Pyrolysis is a recognized alternative for the sustainable management of contaminated organic waste, as it yields energy-rich gas, oil, and a carbon-rich biochar product. Low-volatility compounds, however, such as heavy metals (HMs; As, Cd, Cu, Cr, Ni, Pb, and Zn) typically accumulate in biochars, limiting their application potential, especially for soil improvement. The distribution of HMs in pyrolysis products is influenced by treatment temperature and the properties of both the HMs and the feedstock. There is a significant knowledge gap in our understanding of the mass balances of HMs in full-scale industrial pyrolysis systems. Therefore, the fate of HMs during full-scale relevant pyrolysis (500–800 °C) of seven contaminated feedstocks and a clean wood feedstock were investigated for the first time. Most of the HMs accumulated in the biochar (fixation rates (FR) >70%), but As, Cd, Pb, and Zn partly partitioned into the flue gas at temperatures ≥ 600 °C, as demonstrated by FRs of <30% for some of the feedstocks. Emission factors (EFs, mg per tonne biochar produced) for particle-bound HMs (<0.45 µm) were 0.04–7.7 for As, 0.002–0.41 for Cd, 0.01–208 for Pb, and 0.09–342 for Zn. Only minor fractions of the HMs were found in the condensate (0–11.5%). To investigate the mobility of HMs accumulated in the biochars, a novel leaching test for sustained pH drop (at pH 4, 5.5 and 7) was developed. It was revealed that increasing pyrolysis temperature led to stronger incorporation of HMs in the sludge-based biochar matrix: after pyrolysis at 800 °C, at pH 4, <1% of total Cr, Cu, Ni, and Pb and < 10% of total As and Zn contents in the biochars were leached. Most interestingly, the high HM mobility observed in wood-based biochars compared to sewage-sludge-based biochars indicates the need to develop specific environmental-management thresholds for soil application of sewage-sludge biochars. Accordingly, more research is needed to better understand what governs the mobility of HMs in sewage-sludge biochars to provide a sound basis for future policy-making.

Growing concerns over pollutants in sewage sludge have increased interest in integrating anaerobic digestion (AD) and pyrolysis (Py) to enhance energy recovery, reduce toxic compounds, and minimize environmental impacts. This study evaluates six treatment scenarios: standalone AD or Py, and four integrated AD-Py configurations, with or without aqueous pyrolysis liquid (APL) recycling. Environmental impacts were assessed using ReCiPe 2016 and USEtox® methods. Scenario S4:AD-Py-AD, which recycles APL to AD, achieved the highest climate change mitigation (-300 kg CO₂-eq./ton hydrolyzed sludge (HS)) but also showed elevated freshwater ecotoxicity (increase by 52-fold compared to AD) due to persistent organic pollutants emissions in effluent. Scenario S3:AD-Py, without APL recycling, demonstrated the best overall environmental performance, with low climate impact (-293 CO₂-eq./ton HS) and ecotoxicity (same as AD). In contrast, S6:ADCo-Py-AD, which includes a high-rate AD reactor and APL recycling, had the poorest performance due to high ecotoxicity (980-fold increase compared to AD). These findings highlight that Py of digestate can be a beneficial pathway for sewage sludge treatment, showing the best environmental benefits compared to stand-alone processes.

This study evaluated the feasibility of co-digesting aqueous pyrolysis liquid (APL), a by-product of digestate pyrolysis, with hydrolyzed sewage sludge (HS) in an anaerobic continuous stirred-tank reactor under industrially relevant conditions. APL is characterized by a high concentration of volatile fatty acids, ammonium, and phenolic compounds, offering methane production potential, but also simultaneously introducing inhibitory risks. Results showed that APL addition up to 2.4 % w/w of feed (0.2 g COD/L/d) maintained stable methane production and COD removal (61 %), while higher loadings led to inhibition. Microbial analysis revealed a shift toward Firmicutes and Methanosarcina, indicating adaptation to elevated ammonium and phenol levels. Biomethane potential (BMP) tests confirmed microbial acclimatization, with improved methane yields at higher APL concentrations. BMP tests also revealed that fractional APL exhibited lower inhibitory effects compared to non-fractional APL. These findings demonstrate that APL can be co-digested with HS without compromising reactor performance at moderate loadings, supporting the integration of pyrolysis and anaerobic digestion for enhanced resource recovery. Further research should address long-term stability, effluent quality, and strategies to mitigate inhibition.

Aqueous pyrolysis liquid (APL) is the water‑rich by‑product of pyrolysis with high organic content; however, it is too dilute for economical fuel upgrading, so practical applications remain limited. This study investigates the feasibility of co-digesting hydrolyzed sewage sludge filtrate with APL in high-rate anaerobic reactors to enhance energy recovery and as a treatment step for APL. Two lab-scale reactors were operated for >500 days: one fed with filtrate and APL, and a control fed with filtrate only. Results show that APL can be degraded at loadings up to 0.3 g COD_APL/L/d (≈2.1% w/w) without severe inhibition, achieving 93% CODAPL removal and stable methane production. Compared to previous studies, this work demonstrates substantially higher APL degradation under co-digestion conditions. Higher loadings (≥0.44 g CODAPL/L/d) caused inhibition despite recovery attempts. Microbial analysis revealed a shift from acetoclastic to hydrogenotrophic methanogenesis, dominated by Methanobacterium and Methanosarcina, alongside enrichment of taxa linked to resistance to a toxic environment. Measurements of pollutant concentration in effluent indicated partial degradation of low-molecular-weight polycyclic aromatic hydrocarbons (PAHs) and transformation of some polyfluoroalkyl substances (PFAS), though most pollutants persisted in effluent. These findings highlight the potential of high-rate reactors for APL treatment and underscore the need for strategies such as inoculum acclimation, pretreatment, or additives to achieve industrial-scale loadings. The study provides insights into microbial resilience, pollutant fate, and operational considerations for integrated anaerobic digestion–pyrolysis systems.