# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 4 Digestate Processing  
In agriculture, digestate is often used as fertilizer on fields because it contains various valuable nutrients [47]. Reducing the water content of digestate is beneficial in order to reduce transportation and storage costs [47]. Further, solids can then be used for fertilizing the fields more easily, while the liquid phase remains as nutrient source for microalgae cultivation. To reduce the digestate volume, so-called partial processing is proposed. In contrast, complete processing aims at refining digestate to pure water, fibers, and solids as \begin{table}
\begin{tabular}{c l c c c c c c c} \hline \hline \multirow{2}{*}{**Origin**} & \multirow{2}{*}{**Raw Material**} & \multirow{2}{*}{**pH**} & **TS \({}^{*}\)** & **VS \({}^{b}\)** & **NH\({}_{4}\)-N \({}^{c}\)** & **TKN \({}^{d}\)** & **p \({}^{*}\)** & **References** \\  & & [t] & [s] & [s] & [s] & [g kg\({}^{-1}\)] & [g kg\({}^{-1}\)] & [g kg\({}^{-1}\)] & [g kg\({}^{-1}\)] \\ \hline \multirow{4}{*}{Agricultural residues} & n.d. \({}^{f}\) & 7.5-8.4 & 6.41-24 & 4.42–18.5 & 0.03–4.1 & 0.09–50.0 & 0.46–5.76 & [46]\({}^{*}\) \\  & Co-digestion manure + crops and/or & 5.6-8.3 & 1.5–24 & 0.93–18.5 & 0.01–1.63 & 0.02–12.1 & 0.002–2.4 & [46]\({}^{*}\) \\  & industrial waste & & & & & & & \\ residues/ & Corn silage, manure, agricultural & & & & & & & \\ renewable & residues & 7.7-8.1 & 6.1-8.3 & 4.4-6.3 & 4.9-6.1 & _7.6–9.6_ & n.d. \({}^{f}\) & [17] \\ materials & Crop digestion with manure & 7.7-8.0 & 6.5-8.6 & 4.8-6.4 & 2.3–4.2 & 4.3–6.1 & n.d. \({}^{f}\) & [17] \\  & Crop digestion & 7.4-7.9 & 6.2–8.6 & 4.8-6.2 & 15.2-5.6 & 3.6–52.2 & n.d. \({}^{f}\) & [17] \\  & Crop digestion & 7.2-7.9 & 7.8–9.0 & 5.7–6.7 & 1.3–3.6 & 4.6–6.3 & n.d. \({}^{f}\) & [17] \\  & Corn and grass silage & 7.6-8.0 & 6.6–9.3 & 4.8–6.9 & 1.3–2.4 & 3.6–4.9 & n.d. \({}^{f}\) & [17] \\  & Manure & 7.3-8.6 & 2.2–9.2 & 14.6–9.0 & 0.06–9.3 & 0.01–0.57 & 0.007–0.2 & [46]\({}^{*}\) \\  & Brewers’ spent grains & 7.3-7.5 & 5.3–5.8 & 4.7–5.3 & 1.9–2.3 & 2.3–3.1 & n.d. \({}^{f}\) & [17] \\  & Slaughterhouse waste & 7.9-8.3 & 22–4.9 & 1.6–3.9 & 5.3–7.7 & 6.4–8.1 & n.d. \({}^{f}\) & [17] \\  & Thin silitage–bioethanol by-product & 7.7-8.1 & 1.7-2.8 & 0.9–1.6 & 2.2–2.8 & 3.0–4.3 & n.d. \({}^{f}\) & [17] \\  & n.d. \({}^{i}\) & 7.9-8.3 & 1.4–7.88 & 0.56–57.8 & 0.01–0.67 & 0.01–098 & 0.002–0.1 & [46]\({}^{*}\) \\  & Bo waste & 7.6-8.1 & 2.5–4.7 & 1.4–2.7 & 1.5–5.6 & 3.0–6.8 & n.d. \({}^{f}\) & [17] \\ Food & Bio and food waste, blood & 8.0-8.3 & 3.9–4.1 & 2.2–8.5 & 5.1–2.7 & 6.4–8.1 & n.d. \({}^{f}\) & [17] \\ waste/residues & Bio and food waste, blood & 7.3-7.9 & 1.6–3.3 & 1.0–1.7 & 0.6–1.5 & 1.4–2.3 & n.d. \({}^{f}\) & [17] \\  & Bio and food waste, blood, food & 7.8-8.2 & 5.6–8.1 & 3.0–4.5 & 3.1–4.1 & 4.2–6.7 & n.d. \({}^{i}\) & [17] \\  & Maunre, slaughterhouse, bio, food, & & & & & & & \\  & and kitchen waste & 8.0 & 8.3 & 5.7–7.2 & 4.1–5.6 & 6.8–8.6 & 8.4–10.8 & n.d. \({}^{i}\) & [17] \\  & Kitchen food waste & 8.0 & 5.9 & n.d. \({}^{f}\) & 4.02 & n.d. \({}^{f}\) & 0.67 & [50] \\ \hline \hline \end{tabular}  
* TS—total solids, \({}^{b}\) VS—volatile solids, \({}^{c}\) NH\({}_{4}\)-N—ammonia nitrogen, \({}^{d}\) TKN—total Kjeldahl nitrogen, \({}^{e}\) P—phosphorous, \({}^{f}\) n.d.—not defined/determined, \({}^{*}\) values converted into fresh matter.  
\end{table} 
Table 2: Origin and composition of anaerobic effluents. well as to mineral nutrients. Technologies used for partial processing are rather simple and cheap while complex technologies are necessary for complete processing, which differ in their degree of technical maturity, energy consumption, and costs [17; 47; 48]. In the first step of a partial digestate process, flocculating or precipitating agents are added to improve solids removal. Subsequently, solids can be directly used to fertilize fields, compost, or dried.