Biogas liquid digestate grown Chlorella sp. for biocrude oil production via hydrothermal liquefaction

Microalgae can not only purify and recover the nutrients from wastewater, but also be harvested as wet biomass for the production of biocrude oil via hydrothermal liquefaction (HTL). Chlorella sp. cultivated in the ultrafiltration (UF) membrane treated anaerobic digestion (AD) liquid digestate of chicken manure was used as the feedstock in this study. The present study characterized the products and investigated the elemental migration during HTL of Chlorella sp. fed with AD effluent wastewater (WW) and BG11 standard medium (ST) in 100 mL and 500 mL reactors under different operational conditions. Results showed that the highest oil yield of WW (38.1%, daf) was achieved at 320 °C, 60 min and 15% TS in 500 mL reactor, which was 14.1% higher than that of ST (33.4%, daf) at 320 °C, 30 min and 20% TS in the same reactor. WW had a similar carbon and hydrogen distribution in the four product fractions under HTL conditions compared with ST. 43.4% and 32.4% of carbon in WW11 and ST11 were released into the biocrude and aqueous phase in 500 mL reactor, respectively. As much as 64.5% of the hydrogen was transferred to the aqueous phase. GC–MS results showed that the chemical compounds in the biocrude oil from WW consist of a variety of chemical constituents, such as hydrocarbons, acids, alcohols, ketones, phenols and aldehydes. These two biocrude oils contained 17.5% wt. and 8.64% wt. hydrocarbons, and 63.7% wt. and 79.8% wt. oxygen-containing compounds, respectively. TGA results showed that 69.3%–66.7% of the biocrude oil was gasified in 30 °C–400 °C. This study demonstrates the great potential for biocrude oil production from microalgae grown in biogas effluent via HTL.