## 4 Discussion
This nutrient plays an important role in several metabolic pathways, such as the Calvin cycle, phosphorylation processes and ATP synthesis [82; 83]. Thus, growth media with low phosphorus content will directly affect the microalgae growth and chlorophyll production [65; 80]. In this context, the higher phosphorus concentration found in the SR in comparison to SE can explain the highest chlorophyll content found in _C. vulgaris_ cultivated in SR. According to some authors, the carbon source also influences _C. vulgaris_ metabolism, that can present a photoautotrophic, heterotrophic or mixotrophic metabolism [84]. Due to the absence of a carbon source in the control (with BBM2N growth medium), _C. vulgaris_ show a photoautotrophic metabolism and carbon is provided through the aeration (atmospheric carbon dioxide) [45]. When municipal wastewater is added to the growth medium, an organic carbon food source is also added [65; 85]. In the presence of an organic carbon source, _C. vulgaris_ can present a mixotrophic metabolism. According to [PERSON] et al. [65], during the photoautotrophic growth pH values often increase, whereas in the mixotrophic metabolism a low pH range variation is observed. Based on these findings, the pH range variation and _C. vulgaris_ growth registered herein suggest that the SE and the SR are also an organic carbon source. Ammonia concentration can also affect pH values [86]. Ammonia is a critical nutrient, but in high concentrations can be toxic for microalgae [87]. In fact, a previous study shown that an ammonia concentration higher than 28 mM can reduce the microalgae culture viability [88]. Here, the reported ammonia concentration was significantly lower and did not present a toxic risk for _C. vulgaris_ growth. In light of the results presented here, the application of wastewater for microalgae cultivation revealed to be advantageous, maximizing the growth of _C. vulgaris_, particularly for 25% and 50% SE and 25% SR treatments. The addition of effluent to growth media will reduce the costs inherent to microalgae biomass production enhancing the economic viability of the process. The present study showed that the cultivation of _C. vulgaris_ can be coupled with bioremediation of effluents since removal of nutrients was verified during the 12 days of cultivation. No differences were observed in the total P of the control, which could be caused by the nitrogen phosphorus ratio, whereas high levels of nitrogen are required to ensure the efficient removal of phosphorus [89]. On the other hand, in effluent, phosphorus removal (94.8%) occurred and in this study a higher removal was registered in comparison with the study conducted by [PERSON] and [PERSON] (2007), where _C. vulgaris_ presented a removal rate of 49.6% [90]. Higher phosphorous removal has also been reported by other works. For instance, an 85% decrease in phosphorus content was observed after two days of _C. vulgaris_ cultured in secondary effluent [91]. The cultivation of _C. vulgaris_ in SR also showed to remove considerable amount of nitrogen. Previous research, using centrate as culture medium, demonstrated a total nitrogen and total phosphorus removal of 33.6 and 25.8%, respectively [63]. Furthermore, high removal rates of ammonia and nitrogen, 78.3% and 82.8% respectively, were registered by [PERSON] et al. when _C. vulgaris_ was cultivated in sludge run-off [20]. This approach combining microalgae cultivation-bioremediation shows potential for further application at industrial scale, possibly by integrating algal farms with wastewater treatment. The application of such practice will promote the production of biomass, reducing the cost of the cultivation process, while enhancing the removal of nutrients from effluents improving the efficiency of wastewater treatments. Nevertheless, upscaling the usage of municipal wastewaters for microalgae cultivation currently faces several drawbacks from an economic perspective.