Aqueous Phase from Hydrothermal Liquefaction: Composition and Toxicity Assessment

Abstract

The main obstacle to the widespread use of hydrothermal liquefaction (HTL) for waste and wet biomass recycling is the formation of a significant amount of highly polluted wastewaters. This paper presents an analysis of the chemical composition and toxicity of aqueous phase from the HTL (HTL-AP) of primary and secondary sludge. It was shown that HTL-AP has a high level of organic pollution (total organic carbon (TOC) = 4.2–9.6 g/dm3, chemical oxygen demand (COD) = 7.9–14.0 g/dm3, BOD5 = 6.0–8.1 g/dm3) and high biological toxicity for traditional test organisms (so that dilution ratio, ensuring the death of no more than 50% of organisms (DR50), varied within 64.7–142.2 and 44.9–81.7 for Artemia salina and Paramecium caudatum, respectively). An analysis of HTL-AP composition with NMR-spectroscopy method allowed us to establish that the share of carbon in aliphatic chains was 34.05–41.82% and the content of carbon in carboxyl groups and aromatic rings was 26.42–34.44%. As a result, we can conclude that the main HTL-AP components are fatty carboxylic acids and their derivatives, aromatic carboxylic acids. The content of aldehydes, ketones, and lignin is less than 8%. Biological treatment of HTL-AP in a lab-scale aerobic reactor turned out to be successful, so average COD reduction was 67–95%. Sludge from an industrial waste water treatment plant (petrochemical sector) with a microorganism concentration of 2.7 g/dm3 was used as inoculum. HTP-AP was diluted 1:10 with tap water. The duration of the process was 18 h.