## 4 Conclusions  
Using M-BGS is still strictly in the realm of Research & Development, unlike purely bacterial aerobic and anaerobic granular sludge. Processes that mediate microalgae-bacterial granulation have not yet been fully understood. There is a dearth of research exploring how various operational and environmental parameters affect the process and how to maintain the granules for long-term bioreactor use. Therefore, there is a real need to obtain more data, especially with studies on scales of operation close to those of commercially-run plants. The present study demonstrates that treatment of LF-DSS with _Chlorella_ sp. can produce a growing microalgae + AS microbe community that can merge into M-BGS. The final stage of taxonomic and morphological evolution of the M-BGS was reached after 60 days' PBR operation. Afterwards, no further changes were noted in the morphology, taxonomic structure, chemical composition, LF-DSS treatment performance, and anaerobic digestibility of the M-BGS biomass. The biomass was abundant not only in _Chlorella_ sp. microalgae, but also in filamentous _Microthrix parvicella_, and type 1851 and 1701 filamentous bacteria. Unicellular and protozoan species were much slower to grow. Activated sludge microbes accounted for approx. 43%TS of the entire M-BGS community. Bacteria population growth significantly affected population growth curves, while also increasing the M-BGS biomass growth rate and final concentration in the H-PBR. A significantly higher final biomass concentration was observed after 45 days of LF-DSS treatment. The higher levels of M-BGS biomass and the grown abundance of heterotrophic microbes were the major factors in improving organic pollutant removal from the LF-DSS. On the other hand, no relationship was found between these M-BGS parameters and N/P removal. Nutrients were effectively taken up during all phases of H-PBR operation. Changes in taxonomic composition of the M-BGS affected the chemical characteristics of the biomass, as well as the biogas composition and yield. Higher abundance of bacteria in the M-BGS improves the C/N ratio, which in turn leads to significantly higher anaerobic digestion performance. Future research into M-BGS-based technologies should focus on further understanding of microalgae-bacteria interactions. Data on these mechanisms, supported by reliable and comprehensive research, is necessary to develop technical and technological guidelines for biodegradation and pollutant removal, which can be implemented on a large scale. In-depth knowledge will also allow for the development of optimization procedures and empirical equations enabling the prediction of possible technological effects, including the efficiency of biomass growth, its biochemical composition, the efficiency of pollutant removal and the production of energy carriers or other value-added products. There have been no studies conducted on a pilot scale or in conditions close to real, which limits the possibility of reliable estimation of investment and operating costs of technologies based on M-BGS. The results of experimental work on a larger scale are also necessary to conduct a life cycle analysis (LCA), which in the longer term will allow the verification of the extension of the M-BGS application in other areas, such as bioenergy (e.g., methane, biohydrogen, biodiesel, or bioelectricity) and production biochemicals (e.g., polyhydroxyalkanoates or exopolysaccharides) at reduced process costs. Conceptualization, J.K.; Methodology, J.K. and M.D. ; Validation, J.K.; Formal analysis, J.K.; Investigation, J.K.; Resources, J.K., M.D. and M.Z. ; Software, J.K.; Data curation, J.K., M.D. and M.Z. ; Writing--original draft preparation, J.K. and M.D. ; Writing--review and editing, J.K., M.D. and M.Z. ; Visualization, J.K. and M.D. ; Funding acquisition, J.K. All authors have read and agreed to the published version of the manuscript.