## 3 Results and Discussions
### Cultivation of Microalgae in Outdoor Conditions  
To validate the scenario observed in batch indoor conditions, outdoor microalgae growth was also investigated in photobioreactor of 110 dm\({}^{3}\) applying a liquid digestate dilution 1:50 (Figure S2). Two basic cultivation regimes were investigated as batch and semi-continuous regime. In the batch regime, the culture was inoculated and at a certain point of growth was harvested. In the continuous regime, the culture was harvested continuously according to its growth rate and fresh medium was added [37]. Figure 3a shows the temperature and sunlight hours registered during cultivation time of _Monoraphidium_ sp. for a better understanding of the microalgae-growth behavior and nutrients removal from the media. For the batch mode trials, the temperature ranged from 14 \({}^{\circ}\)C to 25 \({}^{\circ}\)C and the average sun exposition from 5 to 10 h. During the semi-continuous trials, meteorological conditions for the three first weeks were similar to batches but further degraded with temperature ranging from 8 \({}^{\circ}\)C to 13 \({}^{\circ}\)C and an average sun exposition of only 2 h.  
Interestingly, the specific growth rate for the outdoor batches was higher than in indoor conditions with specific growth rate of 0.25, 0.14 and 0.21 d\({}^{-1}\) respectively for B1, B2 and B3 (Figure 3b, Table 2). Biomass productivity of 43.3; 31.7 and 39.8 mg TSS L\({}^{-1}\) d\({}^{-1}\) were obtained for respectively B1, B2 and B3. At steady growth, biomass yields of 0.77, 0.57 and 0.68 g TSS.L\({}^{-1}\) where respectively achieved for the B1, B2 and B3 batches. It is most probably that the higher specific growth rates obtained in outdoor compared to the indoor conditions was due probably to the higher light illumination (Table 2). During the three batches, microalgae outdoor cultivation demonstrated high nutrient-removal capacity of up to 100% NH\({}_{4}\)\({}^{+}\) and 64.77% P (PO\({}_{4}\)\({}^{-3}\)) (Figure 3c,d and Table 2). Only when ammonium was exhausted, did nitrates start to be consumed, as indicated in Figure 3c. Nitrogen is assimilated by the cells in different forms; ammonium (NH\({}_{4}\)\({}^{+}\)) and nitrates (NO\({}_{3}\)\({}^{-}\)), being ammonium the preferred as a nitrogen source [18]. In less amount, phosphorus also played an important role, as it constitutes important molecules for microalgae growth [11; 18; 38]. Once the third batch was completed, the system was shifted in a semi-continuous mode. During three weeks of operation the microalgal performances remained stable with cell densities in the level at \(1.3\times 10^{7}\) cells mL\({}^{-1}\). Then, the microalgae growth started to decline because of the unfavorable meteorological conditions. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Indoor**} & \multicolumn{3}{c}{**Outdoor**} \\ \cline{2-7}  & **L 20** & **L 30** & **L 50** & **B1** & **B2** & **B3** \\ \hline
**Specific growth, \(\mu\) (d\({}^{-1}\))** & 0.03 & 0.05 & 0.13 & 0.25 & 0.14 & 0.21 \\
**Biomass productivity (mg** & - & - & - & 43.3 & 31.7 & 39.8 \\
**TSS L\({}^{-1}\) d\({}^{-1}\))** & 0.74 & 2.2 & 3.6 & 1.7 & 1.3 & 1.6 \\
**Biomass yield (g TSS. L\({}^{-1}\))** & - & - & - & 0.77 & 0.57 & 0.68 \\
**NH\({}_{4}\)+ (\%removal capacity)** & 34 & 66 & 100 & 100 & 100 & 100 \\
**P-PO\({}_{4}\)\({}^{-3}\) (\%removal capacity)** & 0 & 77 & 92 & 78.5 & 46.6 & 75.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main microalgae performances parameters and nutrients removal efficiency (%) for the culture lead in batch in indoor condition at various dilution rate of 1:20; 1:30; 1:50 and outdoor condition (B1, B2 and B3) at a dilution rate of 1:50. B1, B2 and B3 represent the three successive batches lead in outdoor condition prior to the semi-continuous assay. Several other studies have reported microalgae cultivation in outdoor conditions [21; 39; 40]. First of all, [PERSON] et al. (2017) have investigated the cultivation of the _Monoraphidium sp.