## 3 Results and Discussion
### Air Purification Performance  
The results in Section 3.1 suggest that NaHCO\({}_{3}\)-free SOT can be used for our device. To identify an immediate CO\({}_{2}\) removal effect of the developed PBR-based air system using a 1:1 (vol.) mixture of the NaHCO\({}_{3}\)-free SOT and SOT, gas concentrations of the test chamber in the room (Figure 3) were tracked during the first 7 days after installation and the vessel cultivation using the NaHCO\({}_{3}\)-free SOT. The air temperature and humidity in the chamber were 26.9 \(\pm\) 0.8 \({}^{\circ}\)C and 63.3 \(\pm\) 3.8%. As seen in Table 3, we found that the air exchange through the \(S\). _maxima_ PBRs does not necessarily lead to revamping the gas composition other than carbon dioxide. The oxygen almost invariably remained at about 20%, and other gases including volatile organic compounds (VOCs) were not significantly reduced. Figure 11a presents overall trends of CO\({}_{2}\) reduction. Starting from \(\sim\)1100 ppm in a non-treated environmental state, the CO\({}_{2}\) concentration gradually decreased by 55%, less than 500 ppm after 7 days. The regular fluctuations during the lag phase (1-3 days) of algal growth parallel a cyclic metabolic pattern of \(S\). _maxima_; CO\({}_{2}\) decreases due to daytime photosynthesis and increases during the night respiration. Before Day 3, it was seen that CO\({}_{2}\) accumulations were peaked at 12:00-2:00 a.m., but the peak intensity noticeably decreased since Day 4. In Figure 11b, the changes in daily average CO\({}_{2}\) concentration suggest that the exponential cell growth during Day 3 and 4 (see Figure 7) results in a sharp CO\({}_{2}\) decline with a diminutive respiration effect. The reduction rates in Figure 11c confirm  
Figure 10: Transient CFD simulation of the air dispersion in the test chamber: (**a**) Initial, 5 s, (**b**) Stationary, 60 min. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **VOCs (ppb)** & **CO (ppm)** & **O\({}_{2}\) (\%)** & **CH\({}_{4}\)** & **NO\({}_{2}\)** & **NH\({}_{3}\)** \\ \hline \(\mu\) & 129 & 98.6 & 19.98 & 621.8 & 100.4 & 96.7 \\ \(\sigma\) & 93 & 36.5 & 0.07 & 31.9 & 14.0 & 12.7 \\ _Min._ & 0 & 6 & 19.74 & 519 & 82 & 76 \\ _Max._ & 474 & 242 & 20.25 & 704 & 175 & 165 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Gas measurement result (chamber, 7 days). a thriving event on Day 4, which is consistent with study [31] revealing that _Spirulina_ cultures undergo exponential growth regimes in 3-7 days followed by stationary phases. Figure 12 compares CO2 reduction performance of our modified NaHCO3-exclusive SOT and the standard SOT. In Figure 13a-c, CO2 concentrations differed by about 300 ppm, which reveals that early cell growth highly depends on the readily available carbon content. However, note that such gaps were quickly narrowed after Day 4. The Mann-Whitney U test result confirmed that the average concentration in the NaHCO3-exclusive SOT was significantly lower than that in the standard SOT treatment. Also, the Brown-Forsythe test revealed that the null hypothesis could not be rejected, and their variance could be considered similar (Table 4). Figure 11: Test results (all PBRs): (**a**) Chamber CO2 decrease, (**b**) CO2 reduction per day, (**c**) Reduction rate per day. Figure 12: Test results: (**a**) Day 1, (**b**) Day 2, (**c**) Day 3, (**d**) Day 5, and (**e**) Day 7. Figure 13: Test results (forced CO\({}_{2}\) injection): (a) Daily comparison of CO\({}_{2}\) decrease, (b) CO\({}_{2}\) decrease and PBR capacity.