## 3 Results and Discussion
### Effect of Different Carbon Sources on Microalgal Growth, Pigments, and Nutrient Removal
Microalgae biodiesel, a renewable alternative to petroleum diesel with a reduced carbon footprint [7; 8], faces high production costs primarily due to the expensive cultivation process [9; 10]. Utilizing wastewater as a nutrient source for microalgae offers dual benefits, cost-effective nutrient recycling and bioremediation [13], while accumulating lipids for biodiesel [13; 14; 18; 19]. However, the limited pollutant removal efficiency hinders scalability. Mixotrophic cultivation with carbon supplementation in wastewater can enhance biomass productivity [31; 33], as demonstrated through optimal carbon source screening in ADSW systems in our study. Figure 3: Time course profile of chlorophyll \(a\) concentration (**a**) and carotenoid concentration (**b**) for FACHB-8 cultivated in ADSW supplemented with different carbon sources. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Carbon Source**} & \multicolumn{3}{c}{**TN**} & \multicolumn{3}{c}{**TP**} \\ \cline{2-7}  & \begin{tabular}{c} **Initial** \\ **Concentration** \\ **(mg/L)** \\ \end{tabular} & \begin{tabular}{c} **Final** \\ **Concentration** \\ **(mg/L)** \\ \end{tabular} & \begin{tabular}{c} **Removal** \\ **Efficiency (\%)** \\ \end{tabular} & \begin{tabular}{c} **Initial** \\ **Concentration** \\ **(mg/L)** \\ \end{tabular} & \
\begin{tabular}{c} **Final** \\ **Concentration** \\ **(mg/L)** \\ \end{tabular} &
\begin{tabular}{c} **Removal** \\ **Efficiency (\%)** \\ \end{tabular} \\ \hline CK & 332.47 \(\pm\) 41.37 & 37.77 \(\pm\) 5.86 & 88.58 \(\pm\) 2.26 \({}^{\text{b}}\) & 12.40 \(\pm\) 0.91 & 6.93 \(\pm\) 0.41 & 44.16 \(\pm\) 1.13 \({}^{\text{b}}\) \\ NaHCO\({}_{3}\) & 110.20 \(\pm\) 6.00 & 35.40 \(\pm\) 2.69 & 67.84 \(\pm\) 3.00 \({}^{\text{c}}\) & 21.47 \(\pm\) 1.60 & 6.23 \(\pm\) 0.89 & 43.76 \(\pm\) 4.38 \({}^{\text{b}}\) \\ Glucose & 453.13 \(\pm\) 11.35 & 42.43 \(\pm\) 3.84 & 90.62 \(\pm\) 1.19 \({}^{\text{a}}\) & 17.53 \(\pm\) 0.34 & 5.78 \(\pm\) 1.00 & 76.14 \(\pm\) 0.76 \({}^{\text{a}}\) \\ Na\({}_{2}\)CO\({}_{3}\) & 189.80 \(\pm\) 18.37 & 24.95 \(\pm\) 3.60 & 86.56 \(\pm\) 3.89 \({}^{\text{ab}}\) & 20.33 \(\pm\) 1.00 & 4.18 \(\pm\) 0.13 & 23.06 \(\pm\) 6.78 \({}^{\text{c}}\) \\ CH\({}_{3}\)COONa & 189.20 \(\pm\) 22.84 & 25.48 \(\pm\) 3.69 & 87.65 \(\pm\) 0.62 \({}^{\text{a}}\) & 24.53 \(\pm\) 3.87 & 12.44 \(\pm\) 0.90 & 48.32 \(\pm\) 9.76 \({}^{\text{b}}\) \\ Glycerol & 233.87 \(\pm\) 17.50 & 45.37 \(\pm\) 6.15 & 80.32 \(\pm\) 5.02 \({}^{\text{b}}\) & 12.60 \(\pm\) 0.71 & 12.03 \(\pm\) 0.24 & 50.63 \(\pm\) 7.82 \({}^{\text{b}}\) \\ Sucrose & 281.00 \(\pm\) 37.54 & 31.367 \(\pm\) 1.479 & 85.25 \(\pm\) 5.29 \({}^{\text{ab}}\) & 19.00 \(\pm\) 1.88 & 15.81 \(\pm\) 0.77 & 69.93 \(\pm\) 3.78 \({}^{\text{a}}\) \\ \hline \hline \end{tabular} Note: Values with different letters represent significant difference (\(p<0.05\)) between treatments.  
\end{table} 
Table 3: Nutrient removal of FACHB-8 cultivated in ADSW supplemented with different carbon sources. In our carbon source screening experiments, six inorganic and organic carbon sources--sodium carbonate, sodium bicarbonate, sodium acetate, glucose, sucrose, and glycerol--were selected based on literature reports of mixotrophic microalgae cultivation. According to ICIS Chemicals [50], the market prices (USD/ton) for industrial-grade sodium bicarbonate, sodium carbonate, glucose, sodium acetate, glycerol, and sucrose are 120-150, 100-130, 300-380, 800-1000, 600-800, and 450-550, respectively. Our experimental results identified glucose as the optimal carbon source for ADSW supplementation. However, despite its moderate price ranking among the six candidates, glucose contributes approximately 80% of the total medium costs in mixotrophic systems [51; 52], creating economic barriers for large-scale implementation. To address this, agricultural and food industry byproducts rich in glucose or fermentable sugars (e.g., can molasses, sugarcane bagasse hydrolysates, and starch-processing wastewater) have emerged as viable low-cost alternatives [17; 53].