## 3 Results and Discussion
### Nutrients Removal
According to several studies, although nitrate-nitrogen is the most thermodynamically stable form (and the most commonly found in aquatic environments), ammonia is directly assimilated and converted into proteins by microalgae, while nitrate must be reduced to nitrite and then to ammonia before being assimilated by microalgal cells [25]. However, for an adequate comparison of nutrients removal performance, it is important to determine the average removal rate, as this parameter takes into account initial nutrients concentrations and cultivation/treatment time. Comparing average removal rates obtained in the present study and in the reference studies, values in the same order of magnitude were obtained, except in what concerns ammonium-nitrogen removal in the studies performed by [PERSON], et al. [19] and [PERSON] [2]. In these cases, the higher removal rates obtained may be associated with the higher ability of microalgae to assimilate ammonium-nitrogen than nitrate-nitrogen. Considering values of average RR, it is possible to conclude that promising results were obtained in this study. Moreover, differences found in experimental conditions used in this study and in the studies reported in the literature demonstrate that these results can be significantly enhanced. Besides increasing N:P molar ratio and providing an ammonium-nitrogen source, the increase of light supply should also be considered, as values reported in the literature correspond to cultures grown under light intensities of 130-800 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\), whereas results reported in the present study were obtained with light intensities of 30-40 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). In summary, the results obtained in this study for both nitrogen and phosphorus removal evidence that the remediation of paper industry effluents using microalgae is possible, provided that it is properly diluted to avoid inhibitory effects related to the presence of strong color or high concentrations of toxic compounds, typically associated with effluents resulting from this industrial sector [19; 20]. Considering the results obtained for nitrogen removal, the dilution of the effluent to the concentrations present in assays 3 and 4 is the most adequate. In these conditions, nitrogen concentrations were significantly reduced, reaching (\(7.1\pm 0.7\)) mg\({}_{\text{N}}\) L\({}^{-1}\) and (\(2.81\pm 0.05\)) mg\({}_{\text{N}}\) L\({}^{-1}\), respectively (which corresponds to the highest average removal rates: (\(1.31\pm 0.07\)) mg\({}_{\text{N}}\) L\({}^{-1}\) d\({}^{-1}\) and (\(1.26\pm 0.09\)) mg\({}_{\text{N}}\) L\({}^{-1}\) d\({}^{-1}\), respectively). Regarding phosphorus removal, the highest removal rate was obtained for the conditions tested in assay 5: (\(0.20\pm 0.01\)) mg\({}_{\text{P}}\) L\({}^{-1}\) d\({}^{-1}\). Despite the promising nitrogen and phosphorus removal rates, the results obtained in this study demonstrated that the cultures were limited by nitrogen, as nitrogen and phosphorus were assimilated by _C. vulgaris_ at a N:P molar ratio ranging from 10:1 to 24:1. Considering these results and the N:P molar ratios used in this study (between 6:1 and 9:1), nutrients uptake could be enhanced by increasing nitrogen supply. Another alternative to achieve an adequate N:P molar ratio and reduce the toxicity of this effluent would be to dilute it with other effluents, as proposed in other studies [13]. Finally, the remediation process could be further improved by modulating microalgal cultivation conditions. According to [PERSON], et al. [25], light conditions, temperature, and pH are also important parameters that can influence microalgal growth and, hence, the efficiency of the bioremediation process. Thus, from the prospecting of this work, other studies evaluating these parameters should be carried out to further improve nitrogen and phosphorus uptake from paper industry effluents.