## 3 Results and Discussion
### Stage 2--M-BGS Anaerobic Digestion  
Nominal biogas production from the pure _Chlorella_ sp. culture was 440 \(\pm\) 16 cm\({}^{3}\)/gVS (Figure 15). CH\({}_{4}\) fraction was around 57.2 \(\pm\) 1.4% (Table 6). The results indicate that higher abundance of activated sludge microbes in the M-BGS leads to better anaerobic digestion performance (Figure 15). Anaerobic digestion of the 15-day biomass produced 451 \(\pm\) 22 cm\({}^{3}\)/gVS biogas containing 60.2 \(\pm\) 2.1% CH\({}_{4}\) (Table 6). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Indicator**} & \multirow{2}{*}{**Unit**} & \multicolumn{6}{c}{**Phase of Experiment**} \\ \cline{3-8} \multicolumn{2}{c}{} & & \multicolumn{1}{c}{**Start**} & \multicolumn{1}{c}{**15 Days**} & \multicolumn{1}{c}{**30 Days**} & \multicolumn{1}{c}{**45 Days**} & \multicolumn{1}{c}{**60 Days**} \\ \hline \multirow{4}{*}{Biogas} & \multirow{2}{*}{Output} & cm\({}^{3}\)/gVS & 440 \(\pm\) 16 & 451 \(\pm\) 22 & 459 \(\pm\) 29 & 531 \(\pm\) 36 & 506 \(\pm\) 38 \\ \cline{3-8}  & & cm\({}^{3}\)/gTS & 501 \(\pm\) 18 & 530 \(\pm\) 26 & 546 \(\pm\) 34 & 661 \(\pm\) 45 & 615 \(\pm\) 46 \\ \cline{2-8}  & r rate & cm\({}^{3}\)/gVS- d & 57.2 \(\pm\) 1.4 & 63.14 \(\pm\) 2.6 & 59.7 \(\pm\) 3.4 & 79.7 \(\pm\) 5.1 & 75.9 \(\pm\) 4.3 \\ \cline{2-8}  & Rate constant (k) & 1/day & 0.13 \(\pm\) 0.01 & 0.14 \(\pm\) 0.02 & 0.13 \(\pm\) 0.02 & 0.15 \(\pm\) 0.03 & 0.15 \(\pm\) 0.03 \\ \hline \multirow{4}{*}{Methane} & Methane fraction & \% & 59.1 \(\pm\) 2.0 & 60.2 \(\pm\) 2.1 & 60.7 \(\pm\) 3.1 & 66.2 \(\pm\) 2.7 & 65.5 \(\pm\) 3.0 \\ \cline{2-8}  & \multirow{2}{*}{Output} & cm\({}^{3}\)/gVS & 260 \(\pm\) 9 & 271 \(\pm\) 13 & 275 \(\pm\) 15 & 350 \(\pm\) 17 & 329 \(\pm\) 20 \\ \cline{2-8}  & & cm\({}^{3}\)/gTS & 295 \(\pm\) 10 & 318 \(\pm\) 15 & 327 \(\pm\) 18 & 436 \(\pm\) 21 & 400 \(\pm\) 24 \\ \cline{2-8}  & r rate & cm\({}^{3}\)/gVS- d & 36.3 \(\pm\) 0.8 & 37.9 \(\pm\) 1.4 & 38.6 \(\pm\) 1.8 & 52.5 \(\pm\) 2.6 & 49.3 \(\pm\) 2.4 \\ \cline{2-8}  & Rate constant (k) & 1/day & 0.14 \(\pm\) 0.01 & 0.14 \(\pm\) 0.02 & 0.14 \(\pm\) 0.02 & 0.15 \(\pm\) 0.03 & 0.15 \(\pm\) 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Indicators of hyrophyte biomass AD performance. Figure 15: Trends in biogas and methane production in the anaerobic respirometers: (**a**) pure _Chorella_ sp. Culture; (**b**) biomass after 15 days’ PBR operation; (**c**) biomass after 30 days’ PBR operation; (**d**) biomass after 45 days’ PBR operation; and (**e**) biomass after 60 days’ PBR operation. The M-BGS produced after 30 days of experiment was similar in terms of anaerobic digestibility. There were no statistically significant differences in gas output from fermentative bacteria. Biomass production was 459 \(\pm\) 29 cm\({}^{3}\)/gVS, containing 60.7 \(\pm\) 3.1% CH\({}_{4}\) (Table 6). CH\({}_{4}\) output from the biomass in phases 1 (14 d) and 2 (30 d) of PBR operation was similar, at 271 \(\pm\) 13 cm\({}^{3}\)CH\({}_{4}\)/gVS and 275 \(\pm\) 15 cm\({}^{3}\)CH\({}_{4}\)/gVS, respectively (Figure 15). On the other hand, significantly better performance was noted for anaerobic digestion of 45- and 60-day M-BGS. Biogas yields were 531 \(\pm\) 36 cm\({}^{3}\)/gVS from phase 3 biomass and 506 \(\pm\) 38 cm\({}^{3}\)/gVS from phase 4 biomass (Figure 15). The bacteria- and protozoa-rich biomass produced significantly higher rates of CH\({}_{4}\) in the biogas (over 65%). P3 and P4 showed the highest biogas production rates at 79.7 \(\pm\) 5.1 cm\({}^{3}\)/gVS-d and 75.9 \(\pm\) 4.3 cm\({}^{3}\)/gVS-d, respectively (Table 6). The C/N ratio, N levels, TOC, and biomass pH were found to very strongly correlate with AD performance (biogas/methane yields) (Figure 16). There is a lack of reports in the available literature that would describe the effectiveness of M-BGS anaerobic digestion. For this reason, the results achieved in this study were referred to the findings from experiments into co-digestion of microalgal biomass and sewage sludge [50; 51].