# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 7 Use of Microalgae Biomass
### Bioenergy Production  
Like biomass produced using mineral media, biomass grown on digestate can be used for bioenergy production (e.g., biodiesel, biogas, and bioethanol) [78]. Of those, biodiesel has been studied most thoroughly [78; 80]. The accumulation of high amounts of lipids under nitrogen deprivation, among other triggers, makes microalgae a promising feedstock for biodiesel production [80]. The fatty acid profile of the biomass determines the quality of the obtained biodiesel [78]. Using digestate as nutrient source affects not only the lipid content but also the fatty acid pattern of microalgae [78]. While high amounts of ammonia in pure digestate inhibit microalgae growth and negatively affect lipid accumulation, several studies have shown that diluted digestate is a suitable nutrient source that leads to increased lipid and biomass production [78].  
Moreover, biomass derived from numerous strains (e.g., _Dunaliella salina_, _Scenedesmus obliquus_, _Spirogyna_ sp.) is a valuable feedstock for bioethanol production as it contains significant amounts of carbohydrates (up to 64% TS) when grown under stress conditions [78; 81]. It has several advantages over conventional feedstocks (food crops, lignocellulosic materials) [78; 81]. The biomass does not contain lignin, which means that pretreatment is easier [81]. Instead, starch is present at high amounts, which can easily be converted to sugar monomers [78]. Even though bioethanol production based on microalgae grown on digestate medium has not been studied extensively yet, it has been shown that the carbohydrate content of the obtained biomass is similar to that of microalgae grown on mineral medium [78].  
Furthermore, microalgae biomass represents a promising organic source to produce biogas [82]. Anaerobic digestion of _Acutodesmus obliquus_ biomass obtained a CH\({}_{4}\) yield of 200-391 Nm\({}^{3}\) t\({}^{-1}\) VS, and the maximal yield was achieved through digestion of biomass with increased lipid content (+20%, in total 30% TS, due to nitrogen deprivation) [37]. _Synechocystis_ cf. _salina_ resulted in 429 Nm\({}^{3}\) t\({}^{-1}\) VS and 348 Nm\({}^{3}\) t\({}^{-1}\) VS prior and after the extraction of valuable compounds (polyhydroxybutyrate), respectively [79]. These values are comparable to maize silage (340-360 Nm\({}^{3}\) t\({}^{-1}\) VS), a commonly used feedstock for biogas production [83]. The digestion of microalgae together with other substrates (co-digestion) has been investigated in various studies [82; 84; 85], and positive synergies are formed (e.g., improved C/N ratio, reduced viscosity) [82; 84]. However, there are seasonal variations in the availability and composition of microalgal biomass and co-substrates that affect biogas yield. Additionally, microalgae can be used for biogas upgrading to remove CO\({}_{2}\)[78]. In summary, microalgae that were grown on digestate can also be used in other stages of biogas production.  
For biodiesel or bioethanol production, purity requirements of biomass are low, as lipids or carbohydrates, respectively, are extracted from the biomass. This means that residual constituents of digestate will be removed during this process and are not harmful. When used as substrate for biogas production, residual digestate could even be an advantage, as it might introduce beneficial trace elements. Therefore, microalgae grown on anaerobic effluents can contribute to reducing the costs of producing these low-value products.