Closing the loop in aquaponics: biological and physicochemical recovery of nutrients from aquaculture sludge for circular bioeconomy

Aquaculture sludge from recirculating systems contains a substantial share of feed-derived nutrients, yet it is typically discarded despite nutrient limitations in aquaponics. In this study, tilapia sludge from a commercial RAS was treated by sealed storage, gradual acidification (pH 5), ultrasonication, aerobic mineralization, anaerobic digestion (AD), and two AD-based sequential approaches (AD pH, AD US pH). Total and dissolved fractions were quantified to assess nutrient distribution between solid-bound and dissolved pools, and treatment-dependent mineralization performance was calculated. In untreated sludge, >75–90% of nitrogen, phosphorus, calcium, magnesium, and most trace elements were associated with the solid phase, whereas potassium was largely dissolved. AD variants achieved the highest nitrogen mineralization (up to ~47%) with ammonium-dominated supernatants (≈460–480 mg NH4–N L−¹), while aerobic mineralization promoted nitrification and yielded nitrate-dominant nitrogen (≈257 mg NO3–N L−¹). Phosphorus mobilization was primarily pH-driven, peaking under acidification (≈165 mg PO4–P L−¹) and increasing after post-acidification of AD supernatants. Micronutrient responses were element-specific; manganese increased markedly under acidified and unaerated conditions, whereas dissolved iron remained low across treatments. Compared to ideal hydroponic nutrient media, most supernatants required dilution due to elevated ammonium, and iron supplementation remained necessary. Most potentially toxic elements remained in the low µg L−¹ range and below irrigation and reported algal toxicity thresholds. Selected sludge-derived nutrient media were suitable for Arthrospira platensis cultivation to support circular nutrient reuse. In growth trials, the non-acidified AD supernatant (AD S) supported the strongest biomass and pigment performance among sludge-derived media, reaching up to 1.32 g DW L−¹, with a chlorophyll a content of 21.7 mg L−¹ and a C-phycocyanin content of 24.6 mg g DW−¹. Overall, sludge mineralization shifted nutrients from bound to dissolved pools and generated distinct nutrient profiles, highlighting the importance of aligning treatment strategy with the intended downstream application in circular RAS–aquaponic systems.