## 3 Results and Discussion
### Influence of Mix Ratio of Wastewaters on Nutrient Removal  
During the culture process, microalgae were able to assimilate and utilize organic matter, nitrogen and phosphorus in the wastewater to synthesize algal biomass [11]. Therefore, the process of nutrient removal from the mixed wastewater by microalgae cultivation was studied. Variations in the COD, TN and TP concentrations in the mixed wastewaters at different mix ratios of SW to DFW are presented in Figure 2. All of the nutrients in the mixed wastewaters gradually decreased with increased culture time. The reduction degrees and rates were significantly dependent on the mix ratios of SW to DFW. For example, the removal efficiencies of COD in the wastewaters at the mix ratios of SW:DFW = 1:0.5, 1:3, 1:9 and 1:19 were 5.11%, 57.30%, 49.43% and 49.21%, respectively (Figure 3). With an increase in the proportion of FW, the COD removal efficiency initially increased and then slightly decreased. This suggested an optimal mix ratio of 1:3 (SW: DFW). The COD values of mixed wastewaters at the mix ratios of SW:DFW = 1:3, 1:9 and 1:19 reached a stable level and varied only slightly after 6 d. However, we cannot confirm whether the residual COD are recalcitrant COD or not. The main composition of the remaining COD may be the recalcitrant organic substances that cannot be easily absorbed and utilized by microalgae. Another possibility may be associated with microalgae growth. Microalgae did not constantly grow or even died at the late stage of culture process (Figure 1). Thereby, microalgae cells are not able to efficiently utilize organic substance in the wastewater. Thus, even if the biodegradable matters (COD) existed in the mixed wastewater, microalgal cells would be unable to use them. On possible factor limiting microalgae growth at a late stage is the low gas-liquid mass transfer. With the increase of biomass concentration, microalgal cells need more dissolved oxygen or carbon dioxide to grow mixotrophically [35]. However, the gas-liquid mass transfer coefficient is low for flasks without aeration. Thus, microalgae growth is likely hindered by low concentrations of dissolved oxygen or carbon dioxide. Another reason may be the extremely low TPconcentrations in the experimental groups with ratios of SW:DFW = 1:3 and 1:9. The deficiency of TP resulted in the limitation of microalgae growth. Figure 2: Removal performance of COD (**a**), TN (**b**) and TP (**c**) by _C. pyrenoidosis_ cultured in mixed wastewater with different ratios of swine wastewater (SW) to diluted fur  
For TN and TP removals, similar tendencies as for COD removal were observed. There was poor removal of TN and TP from the mixed wastewater at the mix ratio of SW:DFW = 1:0.5. The highest removal efficiencies of TN and TP were 33.76% and 61.93%, respectively, both at the mix ratio of SW:DFW = 1:9. The mixed wastewater with a low (18:1) or high (202:1) COD/TN ratio had low removal efficiency of COD, TN and TP (Figure 3). [PERSON] et al. [36] also observed that nutrient removal was relatively low in synthetic domes  
Figure 3: Removal efficiency and rate of COD (**a**), TN (**b**) and TP (**c**) in mixed wastewater with different ratios of swine wastewater (SW) to diluted furfural wastewater (DFW). Data are shown as mean \(\pm\) SD, n = 3. Different letters in the same column represent significant differences (\(p\leq 0.05\)). tic sewage when the concentration of the carbon source was low (low COD/TN) or the nitrogen source was insufficient (high COD/TN). [PERSON] et al. (2016) studied the effect of carbon/nitrogen ratio on nutrient removal from manure-free piggery wastewater. They found that a low or high carbon/nitrogen ratio (indicated by TC/TN) restricted nutrient removal due to the inhibition of ammonium or glycerol. In the present study, the low amount of organic carbon added to the mixed wastewater at the mixing ratio of SW:DFW = 1:05 (low COD/TN) possibly limited microalgae growth, resulting in low removal efficiency of nutrients.