(562ai) Thermal Treatment of the Aqueous Phase Resulting from the Hydrothermal Liquefaction of the Microalga Chlorella vulgaris

Microalgae are one of the most promising biofuel sources, because, among other characteristics, they capture between 10 and 50 times more CO₂ than terrestrial plants. Since the water content of these microorganisms can reach 95% by weight, hydrothermal liquefaction (HTL), which uses water at supercritical conditions, has emerged as a promising technology to convert wet biomass into biofuel. However, current biofuel yields of about 50% result in the generation of an aqueous phase with high concentrations of organic compounds that could be toxic, which constitute the main residue of the process. This work intends to evaluate the hydrothermal gasification as an alternative treatment of the aqueous phase coming from the HTL of the microalga Chlorella vulgaris, in order to recirculate it to the culture bioreactors.

In the first place, the algal biomass to be liquefied was obtained and biochemically characterized. Then, the HTL was carried out at constant reaction conditions (350ºC, 20 minutes). Subsequently, the aqueous phase obtained was characterized and an experimental plan was designed to establish the adequate reaction conditions (time and temperature) to perform the proposed hydrothermal treatment. Finally, the treated water was characterized, and was diluted to different concentrations to carry out comparative microalgae cultures, in order to determine the reuse potential of the process water.

At the end it was found that the aqueous phase resulting from the HTL contained nutrients necessary for the growth of the microalga, in higher concentrations than those used for the culture medium. However, the presence of toxic compounds such as phenol, which limit and inhibit the growth of microalgae, limited the recirculation of water as large dilutions factors were required. The hydrothermal gasification, used to treat the water, allowed the removal of more than 50% of the phenolic compounds present in the water. Therefore, a major recovery of water and nutrients was achieved, due to a lower dilution factor of the aqueous phase.