## 4 Discussion  
Enhanced microalgal growth was observed when it was cultivated in BG-11 medium supplemented with 50% DSW, corroborating previous studies that highlighted the benefits of combining SW with BG-11 for microalgal cultivation [47]. The utilization of 50% DSW not only capitalizes on the nutrient richness of wastewater but also mitigates the inhibitory effects associated with higher concentrations of heavy metals and other toxins found in less diluted wastewater. The high concentrations of Chl-\(a\) and Chl-\(b\) on day 4 indicated that the 50% DSW mixture provides an optimal balance of nutrients, which is crucial during the exponential growth phase of algae. However, a decline in chlorophyll levels by day 8, observed across all the conditions, likely indicated a transition to the stationary phase, where nutrient depletion or metabolic shifts lead to reduced chlorophyll synthesis [48]. In contrast, the cultures grown in 100% DSW displayed significant morphological stress, characterized by clumped cells and the formation of large vacuoles, which are typical indicators of toxic \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Fatty Acid**} & \multirow{2}{*}{**BG-11**} & \multicolumn{2}{c}{**BG-11 +**} & \multicolumn{2}{c}{**BG-11 +**} & \multirow{2}{*}{**100\% DSW**} \\  & & **25\% DSW** & & **50\% DSW** & \\ \hline C14:0 & - & - & 0.3 & 0.4 \\ C16:0 & 20.2 & 18.2 & 17.0 & 24.4 \\ C16:2 (\(\omega\)6) & - & 1.5 & 1.5 & 1.9 \\ C16:3 (\(\omega\)3) & - & 4.2 & - & - \\ C16:4 (\(\omega\)3) & 11.6 & 12.5 & 13.9 & 11.1 \\ C18:0 & - & 1.5 & 0.9 & 1.7 \\ C18:1 & 18.5 & 14.5 & 10.5 & 14.5 \\ C18:2 (\(\omega\)6) & 10.4 & 8.4 & 8.7 & 9.2 \\ C18:3 (\(\omega\)3) & 39.3 & 39.1 & 44.9 & 36.6 \\ C18:4 & - & - & 2.2 & - \\ \hline Saturated fatty acids (\%) & 20.2 & 19.7 & 18.2 & 26.6 \\ Monousaturated fatty acids (\%) & 18.5 & 14.5 & 10.5 & 14.5 \\ Polyunsaturated fatty acids (\%) & 61.3 & 65.8 & 71.3 & 58.9 \\ \hline Total & 100 & 100 & 100 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Relative abundance of FAMEs in \(H\). _rubscens_ KNUA214 biomass under different concentrations of DSW over an 8-day period. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **BG-11** & **BG-11 +** & **BG-11 +** & **100\%** & **EN14214** & **ASTM D6751** \\  & **25\% DSW** & **50\% DSW** & **DSW** & **Standard** & **Standard** \\ \hline SV & 145 & 178 & 167 & 165 & \\ IV & 134 & 173 & 173 & 141 & \(\leq\)120 & \\ CN & 37.8 & 35.3 & 32.3 & 40.3 & \(\geq\)51 & \(\geq\)47 \\ DU & 104 & 131 & 129 & 109 & \\ CFPP & \(-\)11.8 & \(-\)9.3 & \(-\)10.7 & \(-\)14.2 & \(\leq\)0 & \(\leq\)\(-\)3 \\ OS & 5.8 & 5.3 & 5.2 & 5.7 & \(\geq\)8 & \(\geq\)3 \\ \(\
u\) & 3.58 & 3.48 & 3.40 & 3.63 & 3.5–5.0 & 1.9–6.0 \\ \(\rho\) & 0.89 & 0.89 & 0.89 & 0.89 & 0.86–0.90 & 0.82–0.90 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Biodiesel quality analysis of \(H\). _rubscens_ KNUA214 biomass under different concentrations of DSW after 8 days. stress. These conditions confirmed the adverse effects of high metal ion concentrations in less diluted DSW that surpass the microalgae's tolerance thresholds, leading to inhibited growth and cellular damage [49]. The notable morphological differences between the cultures grown in BG-11 with 50% DSW and those grown in 100% DSW emphasize the critical need for optimizing wastewater dilution. Proper dilution harnesses the benefits of nutrient availability while preventing the detrimental impacts of metal toxicity. Achieving this balance is vital not only for maximizing biomass yield but also for maintaining the physiological health of the algae, which is essential for sustainable bioenergy production. The nutrient removal efficiency of _H. rubescens_ KNUA214 in 100% DSW within 8 days--59.3% of TN, 67.7% of NH\({}_{3}\)-N, 40.7% of TP, and 1.9% of SO\({}_{4}\)2--highlight its potential in wastewater treatment [50; 51]. Mixed cultures can achieve enhanced performance by improving removal rates through synergistic effects.