Tandem acid assisted hydrothermal treatment and inhibited heavy metal extraction towards high-purity phosphorus recovery from sewage sludge

Hydrothermal processing (HTP) is a promising technology for sewage sludge (SS) treatment to facilitate subsequent phosphorus (P) recovery via chemical extraction. This study has proposed a tandem acid-assisted HTP and chemical extraction process to improve P recovery from SS. Organic acids (i.e., formic acid and oxalic acid) were introduced during HTP and resulted in acid mediated hydrochar (HC) (i.e., formic acid treated hydrochar (HC-FA) and oxalic acid treated hydrochar (HC-OA)). Then H₂SO₄ and NaOH were applied for subsequent P extraction from HC-FA and HC-OA. It is found that acid-assisted HTP increased the porosity and surface area of HC, thereby facilitating P extraction. P leaching efficiency from HC-FA was enhanced by 91.8 % with H₂SO₄ leaching and 85.2 % with NaOH leaching. For HC-OA, the efficiency was increased by 88.2 % and 83.7 % for H₂SO₄ and NaOH leaching, respectively. NaOH leaching resulted in significantly lower heavy metal (HM) release, e.g., Cr and Pb extraction being 15-fold lower, Zn and Cd 10-fold lower, and Cu and Ni 5-fold lower than those in H₂SO₄ leaching. This technique indicates its great potential for cleaner P extraction. Furthermore, leachate recirculation was performed to further enhance P recovery with reduced consumption of chemicals. The scenario that involve replenishing the 1 M leaching agent after two cycles has demonstrated the highest P accumulation (96.7 % in H₂SO₄ and 84.4 % in NaOH) and minimal HM release (<10 % in NaOH). These findings highlight the superior efficiency of organic acid assisted HTP and leachate recirculation in enhancing P recovery with inhibited HM extraction, promoting sustainable SS management towards circular economy.