# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 5 Cultivation in Digestate
### Adjustment of Nutrients  
Besides the removal of solids and obtaining a light-permeable cultivation medium, the nutrient concentration in digestate must also be considered. High nutrient loads can simply be lowered by dilution [44; 50; 61; 62]. However, this will not affect the ratio between elements, which is an important variable to induce microalgae reproduction (see Section 2). Thus, if there is a high load of only specific nutrients, the ratio must be adjusted. If solid-liquid separation techniques are applied, it must be considered that some of the nutrients tend to remain in the liquid fractions (supernatant, permeate) while others might remain in the solid fractions (reentate) (see Section 4). Thus, the ratio might change during solid-liquid separation. These techniques can therefore be used for adjusting the nutrients. Alternatively, the digestate could be supplemented with selected nutrients.  
Ammonium nitrogen is the major component of total nitrogen (65-98%) in digestate (see Table 2, [39]). Depending on the concentration of macro-elements, the digestate could be diluted to prevent NH\({}_{4}\)-N inhibition during microalgae cultivation. If the ratio between the elements is out of balance, the share of ammonium should be reduced. An NH\({}_{3}\)-N concentration of 0.3 mM at pH 6 inhibited the growth of _Scenedesmus obliquus_[49]. However, the utilization of this nitrogen source after setting the proper concentration can result in fast growth of microalgae [39]. The NH\({}_{4}\) uptake by _Neochloris oleaabundans_ was efficient (about 90-95%) [13].  
Microalgal growth was also achieved in digestate supernatant resulting from stru-vite precipitation [60]. The highest growth rates were obtained by _Dictyosphaerium ehenbergianum_ (0.144 d\({}^{-1}\)), followed by _Chlorella regularis_ (0.097 d\({}^{-1}\)), _Scenedesmus obliquus_ (0.089 d\({}^{-1}\)), _Arthospira maxima_ (0.084 d\({}^{-1}\)), and _Arthospira subsalsa_ (0.069 d\({}^{-1}\)). In terms of _D. ehenbergianum_ more specifically, higher biomass (161 mg L\({}^{-1}\) d\({}^{-1}\)) and lipid productivity (55 mg L\({}^{-1}\) d\({}^{-1}\)) was achieved compared to mineral medium (85 mg biomass L\({}^{-1}\) d\({}^{-1}\), 20 mg lipids L\({}^{-1}\) d\({}^{-1}\)), when the NH\({}_{4}\)\({}^{+}\)/Mg\({}^{2+}\)/PO\({}_{4}\)\({}^{3-}\) ratio was adjusted to 1:1.2:1.2.  
Besides nutrients, pH value and salinity are important variables for microalgae cultivation (see Section 2). The pH value of digestate usually reaches from neutral to slightly alkaline (see Table 2). This is similar to the required pH value for some commercial microalgae (see Table 1). However, the pH value can change during digestate treatment (e.g., ammonia stripping, struvite precipitation), as described in Section 4. If the pH value of the treated digestate does not fit the microalgae requirements, it needs to be adjusted. The presence of various digestate components or chemicals added during digestate treatment might cause a high salinity, which might have to be lowered through dilution prior to microalgae cultivation.