Development of a multi-objective support tool for optimizing phosphorus recovery from sewage sludge ash: A step towards process feasibility

Phosphorus (P) recovery from Sewage Sludge Ash (SSA) represents a promising solution to P supply concerns, with the main application challenge consisting of the poor economic feasibility of current technologies, requiring Operating Expense (OPEX) reduction to compete with primary P sources. This study aims to provide a multi-objective support tool for the design and operation of P recovery from SSA by combined wet acid leaching and alkaline precipitation to produce bio-based fertilizers. First, in the view of filling literature gaps, lab-scale leaching tests based on the Design of Experiment were performed with SSA from a full-scale mono-incinerator, and multi-variate statistical techniques were applied to generate polynomial regression models for Mg, K, Cu and Zn extraction. Then, small pilot-scale leaching and precipitation tests were conducted, applying HCl and H2SO4 as leaching agents and Ca(OH)2 and a low-grade magnesium oxide mining by-product (LG-MgO) as precipitating agents. Lab- and pilot-scale data were then jointly employed to develop the support tool that was later applied for process optimization based on a set of key performance indicators. The support tool indicated the optimal leaching (HCl, 0.82 N, 10 L/kg, 0.5 h) and precipitation (Ca(OH)2) operating parameters for obtaining EU-compliant fertilizing products while minimizing OPEX in different pricing scenarios. Furthermore, as chemical supply and process solid residue disposal resulted as the most impacting cost items, priority actions for targeting the break-even point were identified.