Toxicity of Various Pyrolysis Liquids From Biosolids on Methane Production Yield

Abstract

Aqueous pyrolysis liquid (APL) is a high-COD byproduct of wastewater biosolids pyrolysis that is comprised of numerous complex organic compounds and ammonia nitrogen (NH 3 -N). One potential beneficial use of APL is as a co-digestate to produce more biogas in anaerobic digesters. However, some APL organics and NH 3 -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 IC 50 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. NH 3 -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.

Inhibition during Anaerobic Co-Digestion of Aqueous Pyrolysis Liquid from Wastewater Solids and Synthetic Primary Sludge

“…In an effort to remove NH 3 -N that could inhibit methanogenesis, some catalyzed and non-catalyzed APL samples (30 mL) were aerated for 9 h at 2 L/min air flow (1 atm, 20 • C) to strip NH 3 -N [7]. Volatile constituents such as ethylbenzene and styrene were also removed during air stripping (Table S1) [13].…”

“…However, previous studies have shown anaerobic digestion is challenging since APL contains organic compounds that are known to inhibit methanogens and reduce or stop digester methane production [10,11,15]. In addition, high ammonia nitrogen (NH 3 -N) concentration in APL may also cause digester inhibition [13,16]. Both catalyzed and non-catalyzed APLs used in this study contained high NH 3 -N concentrations.…”

“…By contrast, APL is water-based, has a low heating value and currently has no known beneficial use [10][11][12]. APL can be environmentally harmful if not managed properly due to its high chemical oxygen demand (COD) concentration and the presence of potentially toxic organic compounds such as cresol, ethylbenzene, phenol and xylene [13].…”

“…The novel autocatalytic process has been shown to increase py-gas energy by more than three times (from 2940 kJ/kg biosolids-pyrolyzed to 10,200 kJ/kg biosolids-pyrolyzed) [14]. Additionally, as illustrated in the graphical abstract, autocatalytic pyrolysis of wastewater biosolids at 800 • C resulted in no bio-oil production, but APL was still produced (catalyzed APL) that had a lower organic content and fewer unsaturated hydrocarbons compared to non-catalyzed APL [13,14].…”

“…One possible APL management strategy involves anaerobic digestion, since APL contains a high concentration of organics (30-300 gCOD/L), including acetic acid (approximately 25 g/L) that possibly could be converted to biogas containing methane for renewable energy generation [11,13]. However, previous studies have shown anaerobic digestion is challenging since APL contains organic compounds that are known to inhibit methanogens and reduce or stop digester methane production [10,11,15].…”