Dual valorization of sewage sludge ash through phosphorus recovery and wastewater remediation

Sewage sludge ash (SSA) can be exploited for phosphorus recovery, as it contains about 5-15% of this element by mass. However, residual waste remains and requires valorization to avoid disposal costs. This study investigates fluidized bed mono-combusted ash from sewage treatment for water remediation within a circular economy framework. Acid treatment, primarily applied for phosphorus extraction and recovery, was here followed by the valorization of the residual ash as an adsorbent, since this step simultaneously increases porosity and surface area. Characterization by SEM, BET, XRD, ICP-OES and pH_PZC provided insights into structural and chemical changes in SSA and its activated form (ASSA). Results showed complete phosphorus removal, surface area increase from 1.51 to 16.18 m²/g, and compositional phase modifications. Batch adsorption experiments were performed on solutions mono-contaminated with four anionic organic dyes bearing sulphonate groups and methylene blue as a representative cationic dye. ASSA resulted in being much more effective in the removal of the anionic dyes, with the adsorption efficiency gap increasing at higher contaminants' concentrations, while no significant difference was observed in the adsorption of methylene blue. Isotherms and kinetics of the decontamination processes allowed elucidating the adsorption mechanisms, which differ for different dyes and in some cases between SSA and ASSA. Sustainability analysis, using the ESCAPE tool, indicates that the produced adsorber may be competitive with activated carbons in terms of overall environmental impact, despite its lower adsorption efficiency. The successful regeneration and reuse of ASSA is contingent upon the types and levels of contaminants present.