## 3 Results and Discussion
### Stage 1--LF-DSS Treatment and M-BGS Production
#### 3.1.3 Chemical Composition of M-BGS Biomass  
The lowest levels of organic substances (expressed as vs. and TOC) were found in the initial, pure _Chorella_ sp. culture, at 87.9 \(\pm\) 1.3% TS and 439 \(\pm\) 30 mg/gTS, respectively (Table 3). As the abundance of activated sludge microbes increased in the evolving M-BGS,  
Figure 11: Characteristic views with microscopic magnification \(\times\)100 of M-BGS after 60 days of H-PBR operation.  
Figure 10: Characteristic views with microscopic magnification \(\times\)100 of the evolving M-BGS after 45 days of H-PBR.  
the percentage of organics in TS significantly decreased. The lowest vs. levels were recorded for the 45 th day of H-PBR operation onwards. The 45-day-old and 60-day-old biomass contained 80.3 \(\pm\) 4.2% TS and 82.3 \(\pm\) 3.5% TS, respectively, as well as 438.2 \(\pm\) 81 mg/gTS and 455.0 \(\pm\) 74 mg/gTS TOC, respectively (Table 3). The formation and maturation of M-BGS improved the C/N ratio--an important parameter for anaerobic digestion. At the beginning of the experimental cycle, the C/N of the microalgal biomass was 11.0 \(\pm\) 1.4, which is at the lower end of the optimum range for AD (Table 3). After the first (15-day) phase of operation, the C/N was around 12.8 \(\pm\) 1.3 and it steadily increased as the PBR continued running. The C/N ratio peaked at 15.6 \(\pm\) 2.4 45 days into the cultivation process (Table 3). As the bacteria and protozoa proliferated in the evolving M-BGS, there was a significant reduction in protein in the biomass--from 24.9 \(\pm\) 1.5%TS at start-up to 17.1 \(\pm\) 2.2%TS after 45 days of cultivation. Lipid levels, on the other hand, were mostly unaffected. The microalgal biomass profiles across the PBR operation phases are given in Table 3.  
Thickening and separation are some of the most difficult and costly processing steps in scaled-up microalgae cultivation. Unlike activated sludge, microalgae do not settle, and thus usually require targeted coagulation, filtration, flotation, centrifugation and other thickening steps. The symbiotically grown M-BGS significantly boosted biomass congealability. The M-BGS had very good settleability, forming dense and heavy aggregates. This structure made it easy to separate the M-BGS from the medium by means of simple drum filtration and sedimentation. The products of the two-step microfiltration and sedimentation are shown in Figure 12 and Table 4. At 60 days into the experiment (last phase), the M-BGS concentration in the PBR was 4.8 \(\pm\) 0.50 gTS/dm\({}^{3}\). The 61 \(\pm\) 4 dm\({}^{3}\) biomass remained after the 1 st filtration pass (10.0 \(\upmu\)m sieve) contained 59 \(\pm\) 3.1 gTS/dm\({}^{3}\), for a total of 3600 \(\pm\) 200 gTS. 939 \(\pm\) 4 dm\({}^{3}\) of the medium was passed through the 2 nd drum filtration (5.0 \(\upmu\)m), containing 1.3 \(\pm\) 0.2 gTS/dm\({}^{3}\) M-BGS. After the 2 nd filtration pass, 84 \(\pm\) 2 dm\({}^{3}\) biomass was thickened to a concentration of 14 \(\pm\) 1.3 gTS/dm\({}^{3}\), for a total M-BGS mass of 1175 \(\pm\) 110 gTS/dm\({}^{3}\). The final effluent contained 0.029 \(\pm\) 0.01 gTS/dm\({}^{3}\) (Table 4).