## 3 Results and Discussion
### Algal Growth at Different Initial Optical Densities  
Algal growth curves of NCU-39 showed similar trends in the pretreated 5\(\times\) diluted ADPE in all initial OD treatments (Figure 6). ODs rapidly reached about 2.1 until day 2 in the pretreated ADPEs at initial ODs of 0.3, 0.4, and 0.6, almost identical to those in Tap medium, while those increased to 1.6 at initial OD as 0.2 (Figure 6). Tap medium contains \begin{table}
\begin{tabular}{c c c c c} \hline
**Nutrient (mg L\({}^{-1}\))** & **Time (Days)** & **3\(\times\)** & **5\(\times\)** & **10\(\times\)** \\ \hline NH\({}_{4}\)+-N & 0 & 653.7 \(\pm\) 7.7 & 434.3 \(\pm\) 3.9 & 216.6 \(\pm\) 14.8 \\ NH\({}_{4}\)+-N & 3 & 586.3 \(\pm\) 7.6 & 380.6 \(\pm\) 12.9 & 153.4 \(\pm\) 7.1 \\  & 0 & 656.8 \(\pm\) 1.0 & 456.6 \(\pm\) 1.9 & 235.5 \(\pm\) 1.2 \\ TN & 3 & 611.0 \(\pm\) 4.3 & 422.6 \(\pm\) 1.8 & 191.8 \(\pm\) 2.5 \\  & 0 & 23.1 \(\pm\) 0.5 & 20.7 \(\pm\) 1.3 & 9.4 \(\pm\) 0.5 \\ TP & 3 & 33.0 \(\pm\) 2.1 & 27.2 \(\pm\) 1.5 & 13.6 \(\pm\) 0.4 \\ COD & 0 & 4986.3 \(\pm\) 66.7 & 4066.7 \(\pm\) 70.7 & 2976.7 \(\pm\) 45.3 \\  & 3 & 3410.2 \(\pm\) 54.3 & 3083.3 \(\pm\) 23.6 & 2443.3 \(\pm\) 30.2 \\ pH & 0 & 4.0 \(\pm\) 0.1 & 4.1 \(\pm\) 0.1 & 4.2 \(\pm\) 0.2 \\  & 3 & 3.1 \(\pm\) 0.1 & 3.1 \(\pm\) 0.1 & 3.2 \(\pm\) 0.1 \\ \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of 3-fold (3\(\times\)), 5\(\times\), and 10\(\times\) diluted anaerobic digestion piggery effluents (ADPEs) before and after three-day fungal decolorization. The results are presented as arithmetic means \(\pm\) standard deviations (\(n\) = 3). inorganic nutrients and amounts of available organic carbon, i.e., acetic acid (Table S4), which can enhance the growth rate and achieve satisfactory maximum _Chlorella_ sp. yield by short-time mixotrophy [34, 35]. In all initial OD treatments, algal yields showed high growth rates in the pretreated ADPEs during the first two days (Figure 6), which might be attributed to amounts of residual added glucose from the pretreated ADPEs during fungal cultivation (see Section 2). Glucose has been demonstrated as an available organic carbon source for achieving a maximum algal yield of _Chlorella_ sp. under mixotrophy [36, 37]. Figure 5: Growth curve of NCU-39 in different diluted anaerobic digestion piggery effluents (ADPEs) (3-fold (3\(\times\)), 5\(\times\), and 10\(\times\) dilution) pretreated by 3-day fungal decolorization. Tap medium was used as a control. Error bars represent standard deviations (\(n=3\)). Figure 6: Growth curves of NCU-39 at different initial optical densities (ODs) (OD values = 0.2, 0.3, 0.4, and 0.6) in 5-fold diluted anaerobic digestion piggery effluents (ADPEs) pretreated by 3-day fungal decolorization. Tap medium was used as a control. Error bars represent standard deviations (\(n=3\)). Due to the nearly consistent maximum algal yields obtained at initial ODs of 0.3, 0.4, and 0.6, initial OD of 0.3 sufficed to achieve maximum algal yield in the pretreated ADPE. Generally, a higher initial cell density brings about a better algal yield. However, the obtained maximum algal yield would accumulate auto-inhibitors and reduce photosynthetic efficiency in culture medium, leading to a self-shading effect of limiting algal growth [38]. Thus, the almost same maximum yield obtained at higher initial ODs of 0.4 and 0.6 might be hindered by the self-shading effect. In order to reduce initial cell density as much as possible for high cost-efficiency, initial OD of 0.3 was therefore selected as inoculum density for subsequent experiments. After day 2, ODs slumped in all pretreated ADPEs until the end of the experiment, while those remained constant in the Tap medium (Figure 6). The trend of algal yields might be related to pH variation. Throughout the experiment, pH rapidly declined from 7.0 to about 3.5 in the pretreated ADPEs but increased to 8.1 in the Tap medium (Figure S3).