From waste to resource: nutrient recovery from aquaculture sedimented sludge for sustainable Arthrospira platensis cultivation in circular aquaculture systems

Introduction: The sustainable recovery and reuse of nutrients from aquaculture waste represent a promising approach to reducing the environmental impact of fish farming while supporting cost-effective microalgal cultivation.Materials and methods: This study investigated the potential of aquaculture sedimented sludge as a nutrient source for cultivating Arthrospira platensis. Chemical and thermochemical treatments were applied to enhance the solubilization of essential nutrients, particularly phosphorus.Results and discussion: Among the tested reagents, 1 M HCl and 2 M NaOH resulted in the highest phosphate release, with increasing sludge dosage up to 20 g L-1 further improving solubilization efficiency. Thermochemical treatments combining ultrasound or autoclaving with chemical conditioning significantly enhanced nutrient recovery, with the acid–autoclave (AUA) and autoclaved water (AUW) treatments achieving the highest phosphate concentrations (270.6 and 210.8 mg L-1, respectively). Recovered nutrient extracts were subsequently tested as culture media for A. platensis, with six treatments evaluated (acidic, alkaline, and water-based extracts with ultrasound and autoclaving) compared to a standard synthetic medium (Spirulina SAG). The AUW extract supported robust biomass accumulation, comparable to that in SAG medium, when supplemented with 2 g L-1 KNO3 and 10 g L-1 NaHCO3. Further optimization of nitrogen supplementation (0–2 g L-1 KNO3) identified 1 g L-1 KNO3 as optimal, yielding a biomass of 1.45 g L 1 and a protein content of 559.3 ± 21.6 mg L-1. Phycobiliprotein yields (C-phycocyanin: 25.12 mg g-1; allophycocyanin: 6.25 mg g-1) were comparable to those in synthetic medium. Nutrient analysis revealed substantial reductions in 1 g L-1 KNO3 medium: PO43- (90.90%), NH4+ (99.2%), NO3- (68.2%), and K+ (79.7%) during cultivation, indicating efficient nutrient assimilation by A. platensis. This study demonstrates a novel, sustainable approach to transforming aquaculture sedimented sludge into an effective culture medium for A. platensis, thereby reducing reliance on synthetic nutrients and advancing circular bioresource utilization in aquaculture systems.