Optimizing energy yields from nutrient recycling using sequential hydrothermal liquefaction with Galdieria sulphuraria

Hydrothermal liquefaction (HTL) provides a promising option for extracting bio-crude oil from wet algal biomass. One of the byproducts of HTL is an aqueous phase (AP) rich in organic carbon and nutrients. This study evaluated the hypothesis that recycling the AP to the cultivation step could enhance biomass productivity and net energy yield. Since the yields of bio-crude and nutrients post-HTL are functions of HTL reaction temperature, this study evaluated the impact of reaction temperature on net energy yield. Nutrient recycle experiments were conducted with a low-lipid, acidophilic strain, Galdieria sulphuraria, being developed for single-step removal of organic carbon, nitrogen and phosphorus from urban wastewater. G. sulphuraria was cultivated in different dilutions of the AP generated by HTL performed between 180 and 300 °C. Biomass productivity recorded in this study with recycled AP was greater than that in the control by a factor as much as 1.94. Estimates of net energy yields indicate the optimum temperature for the second-stage HTL bio-crude oil extraction from G. sulphuraria to be 280 to 300 °C.