Koenraad Muylaert, Dries Vandamme, Imogen Foubert and Patrick V. Brady

Abstract Large-scale production of microalgae for biofuels is still facing several major challenges to become competitive with other forms of renewable and non­renewable energy. A major challenge is harvesting, which requires the separation of a low amount of biomass consisting of small individual cells from a large volume of culture medium. Flocculation is seen as a promising low-cost harvesting method for microalgae biomass. In this chapter, the challenges and potential advantages of using flocculation as a harvesting method for microalgae are reviewed.

12.1 Introduction

Microalgae have attracted attention in recent years as a promising new source of biofuels (e. g., Chisti 2007). In addition to biofuels, microalgae have a high potential for other innovative applications. They have a high content of proteins with a favorable amino acid composition and are potentially interesting for use as animal feed (Draaisma et al. 2013). Microalgae can absorb nitrogen and phosphorus from wastewater and used in wastewater treatment (e. g., Park et al. 2011a, b). Microalgae [3] [4]

can also produce a range of high-value chemicals that cannot be derived from conventional crops (e. g., specific fatty acids, carotenoids, or natural pigments) (Pulz and Gross 2004). Despite this enormous potential, commercial applications of microalgae are today still limited to production of value-added biochemicals or nutritional supplements. To use microalgae for low-value applications such as biofuels, animal feed, or wastewater treatment, the cost and energy inputs in the production process need to be decreased by at least an order of magnitude (Wijffels and Barbosa 2010; Christenson and Sims 2011). One of the major challenges is to develop a low-cost and energy-efficient harvesting method (Mata et al. 2010; Rawat et al. 2012).

Microalgae are generally small (2-20 pm) and have a specific density that is close to that of their culture medium. This precludes the use of simple screening or sedimentation methods for harvesting microalgae. Similar challenges exist with harvesting of other microorganisms, such as yeasts or bacteria produced hetero — trophically in fermentors. An important difference is that biomass concentrations of phototrophically cultivated microalgae are more than an order of magnitude lower than those of bacteria or yeasts. This is because high biomass concentrations in microalgal cultures result in self-shading and light limitation of photosynthesis. Microalgal biomass concentrations range from 5 g L-1 dry weight in closed pho­tobioreactors to only 0.5 g L 1 in open raceway ponds. The low biomass con­centration implies that large volumes of culture medium need to be processed to harvest the biomass. For example, a 10-ha facility that produces microalgae in raceway ponds and that produces 30 tons of dry microalgal biomass ha-1 year-1 should process about 2000 m3 of microalgal broth per day. This is in the same order of magnitude as a water treatment plant processing wastewater of 10,000 people. To realize large-scale production of microalgae, a harvesting technology is required that is capable of processing large volumes of culture medium at a minimal cost (Molina Grima et al. 2003; Uduman et al. 2010; Christenson and Sims 2011; Benemann et al. 2012; Vandamme et al. 2013).