# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 5 Cultivation in Digestate
### Removal of Solids and Increasing Light Permeability  
One of the first steps in digestate pre-treatment could be the removal of solid particles to obtain a homogenous liquid medium. This can be achieved by centrifugation (either with or without adding precipitating agents) and subsequent ultrafiltration of the liquid fraction, as carried out by [PERSON] et al. [44] (digestate from thin stililage treatment) and [PERSON] et al. [61] (digestate from agro-industrial sources). In both studies, the permeate was then diluted with water, which led to a reduced nutrient load and improved light permeability [44; 61]. The microalgae strains _Phaeodactylum tricornutum_ and _Pavlova lutheri_ were selected for further cultivation on the obtained medium. Both strains performed well in both inorganic f/2 medium and treated digestate. Almost the same biomass productivity was achieved in both media (in digestate and f/2 medium _Pavlova lutheri_ reached 15 and 17 mg L\({}^{-1}\) d\({}^{-1}\) and _Phaeodactylum tricornutum_ reached 25 and 24 mg L\({}^{-1}\) d\({}^{-1}\), respectively) [61]. _Synechocystis_ cf. _salima_ achieved biomass concentrations of 1.55 \(\pm\) 0.33 g L\({}^{-1}\) (0.11 \(\pm\) 0.02 g L\({}^{-1}\) d\({}^{-1}\)) and 1.26 \(\pm\) 0.45 g L\({}^{-1}\) (0.09 \(\pm\) 0.03 g L\({}^{-1}\) d\({}^{-1}\)) in digestate supernatant (diluted 1/3 and 1/5 with water) compared to 1.98 \(\pm\) 0.37 g L\({}^{-1}\) (0.14 \(\pm\) 0.02 g L\({}^{-1}\) d\({}^{-1}\)) when cultivated in BG-11 medium [44]. However, the growth rate in ultrafiltration permeate (diluted 1/5) was considerably lower (0.23 d\({}^{-1}\) compared to 0.5 d\({}^{-1}\) in supernatant (diluted 1/5) and 0.86 d\({}^{-1}\) in BG-11), which was assumed to result from low Mg concentrations or low N/P ratios [44]. [PERSON] et al. [50] chose a different order for digestate treatment--first dilution (various concentrations), then sedimentation, and finally filtration steps were performed. _Chlorella vulgaris_ performed better at low concentrations of processed digestate (2.5%). The growth rates in digestate were scalable and converted excess nutrients to biomass. _C. vulgaris_ grew well for 28 days with a maximum growth rate of 0.62 d\({}^{-1}\) and a maximum biomass concentration of 0.86 g L\({}^{-1}\). After each harvest, the culture was supplemented with new digestate medium. As nutrients were not entirely transformed to biomass by these microalgae, an accumulation of ammonium and heavy metals was observed and presumably linked to a decline in growth rate after 28 days.  
Depending on its properties, untreated digestate can be used in some cases. When BG-11 medium was supplemented with unprocessed digestate (from municipal sludge treatment) by 10 and 20% (_r/v_), the growth rate of the wild type _Synechocystis_ sp. PCC 6803 increased significantly. At higher supplementation over 20% (_r/v_), growth was inhibited, probably due to decreased transparency and the culture becoming photo-limited [62]. As mentioned, growth limitation at higher digestate concentration was observed by [PERSON] et al. [50] as well.  
It should additionally be considered how the effluent to-be-used was treated in the preceding process. During the process, e.g., in a wastewater treatment plant, an automatic addition of flocculant might cause cell aggregation when the wastewater is used to cultivate microalgae. This might be overcome by centrifugation of the activated sludge without flocculant addition [15].