# Optimization of Microalgae-Bacteria Consortium in the Treatment of Paper Pulp Wastewater
## 3 Results and Discussion
### Influence of microalgae-Bacteria Inoculation Ratio on Nitrogen and Phosphorus Removal  
The photobioreactors with the highest inoculation concentration, whether of microalgae or bacteria, had the greatest removal of NH4\({}^{+}\)-N, with average RE around 56%, 86%, and 77%, respectively. This indicates the occurrence of simultaneous nitrification or assimilation of nitrogen (N). It is believed that the loss of N in the system due to volatilization is minimal in systems with the microalgae-bacteria consortium, because there is a natural control of the environment with the operating time, and the synergistic effect between the microalgae and bacteria also allows this form of nitrogenous material to be incorporated into the biomass, preventing the formation of greenhouse gases [44]. [PERSON] et al. [45] also reported that the consumption of NH4\({}^{+}\)-N is slightly faster during the light cycle, due to the active production of ATP and NADPH when microalgae undergo photosynthesis. The initial concentration of nitrate (NO3\({}^{-}\)N) in the six photobioreactors was low, around 1.98 \(\pm\) 0.61 mg/L. Accumulation of nitrate in the medium occurred in PBR1 and PBR2 (only with microalgae inoculation and only with bacterial inoculation) with final concentrations of 3.61 mg/L and 2.79 mg/L, respectively. Final nitrate concentrations in the soluble effluent for PBR3 (1:1), PBR4 (1:3), PBR5 (1:5), and PBR6 (3:1) were 1.81 mg/L, 1.17 mg/L, 1.06 mg/L, and 0.86 mg/L, respectively. It is possible that simultaneous nitrification and denitrification occurred in a single granular structure in these systems, during the granulation process with microalgae and bacteria, which coexist with distinct growth rates [46]. Furthermore, [PERSON] et al. [26] verified that the nitrification process carried out by the bacteria in the symbiotic system favored the assimilation of oxidized forms of nitrogen (NO2-NO3-) by the microalgae _Chlorella vulgaris_. TN removal in PBRs that used microalgae/bacteria inoculation yielded results of 41.69%, 40.00%, 56.31%, and 46.86% for PBR3, PBR4, PBR5, and PBR6, respectively. The photobioreactor that started with a 1:5 inoculation ratio (PBR5) performed better, which could be due to a better nitrification process in the medium, as previously mentioned, allowing for more TN removal. The low initial concentration of ammoniacal nitrogen in the  
Figure 3: Variation of soluble COD concentrations in PBR effluents. paper pulp industry effluent may have influenced the lower TN removal values; even in systems with different amounts of microalgae and bacteria, nutrient removal mechanisms may differ [27]. The mechanisms of N removal using a microalgae-bacteria consortia for industrial wastewater treatment need more investigation to clarify all the pathways, since some non-conventional pathways can be installed in specific conditions of pH and DO. Anaerobic ammonia oxidation, algae nitrification, heterotrophic nitrification, autotrophic denitrifies, and chemolithoautotrophs able to nitrify and denitrify in the presence and absence of oxygen can have an important role in N removal [47; 48]. Regarding the performance in the removal of P (PO4\({}^{3-}\)-P) in the photobioreactors, a reduction of 43.32%, 64.99%, 77.38%, 82.15%, 85.83%, and 93.49% for PBR1, PBR2, PBR3, PBR4, PBR5, and PBR6, respectively, was observed at end of the experiments. The PBR6 performed better in the removal of PO4\({}^{3-}\)-P, which could be due to the increased microalgae concentration in the medium. The final concentration value for PO4\({}^{3-}\)-P observed for PBR5 and PBR6 was around 0.93 mg/L and 0.32 mg/L, respectively, indicating that a higher concentration of microalgae/bacteria inoculation allows for better P assimilation and incorporation in the biomass. The lower removal values were observed in PBR1 and PBR2, with inoculation ratio of 1:0 and 0:1 microalgae:bacteria, and, taking into account the observations of [PERSON] et al.