# Can Anaerobically Digested Food Effluent Support _Arthrospira platensis_ Cultivation in Open Ponds?
## 1 Introduction  
According to recent estimates, 359 billion cubic meters of wastewater are produced annually worldwide, with 48% released untreated into the environment [1]. Untreated wastewater poses serious risks to human health and natural ecosystems, as pathogens in water are responsible for approximately 80% of illnesses and deaths in developing countries [2]. Additionally, wastewater from industrial, agricultural, and domestic activities often contains harmful chemicals that contaminate and degrade natural habitats and wildlife ecosystems [3]. Therefore, effective wastewater treatment systems are essential to enable recycling and reuse for environmental conservation. Food waste accounts for a significant proportion of municipal solid waste globally [4]. Anaerobic digestion (AD) is commonly used to process food waste, producing anaerobically digested food effluent (ADFE) as a byproduct [5]. Due to its high concentrations of nutrients such as ammonium and phosphorus, ADFE cannot be discharged directly into the environment, as it may cause eutrophication and other ecological issues. Residual nutrients in ADFE can be removed using physical, chemical, or biological treatment methods [6], but physical and chemical approaches are costly and generate secondary pollution. Biological treatments, by contrast, offer a cost-effective, environment-friendly alternative using microorganisms to remove nutrients [7]. One form of biological treatment is the use of efficient microorganisms for removing nutrients and toxic metals from wastewater, including microalgae [6]. Many microalgal species (e.g., _Chlorella_, _Scenedesmus_[8], and _Desmodesmus_[9]), as well as filamentous microalgae (e.g., _Rhizcolonium_ and _Ulothrix_[10]), have been cultivated in different wastewater effluents to reduce nutrient concentrations, particularly ammonia. In AD effluents, nitrogen typically exists in the form of ammonia, comprising 70-82% of the total nitrogen content. Ammonia-nitrogen (NH\({}_{3}\)-N) concentrations in anaerobically digested effluents range from 1000-6000 mg L\({}^{-1}\), and levels above 100 mg L\({}^{-1}\) have been shown to inhibit microalgal growth [11]. The ability of microalgae to grow on different effluents and tolerate high ammonia levels varies across microalgae species. For instance, it has been shown that _Chlorella_ sp. can grow in anaerobic digestate piggery effluent (ADPE) with up to 1600 mg NH\({}_{3}\)-N [12]. Moreover, the ammonia removal efficiencies vary from 30 to 100% depending on the species and conditions [13]. Therefore, identifying optimal microalgae species for specific wastewater types is critical for improving nutrient removal efficiency and growth rate. Among various microalgae, Spirulina (_Arthospira platensis_) has gained attention due to its applications in food, phycocyanin production, fertilizers, feed, and bioplastics [14]. However, the high cost of chemical-based culture medium for Spirulina cultivation remains a major limitation [15]. It was estimated that the cost at approximately USD 0.08 L\({}^{-1}\), accounting for around 35% of the total Spirulina biomass production cost [16]. Efforts to reduce expenses have included substituting Zarrouk's medium components with more cost-effective alternatives, such as commercial fertilizers and locally available chemicals, which have shown promise for large-scale Spirulina cultivation [17]. Studies have demonstrated that replacing conventional media with wastewater effluents, such as saline whey wastewater, anaerobically digested swine effluent, and lac wastewater, can also significantly reduce cultivation costs [18; 19; 20]. However, these effluents present challenges, including variability in nutrient concentrations, potential toxicity, and turbidity [21]. Consequently, these factors can result in lower nutrient content and biomass productivity compared to conventional media, such as Zarrouk medium [15]. Most importantly, Spirulina has shown its ability to efficiently reduce ammonia levels by 20% to 99% [22; 23].