## 3 Results and Discussion
### Physical-Chemical Characteristics of UWW and Conventional Pollutants Removal
Initial UWW COD concentration values in all PBRs were below the maximum (125 mg O\({}_{2}\) L\({}^{-1}\)) established by the Council Directive 1991/271/EEC [46]. In the C+ PBR, the sterilization process of UWW led to a 34% increase in the initial COD concentration (Figure 6A), since the organic matter degradation may have originated from polyphenic, humin, and humic and fulvic acids [47]. On the other hand, the CV PBR had a significantly lower (_p_ < 0.05) concentration of COD values due to a dilution effect of the _C. vulgaris_ pre-inoculum. Nevertheless, the COD values lowered in the C+ in the first 3 d, as represented in Figure 6A, suggesting that _C. vulgaris_ underwent mixotrophic metabolism. This type of metabolism allows cells to use organic carbon for respiration, besides inorganic carbon for photoautotrophy [48]. In the C\(-\) PBR, the COD decreased in the same period, most likely due to its consumption by the bacteria from the UWW. However, COD decrease by AMS should not be neglected, even though they were at lower concentrations at this first 3 d. After that, COD concentration increased over time, reaching slightly lower values in the C\(-\) PBR (62.5 \(\pm\) 2.3 mg O\({}_{2}\) L\({}^{-1}\)) than in the CV PBR (75.7 \(\pm\) 2.7 mg O\({}_{2}\) L\({}^{-1}\)), but still lower than legislation limits. In wastewater, COD can have many origins. However, in this present scenario, the increase in COD concentration over time probably occurred \begin{table}
\begin{tabular}{c c c c} \hline \hline
**PBR** & **P\({}_{0}\) (mg L\({}^{-1}\))** & **MR (mg L\({}^{-1}\))** & **RE (\%)** \\ \hline CV & 2.40 \(\pm\) 0.02 \({}^{\text{a}}\) & 1.67 \(\pm\) 0.09 \({}^{\text{a}}\) & 69.60 \(\pm\) 3.44 \({}^{\text{a}}\) \\ C+ & 1.83 \(\pm\) 0.02 \({}^{\text{c}}\) & 1.15 \(\pm\) 0.03 \({}^{\text{b}}\) & 62.91 \(\pm\) 0.61 \({}^{\text{b}}\) \\ C\(-\) & 2.20 \(\pm\) 0.01 \({}^{\text{b}}\) & 2.04 \(\pm\) 0.03 \({}^{\text{c}}\) & 92.62 \(\pm\) 0.10 \({}^{\text{c}}\) \\ \hline \hline \end{tabular}  
* MR: mass removal; P\({}_{0}\): initial phosphorus concentration; RE: removal efficiency.  
\end{table} 
Table 3: Initial PO\({}_{4}\)–P concentration and removal parameters determined for each PBR: CV (_C. vulgaris_ + UWW), C+ (_C. vulgaris_ + autoclaved UWW), and C\(-\) (raw UWW). Values with a different letter within the same column are statistically different (_p_ < 0.05). due to the excretion of metabolic products, such as EPS, of which HA, a stable organic matter form, is a component [47]. Some microalgal species naturally secrete humic acids, while others secret humic acid-like substances along with EPS release in the presence of certain stressors [39; 49]. Indeed, several studies related to wastewater treatment report the excretion of EPS by _Chloella_ sp., including HA, although to a lesser extent when compared to other HA hyperproducers such as _Euglena_ sp. [38; 50; 51; 52]. Hence, HA may have contributed to the COD increase. This occurrence is supported since the COD (Figure 6A) and HA content (Figure 6B) increased after day 3, to which a Pearson correlation was found to some extent (r 0.7-0.8). In addition, a high correlation between HA and MCC values (Pearson correlation, r > 0.9, \(p\) < 0.050) in the C\(-\) PBR was observed (Figure S1). In summary, these results led to the conclusion that both _C. vulgaris_ and AMS are efficient microalgal systems for PO\({}_{4}\)-P removal and organic matter stabilization from UWW.