## 3 Results and Discussion
### Physical-Chemical Characteristics of UWW and Conventional Pollutants Removal  
The pH variations were plotted over time (Figure 4). In the CV and C+ PBRs, the pH values started to increase on day 2, while in the C\(-\) PBR, it only occurred on day 3, along with microalgal growth. In contrast with the present study, a pH decrease was reported in photobioreactors with other microalgal species described as hyperproducers of humic acids, such as _Euglena_ sp. [38]. The co-production of other acid compounds such as itaconic acid by these microalgae was responsible for the pH decrease in such photobioreactors [39]. In the present study, the observed pH increase was probably due to the photosynthetic process. As microalgae consume CO\({}_{2}\), a cell internal conversion of HCO\({}_{3}\)\({}^{-}\) occurs to be further used as a carbon source for photosynthesis. This process leads to the accumulation of OH\({}^{-}\) in the cells; to neutralize these ions, an increase in H\({}^{+}\) uptake from the culture medium occurs, leading to a cell-external increase in pH [39]. The correlation between the  
Figure 3: Microscopic observations (40\(\times\)) of UWW autochthonous microalgal species detected in the C\(-\) PBR. Figure 2: Microalgal cell concentration (MCC, average \(\pm\) standard deviation) over time (d) for each PBR: \(\blacksquare\) CV (_C. vulgaris_ + UWW); \(\blacksquare\) C+ (_C. vulgaris_ + autoclaved UWW); and \(\blacksquare\) C\(-\) (raw UWW). microalgal growth and pH is supported by the exponential growth of microalgal cells and pH increase in both the C+ and C\(-\) PBRs, and the stationary phase of _C. vulgaris_ between days 6 and 9. As with any other living organism, microalgae are mainly composed of C, N, and P, typically with molar ratios C\({}_{106}\)H\({}_{181}\)O\({}_{45}\)N\({}_{16}\)P [35]. Consequently, the assimilation mechanisms of these elements are linked [35]. Hence, the advantages of using microalgae-based treatments is the capacity of these organisms to remove conventional pollutants, such as phosphates, nitrates, and COD, frequently detected in the final effluents of conventional UWWTPs, at concentrations up to 40 mg L\({}^{-1}\) of total phosphorus, 30 mg L\({}^{-1}\) of NO\({}_{3}\)-N and 160 mg O\({}_{2}\) L\({}^{-1}\) of COD, respectively [7; 26; 40]. To confirm this ability, variations of PO\({}_{4}\)-P concentration over time were registered (Figure 5), and the phosphorus MR and RE were calculated (Table 3) after 9 d of the microalgal treatment. Variations in the N-NO\({}_{3}\)-concentration were not determined because their content in the initial wastewater was already below the detection limit (0.2 mg L\({}^{-1}\)). The initial PO\({}_{4}\)-P concentration of UWW was close to the EU legislation limit for UWWTPs with less than 100,000 population equivalent (2 mg L\({}^{-1}\)) [41], and a slightly lower concentration was detected in the C+ PBR. Microalgae removed PO\({}_{4}\)-P in all PBRs, revealing REs above 62% (Table 3). The profile of phosphorus removal over time followed  
Figure 4: pH variation (average \(\pm\) standard deviation) over time (d) for each PBR: \(\blacksquare\) CV (_C. vulgaris_ + UWW); \(\blacksquare\) C+ (_C. vulgaris_ + autoclaved UWW); and \(\blacksquare\) C\(-\) (raw UWW). the microalgal growth, with the highest RE and MR attained by AMS in the C\(-\) PBR, 92.6 \(\pm\) 0.1% and 2.04 \(\pm\) 0.03 mg L\({}^{-1}\), respectively. These results are similar to the ones obtained using _Chlorella sorokiniana_ and _C. vulgaris_ by [PERSON] et al. [42], [PERSON] [43], and [PERSON] et al. [44], and 5% higher than those described for _C. vulgaris_ by [PERSON] et al. [25]. Although it was not possible to calculate NO\({}_{3}\)-N removal efficiencies, due to the linkage between the macronutrients, it is expected that both _C. vulgaris_ and AMS could remove this conventional pollutant as well as other species of nitrogen such as NH\({}_{4}\)\({}^{+}\)[25, 31, 45]. Figure 6 depicts the variation of COD and HA concentrations over time in all PBRs.