Hydrothermal liquefaction of Spirulina platensis in post-carbonization wastewater from sewage sludge

Abstract

Wet solid waste can achieve good dewaterability through hydrothermal carbonization. However, massive wastewater rich in organics is also generated during the process. Previous studies reported the anaerobic digestion potential of organic wastewater, but its inhibitors cause challenges. In our early work, the aqueous by-product from hydrothermal liquefaction of microalgae has been successfully recycled back into the liquefaction process, leading to an enhanced oil yield. Herein, this study addresses the possibility of wastewater from sewage sludge dewatering as a solvent to convert microalgae for biocrude production under temperature of 260-320 ° C . Results show that the wastewater owned a chemical oxygen demand value of 14 g/L and a total organic carbon value of 5.89 g/L with a nitrogen content of 10.34%. The biocrude yield in this wastewater was 31.3–38.7 wt%, higher than liquefaction in pure water (27–33 wt%). Over 10% increment was reached, demonstrating the potential of organic wastewater as a solvent for biocrude production. Following a slight decrease of the nitrogen content, the higher heating value of the biocrude from the wastewater solvent was close to that formed in pure water, suggesting a non-significant influence on fuel quality. Besides, the enhancing mechanism of wastewater stream on liquefaction was discussed based on various analytical approaches. Compound analysis indicates a reduction of nitrogenous chemicals in the wastewater during liquefaction, which are more likely to be transformed to oil phase forming amides. Therefore, this recycling strategy would help develop novel routes for utilization of organic wastewater, significant for sewage sludge treatment.