Hydrothermal upgrading of algae paste: Inorganics and recycling potential in the aqueous phase

Hydrothermal Liquefaction (HTL) for algal biomass conversion is a promising technology capable of producing high yields of biocrude as well as partitioning even higher quantity of nutrients in the aqueous phase. To assess the feasibility of utilizing the aqueous phase, HTL of Nannochloropsis sp. was carried out in the temperature range of 275 to 350°C and Residence Times (RT) ranging between 5 and 60min The effect of reaction conditions on the NO3(-),PO4(3-),SO4(2-),Cl(-),Na(+),andK(+) ions as well as Chemical Oxygen Demand (COD) and pH was investigated with view of recycling the aqueous phase for either cultivation or energy generation via Anaerobic Digestion (AD), quantified via Lifecycle Assessment (LCA). It addition to substantial nutrient partitioning at short RT, an increase in alkalinity to almost pH10 and decrease in COD at longer RT was observed. The LCA investigation found reaction conditions of 275°C/30min and 350°C/10min to be most suitable for nutrient and energy recovery but both processing routes offer environmental benefit at all reaction conditions, however recycling for cultivation has marginally better environmental credentials compared to AD.