ABSTRACT
Energy-efficient and cost-effective microalgae dewatering, nutrient recycling, and wastewater treatment are some of the major challenges facing large-scale industrial-scale microalgae production (Benemann 2013; Borowitzka and Moheimani 2013; Milledge and Heaven 2013). Biomass concentrations in microalgal cultures are generally limited from a few hundred mg dry weight L−1 in open raceway ponds to a 254few g L−1 in typical photobioreactors. This low density of biomass of phototrophic algal cultures necessitates that a large volume of water must be processed to harvest the biomass. For example, to harvest 1 ton of dry biomass from an outdoor pond culture with 0.5 g L−1 dry matter content, 2000 m3 of the culture medium needs to be processed. Filtration through fine screens is not practical as they clog rapidly, while the use of wider mesh screens, such as microstrainers, is only feasible with filamentous species such as Spirulina (Arthrospira). Filtration through fine screens may not be practical in many situations due to fouling. Micro-/ultra-/ nanofiltration membrane selection is dependent on the culture medium and level of contamination with various macro- and microsized molecules, elements, and components. However, harvesting microalgal cells from their culture medium by simple gravity sedimentation is generally not feasible due to the small size of the individual cells (typically just under 3 to about 50 µm) and the small difference in density with the medium. Only larger colonies, clusters, or aggregates (“flocs”) will readily settle. Although, microalgal cell suspensions are stabilized by the negative surface charge of the cells; thus, they generally do not spontaneously coagulate to form larger aggregates that sediment rapidly enough by gravity alone. One exception is Haematococcus pluvialis, commercially produced for its astaxanthin content, which typically produces relatively large and immotile cysts with a high astaxanthin content that settle well (Kobayashi et al. 2001; see also Chapter 12, this volume). Another is Pediastrum, a large colonial green alga, which can dominate in high-rate algal wastewater treatment ponds and which has a high sedimentation rate (Park et al. 2011). For commercial Chlorella production, centrifugation is used to harvest the biomass, but is too expensive for all but such high-value products (e.g., nutritional supplements). The development and application of microalgae harvesting and dewatering technologies capable of processing large volumes of culture medium at a minimal cost are thus essential for large-scale and low-cost production of lower-cost commodities such as feeds and fuels (Greenwell et al. 2010; Uduman et al. 2010; Christenson and Sims 2011).
