Recycle of nitrogen and phosphorus in hydrothermal liquefaction biochar from Galdieria sulphuraria to cultivate microalgae

Hydrothermal liquefaction (HTL) of G. sulphuraria (GS) has been demonstrated to produce energy-dense biocrude oil, water-soluble chemicals, and biochar. The recycling of nutrients from HTL water-soluble chemicals for algae cultivation has been demonstrated. The current study evaluated the feasibility of using HTL biochar, a by-product to recover nutrients and use the nutrients to cultivate microalgae without inhibition. GS biomass was liquefied to produce biocrude oil, biochar, water phase, and gasses under subcritical water conditions (300 °C, 15 MPa & 30 min). The N and P content in the HTL biochar were ~5.27 wt.% and ~15.98 wt.%, respectively. Both nutrients were leached from the biochar at varied pH values with ~40.23% of phosphate and ~70.01% of ammoniacal nitrogen recovery obtained at a pH of 0.5. The leached phosphates and ammoniacal nitrogen were added to N and P free Cyanidium medium (CM) for cultivating GS. Growth experiments were carried out in microtiter plates and culture tube reactors at 40 °C and with CO₂ (2–3%) supplementation. Similar growth rates were observed in cultures supplemented with the leached nutrients when compared to controls in standard CM growth media proving the absence of inhibition in the former. Comprehensive characterization of HTL biochar by Inductively Coupled Plasma Optical Emission Spectrometry, bomb calorimeter, and Scanning Electron Microscope is also presented. This study confirmed the feasibility of recovering nutrients from GS biochar and using these nutrients to cultivate algae successfully without inhibition, thus supporting reduced steps for recycling, process optimization, and HTL technology commercialization.