# Digestate as Sustainable Nutrient Source for Microalgae--Challenges and Prospects
## 4 Digestate Processing
### Nutrient Removal and Recovery  
Membrane technologies can be used to remove particles, suspended solids, and dissolved salts. Particles and colloids down to 0.1 \(\upmu\)m (microfiltration), 0.01 \(\upmu\)m (ultrafiltration), and 0.001 \(\upmu\)m in diameter (nano filtration, including microorganisms) can be separated \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Unit** & **Liquid** & **Solids** \\ \hline Mass & [\%] & 80–90 & 10–20 \\ TS a  & [\%] & 40–50 & 50–60 \\ VS b  & [\%] & 35–45 & 55–65 \\ NH\({}_{4}\)-N d  & [\%] & 35–45 & 55–65 \\ K f  & [\%] & 70–80 & 20–30 \\ P e  & [\%] & 30–40 & 60–70 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Distribution of mass and nutrients after solid–liquid separation [17; 47]. with porous membranes. The application of solution-diffusion membranes (reverse osmosis) can separate dissolved salts such as NH\({}_{4}\)[17; 46]. Using microfiltration, 93% of phosphorous and, with a combination of micro-, ultra-, and nano-filtration, 94% of nitrogen could be recovered [50]. Digestate processing via membranes typically includes micro- or ultrafiltration followed by two- to three-stage reverse osmosis, depending on the permeate quality. Permeate (liquid fraction) of membrane processes can for example be used as process or washing water, while reetentate is rich in nutrients. Membrane processes are prone to fouling and are rather expensive and energy consuming [17]. Further options to recover, remove, or adjust nitrogen concentration in liquid digestate include ammonia stripping [17; 46; 57], membrane conductors [51], evaporation [46; 58], and membrane distillation [59], as well as strivite precipitation [17; 46; 60] and ion exchange [17]. The principle of ammonia stripping is removing NH\({}_{3}\) from the liquid by sparging it with gas and recovering NH\({}_{4}\)-N in sulfuric acid in the form of ammonium sulfate ((NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\)). The volatility of ammonia can be enhanced by increasing temperature and pH value. The pH can be increased by degassing and removing CO\({}_{2}\) or adding alkali. Ammonia stripping is carried out in packed columns but also in stirred tank reactors, which are more robust and less prone to clogging due to particles which results in lower maintenance and cleaning effort [17]. Additionally, there is a certain need for chemicals (pH adjustment, sulfuric acid). To recover heat, the utilization of flue gas instead of air is a viable option [57]. One advantage of NH\({}_{3}\)-stripping is that a standardized pure nitrogen fertilizer can be recovered; this liquid could be used to increase the nutrient content of other digestate fractions and therefore their marketability [17]. Another option to recover NH\({}_{4}\)-N in the form of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) is by using membrane conductors. These membranes are directly submerged into the substrate, and sulfuric acid is pumped through their core as extraction solution. Their application at 400 L-scale has been promising; however, cleaning and maintenance need to be improved [51]. Additionally, evaporation is an attempt to remove NH\({}_{4}\)-N or adjust its concentration in digestate, as through addition of sulfuric acid, (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) is formed and less NH\({}_{3}\) evaporates [58]. This way, small organic acids can be removed besides NH\({}_{4}\)-N [46]. To recover nitrogen and phosphorous simultaneously from liquid digestate, research has successfully tested membrane distillation with fractional condensation of permeate on the downstream side of the membrane [59]. Drawbacks of evaporation and other heat-demanding processes are the high energy requirements, and therefore they are only interesting where excess heat is available [17]. Prior to evaporation, solids, especially fibers, often need to be removed and additionally sulfuric acid is needed to adjust the pH value [17].