Optimization of phosphorus wet acid extraction from sewage sludge ashes: Detailed process insight via multi-variate statistical techniques.

To tackle the issue of phosphorus (P) supply, Sewage Sludge Ash (SSA) is a promising secondary source. The goal of this research is the optimization of the wet P extraction process in the view of providing valuable insights for the process scale-up. While most of the studies relied on the "one-variable-changed-at-a-time" approach, this work applied a statistical optimization model based on the Design of Experiments technique for investigating the influence of most relevant operating conditions of extraction process on Phosphorus Extraction Efficiency (PEE) with two types of SSA characterized by different levels of alkalinity. Experimental results highlighted the importance of ash characteristics, making some SSA not suitable for cost-effective phosphorus recovery due to the high acid dosage required in extraction process. Type of acid and contact time become significant factors only for Ca-rich ashes, especially when using H 2 SO 4 as extracting agent. Optimal operating conditions (C H 2 SO4 = 1.3 N, L/S = 20, t = 2 h and C HCl = 2.0 N, L/S = 10, t = 2 h for Ash#1 and C H2SO4/HCl = 0.8 N, L/S = 10 for Ash#2) resulting in high PEE (80–94%) were identified for extraction performed with H 2 SO 4 and HCl as a function of ash characteristics. [Display omitted] • pH and ash alkalinity significantly affect acid dosage in P extraction process. • Reduced order models clarified the effects of factors and their interaction on PEE. • HCl was slightly (5%) more effective than H 2 SO 4 for Ca-rich ash. • The selection of optimal operating conditions is dependent on ash characteristics. • P, Fe and Al dissolution is a quick process when dealing with ash with low alkalinity. [ABSTRACT FROM AUTHOR]