## 3 Results  
### Microalgae Growth  
Generally, _C. vulgaris_ was able to grow in every tested concentration on both effluents (Figure 3), exhibiting a typical growth curve by presenting a lag and exponential phase.  
Figure 2: Correlation between microalgal number of cells per milliliter and optical density of the microalgal culture.  
In both experiments with SE and SR, it was observed the presence of lag phase, however with different time intervals. For instance, in the 50% SR, it was possible to observe the stationary phase between days 8 to 10, and the declining phase until day 12 of the assay. On the other hand, in the 75% SE, the declining phase was observed from day 8 until the end of the assay. Regarding cultivation in different SE concentrations (Figure 3a), no statistically significant differences were found between the number of cells per milliliter until day 8 of the experiment. From that day on, the 100% SE stands out (\(p<0.05\)) from the other treatments, showing the lowest number of cells (day 12; \(2.37\times 10^{7}\) cells/ mL). Since days 8 to 12, no statistically significant differences were observed between the control, the 25 and 50% SE.  
Regarding the cultivation of _C. vulgaris_ in SR (Figure 3b), statistically significant differences (\(p<0.05\)) in the cellular density were only observed at day 12 of the experiment. In this sampling day, the 100% SR (8.42 \(\times\) 10\({}^{6}\) cells/mL) stands out, by differing from all the other treatments (\(p<0.05\)). However, for day 12, no statistical differences were detected when comparing control (\(1.95\times 10^{7}\) cells/mL), the 25% (1.62 \(\times\) 10\({}^{6}\) cells/mL) and the 75% SR treatments (7.91 \(\times\) 10\({}^{6}\) cells/mL) (\(p>0.05\)).  
The specific growth rate is an important parameter to assess the dynamic behavior of the microalgae culture (Figure 4). Except for the 50% SR, it was possible to visualize an overall increase of the specific growth rate using the SR as a growth medium, compared to the utilization of the SE. However, the 75% concentration of both SE and SR stands out from the other treatments (\(p\)-value \(<0.05\)).  
For the SE assay, the highest specific growth rates, and showing statistically significant differences (\(p\)-value \(<0.05\)) from the remaining treatments, was recorded for the 25% and 50% SE concentration (0.47 and 0.46 day\({}^{-1}\), respectively), while the control unveiled a value of 0.39 day\({}^{-1}\). Comparatively, the 75 and 100% SE exhibited the lowest specific growth rate (0.33 and 0.32 day\({}^{-1}\), respectively).  
In the SR assay, the control and 50% SR showed the highest specific growth values (0.53 and 0.56 day\({}^{-1}\), respectively), contrasting with the remaining treatments, where a decreasing trend was observed in the percentages SR, namely 75, 50 and 100, exhibiting respectively the values of 0.46, 0.43 and 0.42 day\({}^{-1}\). No statistically significant differences were observed among different SR concentrations.  
Figure 3: Microalgal growth in the different concentrations of (**a**) secondary effluent (SE) and (**b**) sludge run-off (SR) for 12 days (mean \(\pm\) standard deviation; n = 3). The symbol * indicates statistical differences among data (\(p\)-value \(<0.05\)).