# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 6 Downstream Processing
### Harvesting Techniques  
Harvesting microalgae is challenging as the cultures are usually thin (dry weight \(\approx\)0.5 g L\({}^{-1}\)) and the cells are small (5-20 \(\upmu\)m) [67]. There is no universal harvesting technique; however, even though it is cost- and energy-intensive, centrifugation is the most commonly used method for harvesting large volumes [66; 67]. It is mainly applied when high-value products are aimed for [67]. Centrifugation is suitable for most types of microalgae, excluding some fragile species [66; 68]. Various centrifuge types are available, e.g., tubular bowl, disc-stack, and scroll decameter [66]. Depending on the particle size, biomass can be concentrated up to 150 g L\({}^{-1}\). Most recently, the spiral plate gravitational  
Figure 3: Scheme of microalgae cultivation and downstream processing options.  
technology of Evodos was introduced, which allows to prepare microalgae cells in the form of a paste suitable for lyophilization [67; 69]. Continuously operating centrifuges such as plate separators or nozzle separators are suitable for harvesting high amounts of microalgae for feed or aquaculture applications [66].  
Flocculation usually means the collection of cells as aggregates caused by the addition of some chemicals, or coagulants (multivalent cations, metal salts, or polymers, for example FeCl\({}_{3}\), Al\({}_{2}\)[SO\({}_{4}\)]\({}_{3}\), Fe\({}_{2}\)[SO\({}_{4}\)]\({}_{3}\), polyethyleneenediamine, and chitosan) [66]. Thus, the particle size is increased, which makes it easier to separate them from the culture suspension [67]. Autoflocculation by pH change, or co-flocculation of co-cultures of particular microalgae and bacteria were also successful in some cases [67]. Flocculation is necessary to enhance the efficiency of biomass harvesting, which is especially important at a large scale [66]. The only drawback of flocculation is a potential unsuitability of the flocculant for the desired application or final product, or non-feasibility of scaling up [66; 67]. Considering the use of microalgae for low-value products such as bioenergy, flocculation combined with sedimentation might be a more economically and energetically feasible harvesting technique than centrifugation.  
Recently, new harvesting technologies, e.g., electrocoagulation and magnetic separation, were tested for cell separation (e.g., [70; 71; 72; 73]). The former technique is usually based on coagulation, which is caused by electro-solubilization of iron or aluminum electrodes, but no metal ions remain in the medium [66]. Magnetic separation (the capture of microalgae cells by magnetic particles of Fe\({}_{3}\)O\({}_{4}\)) has been used for removing microalgae in short operation time, but the practical application of this technique is still limited [66].  
Another harvesting technique is filtration, which relies on porous membranes [66]. One advantage of filtration is that shear-sensitive species can be harvested without the addition of chemicals that could contaminate the product [66]. Microfiltration is suitable for harvesting biomass, and ultrafiltration can be used for the removal of metabolites [66].  
When using digestate as a nutrient source, even when most particles were removed during digestate processing, some digestate components (e.g., suspended matter, salts) are still present in the cultivation liquid. Thus, when flocculation and centrifugation are used for harvesting, digestate components and flocculating agents remain in the separated microalgae biomass. This limits potential product applications. Filtration might be able to remove digestate components from the biomass but cleaning the filtration unit is linked to additional costs.