innovate around the harvesting of microalgae for biofuels and bioproducts. This review discusses the historical approaches and recent advances while comparing and contrasting the different methods. An engineering estimate of comparative costs is also provided.

14.1 Introduction

A major challenge facing the microalgae industry is how to economically harvest microalgal biomass from millions of gallons of culture medium containing biomass at densities of less than 1 % total solids. Typically, open pond photoautotrophic production reaches biomass densities ranging from 0.01 to 1 gdw L 1, while cell densities in enclosed photobioreactors (PBRs) range from 4 to 10 gdw L-1 (Chisti 2007; Stephens et al. 2010). The higher levels of biomass reported for PBRs that provided high-intensity artificial light are still low relative to cell densities that can be reached in heterotrophic cultures (which can exceed 100 gdw L 🙂 and also introduce a different hurdle to commercial viability, the operating expense (OpEx) of the electricity for the lights, and availability of inexpensive, durable, and highly efficient lights (Chen et al. 2011). The contribution that harvesting the algal biomass makes toward the overall cost for renewable biofuel production has been estimated

F. C.T. Allnutt (H)

BrioBiotech LLC, P. O. Box 26, Glenelg, MD 21737, USA e-mail: fct. allnutt@gmail. com

B. A. Kessler

Phycal Inc, 51 Alpha Park, Highland Heights, OH 44143, USA © Springer International Publishing Switzerland 2015

N. R. Moheimani et al. (eds.), Biomass and Biofuels from Microalgae,

Biofuel and Biorefinery Technologies 2, DOI 10.1007/978-3-319-16640-7_14 to be between 20 and 30 % and remains a bottleneck for the industry (Brennan and Owende 2010; Dismukes et al. 2008; Gudin and Thepenier 1986).

Microalgae have been harvested by centrifugation in industrial algal applications that, to this point, have mostly focused on higher value nutritional products, such as P-carotene, astaxanthin, edible algal biomass, or higher value nutritional oils— where the high energy costs of centrifugation can be borne by a high-value product (Spolaore et al. 2006; Wijffels et al. 2013). In the case of wastewater treatment applications, where the purpose is to reduce the biological oxygen demand (BOD), algae have mostly been harvested using flocculation and either settling or flotation tanks in order to lower the overall cost. While such methods have already been operated at scales relevant to commercial biofuel production, they do not provide for preservation of the biomass for downstream processing into fuels and co­products.

It is important to note that the performance of the algal mass culturing system and the properties and value of the product portfolio being produced have direct impacts on how much the harvesting or dewatering step can cost. The reasons for this are the following: (1) In most cases, the size of the equipment is based on volumetric throughput, not dry mass throughput, and (2) in many cases, the operating costs (OpEx) are also based on volumetric throughput. Consequently, culturing systems that produce higher densities of algae and higher densities of product per unit volume of culture medium will require smaller equipment and have lower OpEx.

Additionally, the microalgal feedstock being produced cannot be degraded during the harvesting such that it cannot be used for the production of algal biofuels and any co-products necessary for commercial viability. The industry has, to this point, focused on the production of biomass that is directly converted to energy by a number of different processes (e. g., combustion, hydrothermal liquefaction, and catalytic gasification), secretion into the medium (e. g., ethanol), or biomass that is high in lipid as a feedstock for production of biofuels. But it should be noted that many of the value propositions being put forward for commercial production rely on value-added co-products (e. g., animal feeds, single-celled protein) to meet profit targets. Because of the different requirements for the growth systems, productivi­ties, and extraction criteria, it can be difficult to directly compare harvesting tech­nologies and their associated economics. This chapter will present qualitative considerations for the various harvesting options as well as present ranges for the associated economics.

In 1965, a study completed by Golueke and Oswald compared all of the available harvesting methods and concluded that only centrifugation and chemical flocculation were economically feasible (Golueke and Oswald 1965). However, both of these technologies have characteristics that make them less than ideal for algal biofuel production, and their commercial relevance has been reexamined. Centrifugation is energy intensive and requires expensive equipment to carry out at scale. The addition of flocculants adds OpEx in the form of chemicals and addi­tional bulk of harvested material, and the flocculant itself can negatively impact the final product and valuable co-products (e. g., residual metals). In order to reduce the cost impact of harvesting on production of biofuels, a number of technologies or modifications of old technologies have been evaluated and new technologies have been and are currently being evaluated and developed to reduce harvesting as a hurdle to the commercial viability of algal biofuels. This review will briefly provide an historical backdrop on algal harvesting technologies, describe the existing technologies, compare and contrast the developed technologies, and provide a description of new technologies that have begun the process of crossing into scaled use by the industry.