Removing nutrients in recirculating aquaculture system wastewater from Nile tilapia culture via Spirulina cultivation: Optimizing sodium bicarbonate concentration and micronutrient supplementation

Recirculating aquaculture system (RAS) wastewater from Nile tilapia (Oreochromis niloticus) culture raises environmental concerns, necessitating proper treatment prior to discharge. Phytoremediation using Spirulina offers a promising solution by enabling the simultaneous removal of nutrients from wastewater while also producing valuable biomass. This study aimed to optimize the sodium bicarbonate (NaHCO3) concentrations (3.36, 8.40, and 16.80 g/L) for achieving maximum nutrient treatment efficiency and Spirulina production in RAS wastewater from Nile tilapia culture, without external nitrogen and phosphorus supplementation. Additionally, this study examined the effect of different concentrations of nitrate (10 to 120 mg NO3-N/L) that are naturally present in RAS wastewater and Zarrouk's micronutrient supplementation on Spirulina production and nutrient treatment efficiency. The optimal NaHCO3 concentration for achieving maximum Spirulina production and effective nutrient treatment efficiency was found to be 8.40 g/L. At this concentration, high nitrate treatment efficiencies, reaching up to 80%, were observed specifically in RAS wastewater containing 10 to 50 mg NO3-N/L, while excellent phosphate treatment efficiencies, up to 99%, were found across all nitrate concentrations. Moreover, Spirulina biomass (X) reach a saturated growth of 0.398 g DW/L (6.09 mg Chl a/L) in RAS wastewater with a nitrate concentration of 50 to 120 mg NO3-N/L. Furthermore, the Pearson correlation coefficients revealed that the factor determining Spirulina growth in RAS wastewater with nitrate 10-50 mg NO3-N/L was nitrate, whereas in RAS wastewater with nitrate 80-120 mg NO3-N/L, it was NaHCO3. By supplementing Zarrouk's micronutrient at 25% to RAS wastewater containing 120 mg NO3-N/L and 16.80 g/L of NaHCO3, Spirulina biomass could be enhanced to reach 1.525 g DW/L (6.09 mg Chl a/L), while achieving a nitrate treatment efficiency of 86%.