This study investigated the nutrient content and reuse potential of wastewater generated during hydrothermal liquefaction of microalgal biomass. The hydrothermal liquefaction reaction was tested at 270, 300, 330, and 345 °C to determine the effect of temperature on the formation of non-biodegradable dissolved organic nitrogen (nbDON). Total nitrogen, ammonium, color, and toxicity were selected as key characteristics for the reuse of hydrothermal liquefaction wastewater. Results indicated that a higher concentration of nbDON5 (nbDON defined with a 5 day growth assay) and more diverse heterocyclic N-containing organic compounds were associated with greater toxicity as measured by a growth rate assay. For the tested temperature ranges, the total nitrogen content of the hydrothermal liquefaction wastewater slightly decreased from 5020 ± 690 mg L-1 to 4160 ± 120 mg L-1, but the % nbDON5 fraction increased from 57 ± 3 %DON to 96 ± 5 %DON. The temperature of hydrothermal liquefaction reactions can be optimized to maximize carbon conversion and nitrogen recovery.
Keywords: HTL wastewater; Heterocyclic N-Containing organic compounds (HNOCs); Hydrothermal liquefaction (HTL); Nitrogen recovery; Non-biodegradable dissolved organic nitrogen (nbDON); Toxicity.
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