When using flocculation in wastewater treatment or fermentation, the cost of the flocculant is related to the volume of treated water or medium. But when floccu­lation is used for harvesting microalgae, the cost is calculated relative to the amount of biomass that is harvested, not relative to the volume of microalgal broth that is treated. A given dose of flocculant capable of removing 0.5 kg of particulates from an aqueous medium containing 0.5 g L 1 of particulates will yield 1000 L of treated water in the case of wastewater treatment, but only 0.5 kg of biomass in the case of microalgae harvesting. As a result, the cost of the flocculant is a critical factor when selecting a flocculation method for microalgae harvesting.

When evaluating the cost of flocculation-based harvesting, the cost of the chemical flocculant in relation to the dosage needed is obviously important. Some flocculants have been used since many years in various industries and are low-cost commodities that are available in bulk. An example is alum, which is commonly used in wastewater treatment. Although alum is cheap, the high dosage that is required to flocculate microalgae makes its use relatively expensive (about 300 $ ton-1 harvested biomass) (Molina Grima et al. 2003; Uduman et al. 2010). Chitosan requires lower dosages, but it is a more expensive chemical and therefore has a similar overall cost (about 500 $ ton-1 harvested biomass). Autoflocculation by magnesium hydroxide is often proposed as low-cost method for flocculation. As most waters contain sufficient magnesium, the main cost for flocculation is the cost of the base required to increase the pH. This cost is very low when calcium hydroxide or slaked lime is used as a base (about 30 $ ton 1 harvested biomass) (Schlesinger et al. 2012). When evaluating the cost of ECF, not only the electricity cost for electrolysis should be taken into account, but also the dissolution of the sacrificial anode (Lee et al. 2013a). Aluminum anodes generally have a higher flocculation efficiency than iron anodes, yet iron electrodes may nevertheless be preferred due to a lower anode cost and lower energy con­sumption (Dassey and Theegala 2014). Some novel flocculants such as for modified iron oxide nanoparticles are not yet available on the market, which makes it difficult to estimate their cost. No chemicals are required to harvest microalgae using bio­flocculation. When flocculation of microalgae is induced by addition of cultured bacteria, fungi or other microalgae, the cost to cultivate these flocculating microor­ganisms should be taken into account. It is important to realize that the dosage of flocculant needed and thus the cost of the flocculant depend strongly on the species of microalgae that is harvested and on the culture conditions. The amount of algal organic matter excreted in the culture medium can result in a strong increase in the flocculant dosage.

When estimating the cost of a flocculation method, not only the cost of the flocculant should be taken into account, but also the energy demand for mixing and pumping (Elmaleh and Jabbouri 1991; Danquah et al. 2009a). Intensive mixing of the flocculant solution with the microalgal culture broth is essential to achieve good flocculation efficiency (e. g., Lee et al. 2013a, b). Other indirect costs are costs related to the sedimentation or flotation system (construction costs, operational cost, land cost) (Richardson and Johnson 2014). In life cycle analysis or LCA studies, the environmental burden associated with the production of the flocculant should be taken into account as this cost can differ substantially between different flocculants (Brentner et al. 2011). Examples of flocculants with a potentially high environ­mental burden might include those that are Al-based.

The primary aim of using flocculation for harvesting microalgae is to pre­concentrate the microalgal biomass and to reduce the volume of broth that needs to be processed by a mechanical dewatering method such as centrifugation or filtra­tion. The higher the degree of pre-concentration that can be achieved during the pre­concentration stage, the higher the energy savings in the mechanical dewatering stage (Milledge and Heaven 2012). A flocculation method that can produce a small volume of algal sludge will require less energy for mechanical dewatering than a method that produces a large sludge volume. Pre-concentration of the biomass can be achieved by combining flocculation with sedimentation or with flotation. Flo­tation has higher investment costs and requires more energy than sedimentation, but it can achieve a higher degree of biomass concentration (Besson and Guiraud

2013) .

The use of flocculation for harvesting may interfere with other stages of mic­roalgae cultivation and processing. For instance, the flocculant may prevent recy­cling of the culture medium, which can result in high costs for water to prepare fresh culture medium or a high cost for treatment of the spent medium before discharge into the environment. The flocculant can interfere with downstream processing of the biomass, e. g. lipid extraction. Some flocculants form toxic resi­dues in the harvested biomass and can limit the use of the protein fraction as animal feed, and thus limit the income that can be generated from the microalgal biomass. These indirect costs of using a certain flocculation method should also be taken into account. More information on the economics of harvesting and downstream pro­cessing is given in Chap. 14.

12.2 Conclusions

It is clear that the energy requirements for harvesting microalgae could be reduced by at least an order of magnitude if the biomass could be pre-concentrated using floc­culation. There are several technologies available to flocculate microalgae, including metal salt coagulants, electro-coagulation-flocculation, polymer flocculants, or the use of clays or iron oxide nanoparticles. Flocculation can also be induced by a pH increase (autoflocculation; result of precipitation of Ca or Mg salts) or it can occur spontaneously (bioflocculation). The main disadvantage of using flocculation for harvesting microalgae is that, in most cases, the biomass is contaminated with a foreign substance. This can limit the use of the biomass or can interfere with downstream processing. Therefore, the flocculant should ideally be non-toxic or, better still, it should be possible to remove the flocculant from the biomass after harvesting. Because cost reduction is an important issue in production of microalgal biofuels, the cost of a flocculation technology is an important criterion. Microalgae excrete organic matter in the culture medium that may interfere strongly with floc­culation. This interference of microalgal organic threatens the applicability of floc­culation for harvesting microalgae. So far, no ideal universal flocculation method has yet emerged. It is most likely that the optimal flocculation method will be species specific and depends on the final application of the biomass.

Acknowledgements D. Vandamme is a Postdoctoral Researcher funded by the Research Foundation—Flanders (FWO).