Integrated Process for the Production of P-Based Fertilizer from Digested Sludge: Experimental Assessment and Preliminary Cost Estimation

Abstract A process that includes NaCl-assisted co-incineration, wet-chemical extraction, and precipitation for phosphorus (P) recovery from anaerobically digested sludge was developed. In the coincineration step, NaCl addition proved effective in enhancing heavy metal (HM) volatilization, achieving removal efficiencies ranging from 41 to 88%. This treatment also produced ash that was more prone to acid leaching, due to the conversion of apatitic P species into more soluble phases. In the subsequent wet-chemical extraction step, a high P extraction efficiency (∼90%) was achieved via 1 M H2SO4 leaching at L/S = 10, while minimizing HM coextraction. Finally, EDTA was strategically added during the precipitation stage (0.8 g/L), rather than during the leaching stage, to selectively complex the residual HMs and prevent their incorporation into the recovered P-based product. The resulting solid contained ∼21% P2O5 and met the investigated HM limits of Regulation (EU) 2019/1009, corresponding to an overall P recovery efficiency of 85%. The economic assessment, based on a treatment capacity of 60,000 tons/year, showed a CAPEX of 3.62 M€ and an OPEX of 9.56 M€, mainly due to high reagent consumption. The process resulted in a negative net present value (–18.19 M€), indicating a lack of profitability as a standalone solution. However, the total cost was comparable to conventional ash disposal in a landfill over 20 years (23.48 M€), demonstrating that P recovery can be implemented without a significant additional economic burden. This confirms the feasibility of integrating resource recovery into sludge management, in line with the principles of the circular economy and while keeping the overall costs of the system unchanged.