On safety of sewage biosolids valorisation: Distribution of PFAS, PAHs, PCDD/Fs, and heavy metals in low-temperature pyrolysis end-products for agricultural and energetic applications
Felizitas Schlederer, Edgar Martín-Hernández, Céline Vaneeckhaute
Abstract
• Study on micropollutant distribution between biochar, APL and pyrolysis oil. • Determined PFAS in biochar, APL and pyrolysis oil. • Biochar met the guidelines for agricultural valorisation, except for heavy metals. • High methane production was achieved during anaerobic digestion of APL. • CO 2 supply during pyrolysis achieved highest calorific value of pyrolysis oil. Pyrolysis is a suitable process for sewage sludge valorisation while potentially reducing micropollutants. However, previous studies mainly focused on micropollutants in biochar, neglecting their presence of aqueous pyrolysis liquid (APL), pyrolysis oil, and gas. This study analyses the distribution of 66 micropollutants, including PFAS, dioxins and furans (PCDD/Fs), heavy metals (HMs), and polycyclic aromatic hydrocarbons (PAHs) across biochar, APL and pyrolysis oil. Additionally, the impact of a carrier gas supply (N 2 and CO 2 ) on micropollutants distribution was evaluated. In a second stage, the safe use of the pyrolysis end-products for agricultural and energetic purposes was explored. PFAS from biosolids were distributed in biochar (12–13 %), APL (6–7 %), and the oil phase (2–5 %). 63–74 % remained unaccounted for, possibly transferred to the gas phase, or decomposed during pyrolysis. PAHs were predominantly found in the pyrolysis oil, while PCDD/Fs were found in the biochar and pyrolysis oil. HMs were primarily found in biochar. PAH and PCDD/F values in the biochar met the European Biochar Certificate (EBC) guidelines. However, HMs surpassed the thresholds, suggesting either post-treatment or using biochar as a building material instead. Given that pyrolysis oil contains significant quantities of micropollutants, high-temperature combustion could serve for both micropollutant decomposition and energy reclamation. Energetic valorisation of APL assessed by biomethane potential tests, achieved a methane production yield of 432–450 NmL CH4 /g VS . Overall, a combined anaerobic digester and pyrolysis process, including a recirculation of the APL, the valorisation of pyrolysis oil for process heating, and the use of CO 2 from biogas as pyrolysis carrier gas, is suggested for further study.