Phosphorus recovery and adsorbents production from municipal sewage sludge by the integration of slow pyrolysis and chemical leaching

To improve the circularity of the wastewater treatment sector, new strategies should be found to turn sewage sludge into valuable products. This study investigates the integration of slow pyrolysis and chemical leaching to simultaneously convert sewage sludge into (i) a biocoal with enhanced surface area useable as adsorbent and (ii) an inorganic P-rich compound for agricultural applications. Sewage sludge-derived char was produced in laboratory at different temperatures (400–650 °C) and then processed through chemical leaching by HNO3, revealing that pyrolysis temperature negatively impacts char demineralization and phosphorous solubility. As a novel contribution, pyrolysis and chemical leaching integration was subsequently validated at pilot scale, obtaining a biocoal and a phosphorus-rich liquid (leachate). The surface area increased from 12 m2/g in the raw char to 145 m2/g after leaching and up to 350 m2/g after biocoal activation by CO2 at 800 °C. Adsorption tests with methylene blue revealed a significantly higher adsorption capacity of biocoal compared to the char, while the activated biocoal performance was comparable to a commercial adsorbent, demonstrating the positive impact of char post treatment by leaching (and activation) on the material's adsorption performance. By leaching, 97 % of char's phosphorus was extracted, and the P-based salt obtained by precipitation showed a P2O5 content of almost 16 %, with potentialities of application as a low-contaminants P-salt for fertilizing purposes. In conclusion, the study demonstrated that the investigated process is a promising scalable pathway to maximise raw materials recovery from sewage sludge, thus improving its circular valorisation.