Enhancing the phosphorus availability in aquaculture-sludge based fertilizers using chemical and biological strategies

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-sludge
demonstrated 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.