## 3 Results and Discussion
### Stage 1--LF-DSS Treatment and M-BGS Production
#### 3.1.2 Taxonomic Structure  
The microbial community consisted almost exclusively of _Chlorella_ sp. microalgae (almost 100%TS) at PBR start-up (Figure 8). Until day 30, activated sludge microbes in total  
Figure 7: M-BGS biomass production in phases: (**a**) phase 3 (days 31–45) and (**b**) phase 4 (days 46–60) of the H-PBR operation.  
biomass stayed within levels below 30%TS--specifically 22 \(\pm\) 7%TS after 15 days of PBR operation and 29 \(\pm\) 9%TS after 30 days (Figure 9).  
The share of bacteria and protozoa in the evolving M-BGS was identified after 45 days of experiment. Heterotrophic microbes accounted for 47 \(\pm\) 12%TS (Figure 10). After 60 days, activated sludge microbes accounted for approx. 43 \(\pm\) 9%TS of the entire M-BGS community. The taxonomic structure was stable throughout the remainder of H-PBR operation (Figure 11).  
The artificial ecosystem that emerged in the LF-DSS was abundant not only in _Chorella_ sp. microalgae, but also in filamentous _Microthrix parvicella_, type 1851 and 1701 filamentous bacteria, and _Streptococcus_ sp. Unicellular species were much slower to grow and represented a minor fraction of the M-BGS community. _Pseudomonas_ sp., _Nikrosomonas_ sp., _Azotobacter_ sp., _Achromobacter_ sp., _Flavobacterium_ sp., _Microoccus_ sp., _Staphylococcus_ sp., _Bacillus_ sp., and _Mycobacterium_ sp. bacteria were detected. Protozoa were mainly represented by ciliates, _Aspidisca cicada_, _Drepanomonas revoluta_ and _Vorticella infusionum_. On average, their shares in the total protozoa population were 61, 15 and 6%, respectively. Also found were sapropelic flagellates _Trigonomonas_, _Paramecium caudatum_, and unsupported Rhizopoda.  
Figure 8: _Chorella_ sp. biomass at the beginning of the experimental cycle with microscopic magnification (**a**) \(\times\)10, (**b**) \(\times\)100.  
Figure 9: Characteristic views with microscopic magnification \(\times\)100 of microalgal-bacterial consortia produced after (**a**) 15 and (**b**) 30 days of H-PBR operation.  
Thus far, research works have demonstrated the self-aggregation of microorganisms in photo-bioreactors to be the major mechanism of M-BGS formation [35]. It has been proved that granules formed in this way feature high stability, content and density, which allows for effective pollutant removal and separation of M-BGS biomass during gravitational sedimentation or simple filtration [36]. It has also been found that the presence of filamentous bacteria in the bacterial biocenosis of activated sludge is an important element in the formation of stable and compact granules. They constitute a backbone and a construction of a granule, to which further cells of bacteria and microalgae are attached by means of exogenous polymeric substances [37]. In the study by [PERSON] et al. (2021) [38], the major functional groups classified at the genus level in the taxonomic structure of M-BGS included: _Pseudomonas_, _Thuera_, _Acinetobacter_, _Flavinumbacter_, _Pseudoxanthomonas_, _Aquimonas_, _Gematimomas_, and _Leptolynspyales_[38]. As reported by [PERSON] et al. (2021) [39], the prevailing communities of M-BGS at the genus level were _Rhizobium_ and _Proteiniclastcium_. The community of eucaryotic algae was dominated by the genus _Chlorella_ belonging to the class Chlorophyta [39]. It needs to be emphasized, however, that the cited authors analyzed synthetic municipal wastewater.