Evaluating aqueous pyrolysis liquid as co-substrate in anaerobic digestion of hydrolyzed sewage sludge

• APL from pyrolyzed digestate co-digested with hydrolyzed sludge in CSTR. • Stable performance at APL loadings ≤ 2.4 % w/w; higher caused FAN-related inhibition. • Microbial shift toward Firmicutes and Methanosarcina indicates adaptation. • BMP tests show improved methane yields at higher APL, especially fractional APL. • Fractional APL is less toxic and more digestible than non-fractional, aiding AD. 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.