Anaerobic Digestion of Hydrothermal Liquefaction Wastewater and Biochar from Spent Coffee Grounds

Purpose Spent coffee grounds (SCG) are an underused lignocellulosic residue generated by the instant coffee industry. Hydrothermal liquefaction (HTL) is an alternative thermochemical process applied to high-water-content and renewable biomasses, such as SCG, to produce biocrude oil, biochar, and a high-organic-content wastewater, named PHWW. These last two HTL coproducts associated with the anaerobic digestion process were investigated. This research aimed to reduce the soluble organic content and recover methane from the post hydrothermal liquefaction wastewater (PHWW) of SCG through the anaerobic digestion process enhanced by the addition of activated carbon and biochar (coproduct of the HTL process), as adsorbents. Methods The HTL of the SCG was carried out under literature selected conditions in a laboratory-scale batch reactor to recover PHWW and solid residues. The solid residue was cleaned and activated before being used as an adsorbent during the anaerobic digestion assay. PHWW was characterized and stored to be further anaerobically digested with activated solid residue (ASR) and activated granular carbon (GAC) for comparison. Results The PHWW and ASR association enhanced the anaerobic digestion of the PHWW organic compounds, increasing phenols removal efficiency from 36 (PHWW-AD) to 45% (PHWW-AD with ASR) and Methane Yield by 62%. Furthermore, the addition of ASR to the PHWW-AD process has increased the methane production rate by 40%. Conclusion The results also indicated that PHWW digestion in the presence of ASR potentially reduced inoculum acclimatization periods to the recalcitrant PHWW compounds; thus, it is operationally practical and economically feasible. [GRAPHICS] .

Automated summary

The purpose of this research is to reduce the soluble organic content in spent coffee grounds hydrothermal liquefaction wastewater through anaerobic digestion process while also recovering methane by the addition of activated carbon and biochar as adsorbents.