Low temperature pyrolysis of centralized and decentralized sewage sludges: Biochar production, per-and...

In the urban periphery, applying sludge from decentralized sewage treatment system to farmlands is increasingly adopted in response to demand of nutrient recovery and local crop promotion, but potential risks pertain due to persistent micropollutants. We elucidate thermal destruction of multiple per- and polyfluoroalkyl substances (PFAS) species after slow pyrolysis (∼300 °C) of real sewage sludge samples from both decentralized and centralized treatment plants to determine how low-temperature pyrolysis mitigates PFAS persistence and post-treatment release for edible crop production. Pyrolysis reduced total PFAS concentrations by up to 85%, regardless of initial sludge concentrations but strongly dependent on functional group and chain length. In both sludge types, perfluorooctanesulfonic acid (PFOS) exhibited the highest thermal stability, with reductions of only 30-68.7%. In contrast, perfluorooctanoic acid (PFOA) and other short-chain perfluoroalkyl carboxylic acids (PFCAs) declined substantially (95-96%) while long-chain PFCAs remained detectable in the resulting biochar. Leaching experiments showed greater PFAS release under acidic conditions, with PFOA consistently the most mobile compound (43-168%). Chain-length-dependent leaching trends under both acidic and basic conditions indicate that hydrophobic interaction contributes to PFAS retention on biochar. Despite differences in total PFAS burdens and composition, biochar derived from decentralized sludge exhibited leaching behavior comparable to centralized sludge, suggesting that contaminant mobility rather than bulk concentration governs environmental risk. Evaluation of leachate metal and PFAS concentrations infers that low-temperature pyrolysis alone, without further optimization, may be insufficient to ensure safe agricultural reuse.