## 3 Results and Discussion
### Production of Biomethane from Microalgae-Bacteria Flocs  
Daily and cumulative methane production measurements in batch experiments, presented in Figures 4 and 5, were performed for two inoculums, digestate and slurry, and one substrate, which is Ma-B flocs. The production is noted in terms of mL CH\({}_{4}\cdot\)g\({}^{-1}\)VS of the substrate. In addition to a control containing only digestate or slurry, three substrate/inoculum ratios (gVSsubstrate/g VSinoculum) were tested: 0.2, 0.5, and 0.8, where the substrate was Ma-B flocs. Anaerobic digestion was carried out in a mesophilic operation at 38 \({}^{\circ}\)C. Cumulative methane production related to these ratios is shown in Figures 4a and 5a below. In all cases, methane production was monitored for 30 days.  
Cumulative and Daily Biogas Production for Slurry and Digestate  
The cumulative methane yield for the initial biomass (Digestate) reached the highest value of 245.6 mL CH\({}_{4}\cdot\)g\({}^{-1}\) VS substrate after 30 days of operation. However, for mesophilic digestion with Ma-B flocs as the substrate, the CH\({}_{4}\) production reached 165.7, 30.9, and 14.3 mL CH\({}_{4}\cdot\)g\({}^{-1}\) VS substrate for the S/I ratio of 0.2, 0.5, and 0.8, respectively, at the end of the batch. According to these data, the highest methane yields were observed at a ratio of 0.2.  
Figure 4b shows that daily methane production starts from day 1 and increases to the maximum of 32.2 and 21.9 mL CH\({}_{4}\cdot\)g\({}^{-1}\) VS substrate, respectively, for the digestate control (Dg0) and digestate plus Ma-B flocs digestion at a ratio of 0.2 after only 4 days.  
In the case of the slurry control, methane production increased from 35.2 mL CH\({}_{4}\cdot\)g\({}^{-1}\) VS substrate to 726.7 mL CH\({}_{4}\cdot\)g\({}^{-1}\) VS substrate after 30 days, which is significantly higher than the values reported for the digestate control (Figure 4a). With slurry plus Ma-B flocs digestion, the highest cumulative methane yield of 317.2 mL CH\({}_{4}\cdot\)g\({}^{-1}\) VS substrate was reached at a VS substrate (g)/VSinoculum (g) ratio of 0.2. For the 0.5 and 0.8 ratios, due to a higher addition rate of Ma-B flocs to the reaction medium, lower yields of cumulative methane were observed (Figures 4a and 5a). From the daily methane production profiles, various fluctuations were noted in Figure 5b for the slurry inoculum. As assumed by [13], these fluctuations could be related to insufficient contact between the anaerobic microbial flora and the substrate. Methane productivity is also influenced by the characteristics and thickness of the cell wall and the macromolecular distribution of the substrate [6]. Thus, the fine structure, high cytoplasmic content, and low resistance to biological degradation of the microalgae cell wall led to an increase in methane volume by anaerobic digestion [28]. In conclusion, slurry gave excellent results compared to the good values obtained for the digestate. It is difficult to find a satisfactory explanation for the better performance of slurry, especially because the composition of slurry is not very different from that of digestate. As indicated by [29], the only significant difference is in dry matter, with 9% for the cattle slurry versus 4.9% for the digestate. Dry matter is considered an indicator of available nutrients because it includes fiber, protein, ash, water-soluble carbohydrates, and lipids, etc. This means that the slurry would contain more nutrients essential for the growth of Ma-B flocs.