The fast-growing aquaculture industry, producing 46% of the global fish consumption, generates significant waste, including animal by-products and sludge, such as those coming from recirculating aquaculture systems (RAS). To comply with the New European Circular Economy Plan, this work assessed the production of valuable bio- based fertilisers (BBFs) from a starting biodrying process of RAS-sludge, coupled with different innovative post-processing technologies. The biodried RAS-sludgedemonstrated to have a substantial total P-content (4.31% as P2O5), but a low phosphorus bioavailability (15% of soluble phosphorus). Two strategies were proposed to tackle this issue: i) chemical and biological solubilisation of phosphorus; ii) thermal treatment and chemical phosphorus recovery from ash by acidic treatment.The experimental design of phosphorus solubilisation trials directly with the biodried product were performed via incubation tests. Soil incubations (150 g of low-nutrient soil + 3 g of biodried product) were performed for 30 days in controlled temperature and humidity conditions. Chemical and biological solubilisation of phosphorus was assessed by adding either sulphuric acid (10% addition at 96% m/v) or biological inoculum composed by Pseudomonas putida at different dosing (104-106 CFU/g of soil).After complete combustion of biodried RAS-sludge product, the acidic phosphorus extraction from ash was performed. First, a screening of optimal operational conditions was performed using different concentrations of sulphuric and citric acid (1 M, 0.75 M, 0.5 M and 0.25 M) at 1 to 20 ash to acid ratio. Concentration of acids was selected after the elemental analysis of ash and following the stoichiometric acid requirement (Donatello et al., 2010). After 2 h extraction at continuous stirring at ambient temperature (25oC), 80% of phosphorus recovery was achieved with the most concentrated sulphuric acid while over 50% was achieved with sulphuric acid at 0.25M. Over 40% of phosphorus was recovered with the most concentrated citric acid solution (1 M). To conclude, this work demonstrated the satisfactory enhancement of the quality of RAS-sludge derived biodried product in terms of phosphorus availability.