Aqueous pyrolysis liquid (APL) is a high-COD byproduct of wastewater biosolids pyrolysis that is comprised of numerous complex organic compounds and ammonia nitrogen (NH₃-N). One potential beneficial use of APL is as a co-digestate to produce more biogas in anaerobic digesters. However, some APL organics and NH₃-N are known to inhibit methane-producing microbes. Autocatalytic pyrolysis which uses previously-produced biochar as a catalyst during biosolids pyrolysis, increases energy-rich py-gas while eliminating bio-oil production and reducing COD concentration in the APL (catalyzed APL). However, the catalyzed APL still has a high organic strength and no suitable treatment strategies have yet been identified. In this study, the methane production yields and methanogenic toxicity of non-catalyzed and catalyzed APLs were investigated. Both non-catalyzed and catalyzed APLs were produced at 800°C from a mix of digested primary and raw waste activated sludge from a municipal water resource reclamation facility. Using the anaerobic toxicity assay, APL digester loading rates higher than 0.5 gCOD/L for non-catalyzed and 0.10 gCOD/L for catalyzed APL were not sustainable due to toxicity. The IC₅₀ values (APL concentration that inhibited methane production rate by 50%) for non-catalyzed and catalyzed APLs were 2.3 and 0.3 gCOD/L, respectively. Despite having significantly fewer identified organic compounds catalytic APL resulted in higher methanogenic toxicity than non-catalytic APL. NH₃-N was not the main inhibitory constituent and other organics in APL, including 3,5-dimethoxy-4-hydroxybenzaldehyde, 2,5-dimethoxybenzyl alcohol, benzene, cresol, ethylbenzene, phenols, styrene, and xylenes as well as nitrogenated organics (e.g., benzonitrile, pyridine) ostensibly caused considerable methane production inhibition. Future research focused on pretreatment methods to overcome APL toxicity and the use of acclimated biomass to increase methane production rates during APL anaerobic digestion or co-digestion is warranted.

Pyrolysis can convert wastewater solids into useful byproducts such as pyrolysis gas (py-gas), bio-oil and biochar. However, pyrolysis also yields organic-rich aqueous pyrolysis liquid (APL), which presently has no beneficial use. Autocatalytic pyrolysis can beneficially increase py-gas production and eliminate bio-oil; however, APL is still generated. This study aimed to utilize APLs derived from conventional and autocatalytic wastewater solids pyrolysis as co-digestates to produce biomethane. Results showed that digester performance was not reduced when conventional APL was co-digested. Despite having a lower phenolics concentration, catalyzed APL inhibited methane production more than conventional APL and microbial community analysis revealed a concomitant reduction in acetoclastic Methanosaeta. Long-term (over 500-day) co-digestion of conventional APL with synthetic primary sludge was performed at different APL organic loading rates (OLRs). Acclimation resulted in a doubling of biomass tolerance to APL toxicity. However, at OLRs higher than 0.10 gCOD/L_r-d (COD = chemical oxygen demand, L_r = liter of reactor), methane production was inhibited. In conclusion, conventional APL COD was stoichiometrically converted to methane in quasi steady state, semi-continuous fed co-digesters at OLR ≤ 0.10 gCOD/L_r-d. Undetected organic compounds in the catalyzed APL ostensibly inhibited anaerobic digestion. Strategies such as use of specific acclimated inoculum, addition of biochar to the digester and pretreatment to remove toxicants may improve future APL digestion efforts.