Anaerobic digestion of wastewater generated from the hydrothermal liquefaction of Spirulina: Toxicity assessment and minimization

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.