# Can Anaerobically Digested Food Effluent Support _Arthrospira platensis_ Cultivation in Open Ponds?
## 4 Discussion
This controlled nitrogen availability likely reduced ammonia stress while sustaining prolonged biomass accumulation. [PERSON] et al. [31] found that increasing digestate concentration reduced initial growth rates but significantly enhanced biomass production, aligning with our findings. The staged addition strategy in 50% and 70% ADFE cultures optimized nitrogen assimilation, ensuring stable _A. platensis_ productivity. The residual ammonium concentrations in the 50% and 70% ADFE treatments correlated with increased biomass productivity. The 70% ADFE treatment achieved the highest volumetric (44.2 mg L\({}^{-1}\) d\({}^{-1}\)) and areal productivity (8.83 g m\({}^{-2}\) d\({}^{-1}\)), while a gradual ammonium decline sustained nitrogen availability. In contrast, the 37.5% ADFE treatment exhibited lower productivity despite efficient initial ammonium uptake, likely due to early nitrogen depletion. This aligns with findings by [PERSON] and [PERSON] (1996) [32], who reported reduced microalgal growth at both low (10 mg N L\({}^{-1}\)) and excessive (750-1000 mg N L\({}^{-1}\)) ammonium concentrations, emphasizing the need for an optimal nitrogen range. Although 50% and 70% ADFE showed similar productivity trends, the slightly higher yield in 70% ADFE suggests that increased nutrient availability provided a minor advantage without inducing ammonia stress. The maximum biomass yield in this study (1.38 g L\({}^{-1}\)) is comparable to previous outdoor raceway pond studies. For instance, [PERSON] et al. [33] reported a biomass yield of 1.24 g L\({}^{-1}\), while Earthrise Farms achieved 8.2 g m\({}^{-2}\) d\({}^{-1}\) under optimized conditions [34]. Similarly, [PERSON] et al. [33] reported 11.75 g m\({}^{-2}\) d\({}^{-1}\), closely aligning with our incremental ADFE addition strategy, which yielded 8.83 g m\({}^{-1}\) d\({}^{-1}\). The stable chlorophyll \(a\) content observed across all ADFE treatments suggests that _A. platensis_ adapted to variable nutrient conditions without compromising pigment production. This aligns with previous studies demonstrating Spirulina's resilience in wastewater-based media. For instance, [PERSON] et al. [35] reported unchanged chlorophyll levels in Spirulina cultivated in digested distillery effluent, supporting its adaptability to alternative nutrient sources. Additionally, 10% tofu wastewater was found to enhance chlorophyll and carotenoid content [36], further confirming _A. platensis_'s ability to utilize organic and inorganic nitrogen sources effectively. The adaptive responses of _A. platensis_ in ADFE-supplemented media are comparable to those observed in other microalgae cultivated in nutrient-rich wastewaters. For instance, _A. platensis_ grown in digested distillery effluent demonstrated efficient nitrogen assimilation and stable chlorophyll content, similar to our findings with ADFE [37]. Likewise, cultures cultivated in tofu wastewater showed enhanced pigment synthesis and biomass accumulation [36], indicating effective utilization of organic and inorganic nitrogen sources. These studies suggest that _A. platensis_ exhibits metabolic flexibility across different wastewater types, with staged nitrogen availability playing a key role in optimizing growth and productivity. These findings highlight ADFE's potential as a sustainable and cost-effective alternative for large-scale Spirulina cultivation. The staged addition strategy effectively managed nitrogen levels, reducing ammonia toxicity while maintaining stable productivity [38]. Scaling up this method will require optimizing ADFE dosing and assessing long-term stability in continuous systems. Real-time ammonium monitoring and automated dosing could enhance efficiency, ensuring sustained nutrient availability. Additionally, integrating ADFE-based cultivation with wastewater treatment systems could provide a dual benefit of biomass production and nutrient recycling [39].