# Optimization of Microalgae-Bacteria Consortium in the Treatment of Paper Pulp Wastewater
## 3 Results and Discussion
### Influence of the microalgae-Bacteria Inoculation Ratio on the Growth and Formation of Biomass  
Figure 4 shows a high variation in VSS concentration for the different PBRs, which is due to different inoculation ratios that impacted on the system's starting values. Biomass produced in photobioreactors may contain a mixture of microalgae, bacteria, and other debris. PBR1 and PBR2 started with the lowest biomass concentration (445.98 \(\pm\) 15.57 mg/L) associated only with microalgae inoculation and aerobic sludge inoculation, respectively, but these values also include a VSS fraction that came from the paper pulp wastewater. PBR1 showed lower biomass growth due to the lack of bacteria inoculation, despite the fact that this system start-up strategy provides a greater synergistic process and biomass growth in the system. During the 14 days of operation, the mean VSS concentration in PBR1, PBR2, PBR3, PBR4, PBR5, and PBR6 was 434.97 \(\pm\) 104.99 mg/L, 520.00 \(\pm\) 88.20 mg/L, 685.00 \(\pm\) 135.01 mg/L, 768.11 \(\pm\) 122.69 mg/L, 1100.00 \(\pm\) 227.65 mg/L, and 648.37 \(\pm\) 137.97 mg/L mg/L, respectively. The biomass seems to stabilize around the sixth day of operation, and afterwards, there was no significant variation in the PBRs medium. As discussed by [PERSON] et al. [55], this circumstance can be explained due to the good formation of the microalgae-bacteria consortium. As the biomass increases in size and becomes increasingly dense and compact after formation and structuring, biomass loss from variations in control parameters and substrate concentrations is low. According to [PERSON] et al. [56], in continuous feed systems, better maturation and granular density can only be detected after the 30 th day of operation. In addition, biological aggregates are considered predominantly granular when at least 50% of the particles present in the systems are larger than 0.20 mm [57], enabling a better understanding of this process of biomass generation. [PERSON] et al. [53] and [PERSON] et al. [55] applied variations in aeration and lighting to photobioreactors in sequential batches, and obtained dense granules with a high abundance of microalgae from the seventh day of operation. The photogranules maturation conditions aided in determining the critical factors for growing the granular consortium. As the photobioreactors were only used for 14 days and with a single batch, the results obtained are only indicative of how the symbiotic process of microalgae and bacteria occurs in the treatment of industrial paper pulp wastewater. Bacterial algal biomass is a consortium of photoautotrophs, chemoautotrophs, and heterotrophs that is stable and unified inside an EPS matrix [58] and can be classified as a spectrum of biogranules with applications in environmental engineering [59]. Figure 4: Variation of VSS concentration throughout the experimental period. Another important factor for evaluating the formation of biomass is the chlorophyll content present in the biomass, helping in the evaluation of biomass productivity in photobioreactors. This factor is also important for the evaluation of satisfactory values of the microalgae:bacteria ratio used in the inoculation for starting the system. For a better understanding of the behavior of the systems, the total chlorophyll (chlorophyll-a and chlorophyll-b) was extracted from the samples, and their concentrations were calculated through Equations (1) and (2). Results are shown in Figure 5. This parameter allows for examining the degree of pigment production in microalgae [60]. For PBR1, PBR2, PBR3, PBR4, PBR5, and PBR6, the final mean values obtained from the sum of chlorophyll-a and chlorophyll-b were 0.67 \(\pm\) 0.11 mg/L, 0.31 \(\pm\) 0.03 mg/L, 0.59 \(\pm\) 0.04 mg/L, 0.43 \(\pm\) 0.01 mg/L, 0.54 \(\pm\) 0.06 mg/L, and 0.69 \(\pm\) 0.13 mg/L. The highest concentration of total chlorophyll was found in PBR1 and PBR6, where a higher concentration of microalgae was inoculated, which may have contributed to the greater development of these microorganisms in the medium.