## 3 Results and Discussion
### Nutrients Removal
Regarding nitrogen removal rates (Figure 4B) and mass removal (Figure 4C), the highest values were determined in assays 3 and 4 and no statistical differences were observed (\(p>0.05\)): (i) average removal rates were \((1.31\pm 0.07)\) mg\({}_{\rm N}\) L\({}^{-1}\) d\({}^{-1}\) and \((1.26\pm 0.08)\) mg\({}_{\rm N}\) L\({}^{-1}\) d\({}^{-1}\), respectively; and (ii) mass removal values were \((14.4\pm 0.8)\) mg\({}_{\rm N}\) L\({}^{-1}\) and \((13.9\pm 0.9)\) mg\({}_{\rm N}\) L\({}^{-1}\), respectively. These results are in accordance with maximum biomass concentration achieved and indicate higher photosynthetic activity of _C. vulgaris_ in these intermediate conditions. A different behavior was observed for phosphorus. In this case, average removal rates (Figure 5B) and mass removal values (Figure 5C) determined for assays 1 to 4 were not statistically different (\(p>0.05\)), but values determined for assay 5 were statistically higher (\(p<0.05\)), reaching an average removal rate of (\(0.20\pm 0.01\)) mg\({}_{\rm P}\) L\({}^{-1}\) d\({}^{-1}\) and a mass removal of (\(2.2\pm 0.1\)) mg\({}_{\rm P}\) L\({}^{-1}\). Figure 4: _Cont._  
Figure 3: Temporal variation of (**A**) nitrogen (nitrate + nitrite) and (**B**) phosphorus (phosphate) concentration determined in _C. vulgaris_ cultures grown in raw and diluted secondary-treated paper industry effluent (Assays: 1–  
Figure 4: Nitrogen (nitrate + nitrite) removal parameters obtained by _C. vulgaris_ cultures grown in raw and diluted secondary-treated paper industry effluent (assays 1–5): (**A**) removal efficiency (%R\({}_{\rm N}\)); (**B**) average removal rate (RR\({}_{\rm N}\)); and (**C**) mass removal (R\({}_{\rm N}\)). Error bars correspond to the standard deviation of the mean obtained from two independent experiments. Mean values sharing at least one common letter (shown above the bars) are not statistically different (\(p>0.05\)). Figure 5: Phosphorus (phosphate) removal parameters obtained by _C. vulgaris_ cultures grown in raw and diluted secondary-treated paper industry effluent (assays 1–5): (**A**) removal efficiency (%R\({}_{\rm P}\)); (**B**) average removal rate (RR\({}_{\rm P}\)); and (**C**) mass removal (R\({}_{\rm P}\)). Error bars correspond to the standard deviation of the mean obtained from two independent experiments. Mean values sharing at least one common letter (shown above the bars) are not statistically different (\(p>0.05\)). Nutrients removal from paper industry effluents has already been reported in the literature. Table 3 highlights nitrogen and phosphorus removal efficiencies and removal rates obtained in these studies. According to these data, removal efficiencies reported by [PERSON], et al. [19] and [PERSON] [2] are significantly higher than those obtained in this study, whereas values reported by [PERSON], et al. [1] were closer to those obtained in the present study, especially in assays 3-5. The lower removal efficiencies obtained in this study when compared with those reported by [PERSON], et al. [19], may be associated with the higher N:P molar ratio used in the reference study, which was \(\sim\) 66:1. On the other hand, the higher removal efficiencies reported by [PERSON] [13] may be associated with the use of other effluents to achieve the dual role of providing the required nutrients for microalgal growth while contributing to a reduction in the toxicity of the paper industry effluent. Another explanation for the increased efficiencies obtained in these studies is the nitrogen source used. As in the present study, [PERSON], et al. [1] cultivated microalgae in an effluent with nitrate-nitrogen as the main nitrogen source. On the other hand, [PERSON], et al. [19] tested an effluent with ammonium as the main nitrogen source (digestate obtained from the treatment of a pulp and paper industry effluent) and [PERSON] [13] evaluated this treatment with both nitrogen forms present.