## 3 Results and Discussion  
### Microalgal Growth  
The microalgal growth profile along the UWW treatment is depicted in Figure 2, and the calculated growth parameters are presented in Table 2. In this experiment, light was provided by an artificial source, LED, with continuous illumination (24:0 LDC). Although it may not represent a real-life scenario, considering that sun is the main cost-effective light source, it has been proven that microalgal N and P uptake efficiencies are higher when a continuous light supply is applied, and the absence of a dark period reduces the proliferation of bacteria [35]. The possibility of using artificial light even in open-space photobioreactors (open raceways) was shown recently [36]. Hence, to improve microalgal efficiency, this experiment was designed considering microalgal systems operated in closed photobioreactors with light provided by a low-cost artificial light supply (LEDs).  
PBRs inoculated with _C. vulgaris_ (CV and C+) showed similar growth behavior until day 6, with no significant differences (\(p>0.05\)) in the \(\upmu_{\mathrm{max}}\) values, 0.087 \(\pm\) 0.002 d\({}^{-1}\) and 0.115 \(\pm\) 0.045 d\({}^{-1}\), respectively. No lag phase was observed in any of these PBRs; however, in the CV PBR, a deceleration and a possible stationary phase started after day 6. On the other hand, microalgal growth in the C+ reactor was in a plain exponential phase at the end of the experiment (day 9). In terms of productivity, microalgae grown in the C+ PBR attained a 3-fold higher \(\mathrm{P_{X,avg}}\) than in the CV PBR. Moreover, _C. vulgaris_ on C+ PBR  
\begin{table}
\begin{tabular}{c c c c} \hline
**PBR** & **MCC\({}_{\mathrm{max}}\) (Cells mL\({}^{-1}\))** & \(\upmu_{\mathrm{max}}\) (d\({}^{-1}\))** & **P\({}_{\mathrm{Xavg}}\) (Cells mL\({}^{-1}\) d\({}^{-1}\))** \\ \hline CV & (6.3 \(\pm\) 0.4) \(\times\) 105 \({}^{\mathrm{a}}\) & 0.087 \(\pm\) 0.002 \({}^{\mathrm{a}}\) & (1.7 \(\pm\) 0.6) \(\times\) 104 \({}^{\mathrm{a}}\) \\ C+ & (8.1 \(\pm\) 0.2) \(\times\) 105 \({}^{\mathrm{b}}\) & 0.115 \(\pm\) 0.045 \({}^{\mathrm{a}}\) & (5.1 \(\pm\) 0.2) \(\times\) 104 \({}^{\mathrm{b}}\) \\ C\(-\) & (5.5 \(\pm\) 0.1) \(\times\) 105 \({}^{\mathrm{a}}\) & 0.687 \(\pm\) 0.065 \({}^{\mathrm{b}}\) & (9.0 \(\pm\) 0.3) \(\times\) 104 \({}^{\mathrm{c}}\) \\ \hline \end{tabular}  
\(\upmu_{\mathrm{max}}\): maximum specific growth rate; MCC\({}_{\mathrm{max}}\): maximum microalgal cell concentration; P\({}_{\mathrm{X,avg}}\): average biomass productivity.  
\end{table}
Table 2: Growth parameters determined for each PBR: CV (_C. vulgaris_ + UWW), C+ (_C. vulgaris_ + autoclaved UWW), and C\(-\) (raw UWW). Values with a common superscript letter and within the same column are not statistically different (\(p>0.05\)).  
reached the highest X\({}_{\rm max}\), (8.1 \(\pm\) 0.2) \(\times\) 10\({}^{5}\) cells mL\({}^{-1}\) (\(p\) \(<\) 0.05), probably due to the greater availability of organic carbon from the UWW sterilization process.  
After a lag phase of 3 d, autochthonous microalgal species (AMS) belonging to microalgal classes _Cunophyaceae_, _Chlorophyaceae_, and _Bacillariohypaceae_ (Figure 3), started to proliferate exponentially in the C\(-\) reactor until the end of the experiment, attaining the highest \(\mu_{\rm max}\) (0.687 \(\pm\) 0.065 d\({}^{-1}\)) and \(\rm{Px}_{\rm{Avg}}\) ((9.0 \(\pm\) 0.3) \(\times\) 10\({}^{4}\) cells mL\({}^{-1}\) d\({}^{-1}\)) (\(p\) \(<\) 0.05). The best results in terms of microalgal growth in C\(-\) can be associated, to some extent, with the cooperation effects between bacteria and microalgae and greater adaptation of AMS to the UWW than _C. vulgaris_[37].