# Can Anaerobically Digested Food Effluent Support _Arthrospira platensis_ Cultivation in Open Ponds?
## 4 Discussion  
The utilization of food waste as a nutrient source for microalgae cultivation provides a sustainable alternative to synthetic media while offering a rich source of inorganic nutrients, such as ammonium. Previous studies have successfully cultivated _Scenedesmus_ and _Chlorella_ species using ADFE [28]. However, research on ADFE's feasibility for _Arthrospira platensis_ growth, particularly under outdoor conditions, remains limited. This study aimed to assess the effects of incremental ADFE addition on _A. platensis_ productivity, optimizing nitrogen utilization while mitigating potential ammonia toxicity. One of the key challenges in cultivating _A. platensis_ using wastewater-based media is ammonia toxicity, which inhibits growth and can lead to culture failure. High ammonia-nitrogen concentrations disrupt cellular metabolism [29] and, in open raceway ponds with high pH (>9), ammonia stripping can further reduce nitrogen availability and lower growth efficiency [12]. [PERSON] et al. [28] demonstrated that at higher pH, ammonium (NH4\({}^{\circ}\)) converts into volatile ammonia (NH3), which is lost to the atmosphere, potentially limiting nitrogen uptake. In this study, _A. platensis_ cultures were maintained at pH-10, suggesting that some ammonia loss may have occurred. To mitigate ammonia stress, incremental ADFE addition was employed for 50% and 70% treatments instead of a single-dose approach. This approach minimized ammonia \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Treatment**} & **Overall Volumetric** & **Overall Areal** & **Maximum Specific** & **Chlorophyll _a_** \\  & **Productivity** & **(g m\({}^{-2}\) d\({}^{-1}\))** & **Growth Rate (\(\mu_{\max}\))** & **(mg/g Biomass)** \\ \hline Zarrouk & 22.9 \(\pm\) 1.76 \({}^{\text{a}}\) & 4.58 \(\pm\) 0.35 \({}^{\text{a}}\) & 0.093 \(\pm\) 0.03 \({}^{\text{a}}\) & 6.95 \(\pm\) 0.76 \({}^{\text{a}}\) \\ \hline
37.5\% ADFE & 28.3 \(\pm\) 1.71 \({}^{\text{b}}\) & 5.66 \(\pm\) 0.34 \({}^{\text{b}}\) & 0.13 \(\pm\) 0.03 \({}^{\text{a}}\) & 6.98 \(\pm\) 0.15 \({}^{\text{a}}\) \\ \hline
50\% ADFE-addition & 41.0 \(\pm\) 0.61 \({}^{\text{c}}\) & 8.20 \(\pm\) 0.12 \({}^{\text{c}}\) & 0.11 \(\pm\) 0.01 \({}^{\text{a}}\) & 7.17 \(\pm\) 0.98 \({}^{\text{a}}\) \\ \hline
70\% ADFE-addition & 44.2 \(\pm\) 3.48 \({}^{\text{c}}\) & 8.83 \(\pm\) 0.69 \({}^{\text{c}}\) & 0.13 \(\pm\) 0.01 \({}^{\text{a}}\) & 7.09 \(\pm\) 0.80 \({}^{\text{a}}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Average areal and volumetric productivity, maximum yield (AFDW), and chlorophyll \(a\) of _A. platensis_ cultures in different treatments. Data are mean \(\pm\) SD, n = 3. Different letters indicate the significance of data in each column (One way ANOVA \(p\) < 0.05). shock while ensuring a sustained nitrogen supply. Results demonstrated that incremental ADFE addition significantly increased biomass productivity, with 70% ADFE achieving the highest biomass yield (1.38 g/L), outperforming Zarrouk medium and 37.5% ADFE cultures. Specifically, the 70% and 50% ADFE treatments increased biomass productivity by 0.92 g/L and 0.55 g/L, respectively, compared to Zarrouk medium. A similar study by [PERSON] et al. [30] evaluated _A. platensis_ cultivated in media containing 0%, 10%, 20%, 30%, 40%, and 50% distilled liquid digestate, finding that 50% digestate yielded the highest biomass productivity. Our study further demonstrates that incremental addition plays a crucial role in nutrient balance, avoiding ammonia toxicity while sustaining growth. The initial ammonium concentration significantly influenced _A. platensis_ nitrogen uptake. In the 37.5% ADFE treatment, where all ADFE was added at once, ammonium started at 115 mg/L and was rapidly depleted by day 9, indicating efficient utilization but risking early nitrogen depletion. In contrast, the 50% and 70% ADFE treatments, with incremental additions, had much lower initial ammonium concentrations (34-37 mg/L) at day 0, leading to a more gradual decline.