## 3 Results and Discussion
### Lipid Production of Microalgae Cultured in the Mixed Wastewater  
There were no significant differences in the lipid contents of the experimental groups (Figure 4). The lipid content of microalgae only slightly increased with an increased proportion of FW. The possible reasons were the relatively lower nitrogen concentration or the higher organic carbon source in the wastewater when the mix ratio of SW to DFW decreased. Nitrogen deprivation is an effective approach to stimulate lipid accumulation in microalgal cells ([PERSON] et al., 2016). However, the final TN concentrations of wastewater at mixture ratios of 1:0.5, 1:3, 1:9 and 1:19 were 242.34, 99.06, 42.09 and 18.03 mg L\({}^{-1}\), respectively. This indicated that there was sufficient TN in the mixed wastewater, which did not provide a nitrogen-depletion condition for microalgae cultivation. Therefore, the lipid content of the microalgae was not significantly different between each experimental group. The addition of an organic carbon source to wastewater can also affect lipid accumulation ([PERSON] et al., 2016). The influence of carbon sources on lipid accumulation in microalgal cells was dependent on the amounts of carbon sources in the culture medium under the mixotrophic culture mode. [PERSON] et al. (2016) observed that a small amount of organic carbon source in the medium could not promote the accumulation of lipids in algal cells. [PERSON] et al. (2016) also found that there was no significant increase in the lipid content of _Chlorella sorokiniana_ cultured mixotrophically with low glucose addition. However, a relatively high concentration of an organic carbon source added to the wastewater can significantly stimulate lipid accumulation in microalgal cells ([PERSON] et al., 2016; [PERSON] et al., 2016; [PERSON] et al., 2016). Some microalgae species, such as _Nantochloropsisulata_[42], _Chorella sorokiniana_[39], _Chlorella pyrenoidosa_[24] and _Chlamydomonas reinhardtii_[41], cultured in mixotropical mode, can not only achieve a high microalgal biomass but also accumulate more lipids within cells in comparison with autophototrophic mode. Therefore, it is possible to improve algal lipid content and the productivity of microalgae grown in wastewater by adding an organic carbon source. [PERSON] et al. [39] found that the biomass concentration of _C. pyrenoidosa_ increased from 0.218 g L\({}^{-1}\) to 0.518 g L\({}^{-1}\) when actual municipal wastewater (MW) was added with 2.0 g L\({}^{-1}\) NaAC, and the corresponding lipid content was increased by 1.48 times. Acetyl-CoA is a well-known substantial material for lipid accumulation of microalgae [39]. In addition, lipid accumulation is an energy storage process that requires ATP and NADPH. The biosynthesis of triacylglycerlycerol (TAG) in the green alga _Chlorella_ is preceded by a large increase in acetyl CoA via upregulation of plastic pyruvate dehydrogenase [42]. [PERSON] et al. [43] believed that the acetyl CoA was likely produced by acetyl CoA synthase when microalgal culture medium was added by acetate as a substrate, which supplies sufficient acetyl CoA for algal growth and lipid synthesis in mixotrophic conditions. Compared to photoautotrophic cultivation, acetate supplies sufficient carbon molecules for algal mixotrophic growth. This leads to the reduction of ATP and NADPH that are used to fix CO\({}_{2}\) as a carbon source, resulting in more ATP and NADPH available for lipid synthesis [43]. Hence, the addition of organic carbon such as acetate can promote lipid accumulation and increase lipid productivity. In the present study, acetate was indirectly added to the SW by mixing with the FW that contained large amounts of acetate and which may improve the lipid accumulation of microalgae. However, some microalgal species can accumulate more lipid in photoautotrophic than in mixotrophic cultivation [27]. Thus, lipid accumulation behavior in the different trophic modes may be strain dependent. The lipid production was significantly affected by the mix ratio of SW to DFW.