## 3 Results and Discussion
### Growth Performance of Microalgae Cultured in the Mixed Wastewater  
Figure 1 shows that the growth performance of _C. pyrenoidosa_ varied greatly at different mix ratios of SW to DFW. Microalgae grew poorly at the mix ratio of SW:DFW = 1:0.5, which was possibly due to the high concentration of TN (249.55 mg L\({}^{-1}\)). Ammonia nitrogen is \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & **Swine** & **Furfural** & & & & \\  & **Wastewater** & **Wastewater** & **SW:DFW * = 10.5** & **SW:DFW = 1.3** & **SW:DFW = 1.9** & **SW:DFW = 1.19** \\  & **(SW)** & **(FW)** & & & & \\ \hline COD (mg L\({}^{-1}\)) & 4316.40 \(\pm\) 396.56 & 50,978.67 \(\pm\) 544.95 & 4394.00 \(\pm\) 207.12 & 5026.01 \(\pm\) 65.91 & 5056.67 \(\pm\) 42.34 & 4996.33 \(\pm\) 21.91 \\ TN (mg L\({}^{-1}\)) & 4254.1 \(\pm\) 17.82 & 33.47 \(\pm\) 3.52 & 249.55 \(\pm\) 117.3 & 147.56 \(\pm\) 4.71 & 63.73 \(\pm\) 2.34 & 249.21 \(\pm\) 1.67 \\ TP (mg L\({}^{-1}\)) & 14.18 \(\pm\) 1.56 & 1.01 \(\pm\) 0.04 & 8.88 \(\pm\) 0.06 & 3.68 \(\pm\) 0.03 & 1.52 \(\pm\) 0.01 & 1.00 \(\pm\) 0.01 \\ pH & 6.52 \(\pm\) 0.12 & 3.10 \(\pm\) 0.51 & 7.1 \(\pm\) 0.20 & 7.1 \(\pm\) 0.21 & 7.1 \(\pm\) 0.18 & 7.1 \(\pm\) 0.20 \\ COD/TN ratio & 10 \(\pm\) 1 & 1540 \(\pm\) 157 & 18 \(\pm\) 1 & 34 \(\pm\) 3 & 79 \(\pm\) 5 & 202 \(\pm\) 20 \\ \hline \hline \end{tabular}  
* DFW means the diluted furfural wastewater. The furfural wastewater was diluted by 10 times with water to obtain the DFW.  
\end{table} 
Table 1: Physicochemical characteristics of swine wastewater (SW), furfural wastewater (FW), and the mixed wastewater at different ratios of SW to DFW. Data are shown as mean \(\pm\) SD, n = 3.  
toxic to microalgal cells and can inhibit microalgae growth [26]. In this study, although the ammonia nitrogen concentration of the mixed wastewater was not measured, it is probable that the mixed wastewater had a high ammonia nitrogen concentration. This is because the nitrogen in the mixed wastewater was mainly sourced from the SW, which had a large proportion of ammonia nitrogen [16]. Figure 1: Growth performance of _C. pyrenoidosa_ cultured in the mixed wastewater of swine wastewater (SW) and diluted furfural wastewater (DFW) with different proportions. Data are shown as mean \(\pm\) SD, n = 3. Different letters in the same column represent significant differences (\(p\leq 0.05\)). Microalgae grew well in the mixed wastewater at the other mix ratios. In general, microalgal biomass concentration increased rapidly, after cells acclimatized to the environment of the mixed wastewater, until to a maximum point and then slightly decreased, or were maintained at a stable level. The maximum biomass concentrations at mixing ratios of SW:DFW = 1:3, 1:9 and 1:19 were 0.87, 0.93 and 0.99 g L\({}^{-1}\), respectively, which were approximately three times higher than at mixing ratios of SW:DFW = 1:0.5. The specific growth rate strongly decreased when the mix ratio of SW:DFW was 1:0.5. In comparison with the other experimental groups, the TN concentration at the mix ratio of SW:DFW = 1:0.5 was much higher. Thus, the low specific growth rate may be caused by the high concentration of ammonia nitrogen in the mixed wastewater due to its toxicity to the microalgae cells [16; 26; 31]. The increased proportion of FW in the mixed wastewater can promote the growth of microalgae because FW contains organic acids. The addition of organic matter into wastewater in a certain range is beneficial to the mixotrophic growth of microalgae [15; 32]. However, we observed no significant differences in the biomass concentration and algal growth rate at the mix ratios of SW:DFW = 1:9 and 1:19. The specific growth rate of microalgae was even decreased slightly with an increase of FW proportion in the mixed wastewater, suggesting that the further increase in organic matter in the mixed wastewater cannot further promote microalgae growth. A high carbon/nitrogen ratio can decrease the microalgae growth rate and reduce cell viability due to substrate inhibition [33; 34].