Impacts of aquaculture RAS effluent source on treatment performance, microalgal growth, and biomass feed value

This study investigates the potential of four microalgal species (Chlorella vulgaris, Chlorella sorokiniana, Nannochloropsis oculata, and Haematococcus pluvialis) to grow in and remediate two types of aquaculture wastewater: raw fish tank effluent (Raw AWW; 20 mg/L NH₄⁺-N, 20 mg/L NO₃^--N, 100 mg/L COD) and the final effluent from a recirculating aquaculture system (RAS effluent; 40 mg/L NO₃^--N only). This study provides a novel comparative assessment of how nitrogen speciation and organic carbon availability affect microalgal performance at different points within an RAS setup. C. vulgaris and C. sorokiniana exhibited enhanced growth in Raw AWW, with final VS values 25% (p < 0.05) and 13% (p > 0.05) higher, respectively, than in RAS effluent. This improvement, along with more effective nitrogen removal and pigment accumulation, was likely driven by the simultaneous presence of ammonium and organic carbon, which supported mixotrophic metabolism. N. oculata grew similarly in both media but produced more protein (84 vs. 45 mg/g, p < 0.05) and pigments in RAS effluent. H. pluvialis exhibited minimal growth under both conditions, reaching final volatile solids (VS) concentrations of 581 ± 41 mg L^-1 in RAS effluent and 512 ± 48 mg L^-1 in raw aquaculture wastewater, despite its high intracellular protein content (∼150 mg g^-1). Carbohydrate accumulation was generally higher in Raw AWW than in RAS effluent, with C. vulgaris, C. sorokiniana, and N. oculata showing 2-3 fold increases (e.g., 560 vs 171 mg/g in C. vulgaris). In contrast, H. pluvialis exhibited slightly higher carbohydrate content in RAS effluent (250 vs 199 mg/g). Overall, the findings highlight the importance of effluent composition in optimizing nutrient recovery and algal biomass quality, and provide a basis for future validation of these results using real RAS effluents under continuous operation.