Continuous treatment of hydrothermal liquefaction wastewater in an anaerobic biofilm reactor: Potential role of granular activated carbon

High strength hydrothermal liquefaction wastewater (HTL-WW) is produced during the hydrothermal liquefaction of biowaste into bio-oil. The presence of complex and refractory organics in HTL-WW limited the conversion efficiency during the anaerobic digestion. An anaerobic biofilm reactor, packed with granular activated carbon (GAC) and polyethylene (PE) rings, was continuously operated over 160 days for HTL-WW treatment in this study. The performance was higher than previous researches in terms of organic loading and methane production, resulting in 285 ± 22 mL/g COD (83.4% of theoretic value) methane yield when operated at a concentration of 10 g COD/L under a 1.7 d HRT. The potential role of GAC on HTL-WW conversion, including absorption, microbial enrichment, and metabolic flux variation via enhancement of direct interspecies electron transfer (DIET), was also discussed. It was found that the GAC played a very important role in the beginning, acting as an adsorbent for the reduction of inhibitors and color. In addition, biofilms developed on the GAC enriched the syntrophic metabolism of bacteria responsible for inhibitors degradation. Over 80% of the COD and 75% of color in HTL-WW were removed by microbes. Furthermore, the difference in microbial structure between the PE and GAC biofilms revealed the potential of establishing DIET on the GAC via enrichment of Anaerolineaceae, Syntrophomonas and Methanosaeta.