Previous studies demonstrate anaerobic digestion of hydrothermal liquefaction wastewater (HTL-WW) is significant to the sustainability of algal biofuel development for nutrient reuse and residual energy recovery. HTL-WW contains substantial amounts of residual energy but is toxic to anaerobes. With 6% HTL-WW converted from cyanobacteria (e.g. Spirulina), anaerobes were 50% inhibited. In this study, zeolite, granular activated carbon (GAC), and polyurethane matrices (PM) were used during a two-round anaerobic batch test with HTL-WW, and in the presence of each material, the total methane yields were 136 mL/g COD, 169 mL/g COD, and 168 mL/g COD, respectively, being 11%, 37% and 36% higher than the control. GAC was considered promising due to its highest methane yield of 124 mL/g COD at the second feeding, indicating a good recovery of adsorption capacity. The observed low methane production rates indicated the necessity for anaerobic process optimization. The physicochemical analysis of the digestates demonstrated that most of the compounds identified in the HTL-WW were degraded.

This study demonstrates that water can serve as a green solvent to achieve denitrogenation of biocrude oil converted from wet biowaste via hydrothermal liquefaction (HTL) under previously determined optimal reaction conditions (300°C with a 1 h reaction time for swine manure and Chlorella; 300°C with a 0.5 h reaction time for Spirulina). It was hypothesized that water could extract relatively polar chemicals, such as some of the nitrogen-containing compounds, from HTL biocrude oil. This hypothesis was examined by different extraction techniques using water as a solvent to HTL biocrude oil converted from swine manure (SM) and low-lipid algae. The water solubilities of biocrude oil and product yields (water extract, biocrude oil, solid residue, and aqueous and gas products) were measured. Compared to that of the biocrude oil without extraction by water, the nitrogen content of biocrude oil converted from SM decreased from 4.32 to 3.23%. Gas chromatography−mass spectrometry (GC−MS) analyses of treated biocrude oil and water extract demonstrated that water can extract nitrogen-containing compounds and separate fatty acid derivatives that were originally in biocrude oil. When an ultrasonically assisted extraction by water was conducted, the nitrogen content in algal biocrude oil decreased from 6.83 to 5.75% with improved carbon and hydrogen content. In addition, GC−MS analyses of treated biocrude oil and water extract suggest that fatty acid derivatives may behave like surfactants and help in emulsifying nitrogen-containing compounds into water.

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