## 3 Results and Discussion
### Performance of Individual PBRs under High-Grade CO\({}_{2}\)  
We conducted an additional test to identify the removal effect under a very high CO\({}_{2}\) level, according to a date of cultivation (Day 1, 2, and 5) and volumetric capacity (Figure 14, Table 5). A 10,000 and 6000 ppm dose of CO\({}_{2}\) (for daily observation and separate PBR test, respectively) were initially supplied in 30 s and mechanically reinforced to circulate the system through the air tubes at a rate of 2 L/min. Each test was started at 12:00 PM and continued for 6 h. Figure 13 displays the measured data distributions and their polynomial approximation. As the profiles show in Figure 13a, for all dates, the CO\({}_{2}\) concentrations in the chamber were shortly decayed in 25 min and leveled off afterwards. Nevertheless, note that their absorption behaviors were different. Estimated from the fit curves, they began to stabilize at 24 min 37 s (Day 1), 17 min 7 s (Day 2), and 16 min 48 s (Day 4), respectively. The mean value of 871.2 ppm on Day 5 was significantly lower than that of Day 1 and 2 (1020.4 and 1014.5 ppm). The Kruskal-Wallis rank sum test followed by the Dunn test using the Bonferroni method supported this difference, which confirmed better performance of the system on Day 5.  
As expected in the developmental states and Figure 11a, this suggests that the system improves the IAQ more efficiently after full-fledged _S. maxima_ growth. However, note that the Dunn test _p_-values showed that variances were significantly different in all the cases, but it finds that the variance of Day 1 and Day 2 was statistically identical. Figure 13b characterizes an impact of the culture volume and light on individual PBRs' performances on Day 7. The largest volume (7 L) showed a stable settlement at the minimum of 515 ppm at the end. Also, the 7L-PBR performance more continuously saturated until the end of the test, while the 2L- and 4L-PBR tended to reach a plateau earlier (at ~2 h 43 min and ~2 h 21 min, respectively). It is interesting to note that the quickest CO\({}_{2}\) drop (Figure 14b) was detected in the 4L-PBR illuminated by the highest power (28.8 W) yellow LED. This result parallels the fact that increased energy intensity rapidly activates photosynthetic  
Figure 14: Test results (forced CO\({}_{2}\) injection): bar charts: (**a**) Daily, (**b**) PBR capacity.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Test** & **Parameter** & **Day 1** & **Day 2** & **Day 5** & **2 L** & **4 L** & **7 L** \\ \hline \(\mu\) & 1020.4 & 1014.5 & 871.2 & 1337.0 & 1096.3 & 1305.3 \\ \(\sigma\) & 982.9 & 940.1 & 1112.6 & 1028.5 & 349.2 & 1231.0 \\ _Min._ & 584 & 686 & 504 & 695 & 949 & 515 \\ \hline Kruskal _p_-Value & & *** & & & *** \\ \hline \hline \end{tabular}
\end{table}
Table 5: Forced CO\({}_{2}\) injection: Statistical analysis of the carbon dioxide density in the chamber.  
production of microalgae before saturation [59]. Meanwhile, one may assume that the 7-L PBR should have showed better CO\({}_{2}\) mitigation during the early stage, since the cell's vital response is generally the most sensitive at red LED wavelengths of 640-700 nm [60]. However, this result does not contradict the existing findings [52; 61] that light source color has a far lesser impact on cultivation than the amount of photon energy reception.