# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 9 Concluding Remarks and Future Perspectives  
To obtain optimal productivity, the composition and consistency of digestate must be suitable. These strongly depend on the feedstock and parameters used in anaerobic digestion. Digestates from certain biogas production processes might be better suited for microalgae cultivation than others; thus, also the anaerobic digestion itself must be considered during process development. Digestate composition, especially when received from an industrial biogas plant, hardly changes during seasons. This should allow stable process conditions as well as consistent product quality. However, the raw digestate needs to be processed prior to its use in microalgae cultivation in order to adjust total solids and nutrients. At the very beginning of the process chain, it is important to already have the desired product in mind. Product price and quality requirements might determine the necessary and possible amounts of resources spent on digestate processing. Since the aim is to use digestate as sustainable and economically advantageous option, the effort of digestate treatment as well as the application of chemicals and use of energy should be kept as low as possible, especially for low-value products and bioenergy. Considering a biorefinery approach, the aim should be to use all side fractions that arise during digestate treatment and microalgae processing. The use of the solid phase of digestate as fertilizer enables synergies with agriculture. Residues arising during microalgae downstream processing (process water, residual microalgae biomass) can be recycled into the microalgae cultivation or biogas production process. As a consequence, a suitable combination of digestate treatment as well as downstream processing steps must be applied. This way, waste and loss of valuable resources can be avoided. Simultaneously, a suitable and sustainable cultivation medium and, above all, marketable microalgae products with adequate quality may be obtained. Depending on the application, the quality requirements differ, and additional separation or washing steps might be necessary to remove remaining digestate substances for high value products or applications where the whole biomass is used. Additionally, legislative issues must be considered, especially for the use in the food industry. Aquaculture, agriculture, and biofuels are large markets with medium, low, and almost no safety requirements, respectively. These markets are therefore interesting for microalgae products derived from cultivation in digestate. The use of digestate as nutrient source for microalgae cultivation is advantageous when considering the environmental impact. However, only a few studies dealing with downstream processing of microalgal biomass grown in digestate media compared to mineral media are available. Further research must be conducted to identify the most suitable digestate processing methods and downstream techniques. Moreover, the biomass of algae grown in diverse digestate media should be characterized in order to determine suitable application areas. In this regard, both beneficial and harmful substances must be identified. The most important parameters that affect the productivity of the process must be identified and adapted in a way that leads to lowest possible production costs. Finally, digestate treatment and microalgae downstream processes should receive increased attention because they represent major steps in the production process. Conceptualization, L.B., K.M. ; writing--original draft preparation, L.B., K.R., J.M., K.M. ; writing--review and editing, L.B., K.R., J.M., K.M., B.D. ; project administration, B.D. ; funding acquisition, B.D. All authors have read and agreed to the published version of the manuscript. The research leading to these results has received funding from the COMET program managed by the Austrian Research Promotion Agency under grant number 869341. The program is co-financed by the Republic of Austria and the Federal Provines of Lower Austria, Styria, and Vienna.