Nutrient recycling of aqueous phase for microalgae cultivation from the hydrothermal liquefaction process

Abstract

Two major considerations of the emerging algae biofuel industry are the energy intensive dewatering of the algae slurry and nutrient management. The proposed closed loop process which involves nutrient recycling of the aqueous phase from the hydrothermal liquefaction of microalgae offers a solution to both aspects. Hydrothermal liquefaction has been shown to be a low energy process for bio-crude production from microalgae. For the purpose of this research, microalgae strains of Chlorella vulgaris, Scenedesmus dimorphus and the cyanobacteria Spirulina platensis and Chlorogloeopsis fritschii were processed in batch reactors at 300°C and 350°C. Following liquefaction the product phases were separated and the water phase recovered. The bio-crude yields ranged from 27 to 47wt.%. The bio-crudes were of low O and N content and high heating value making them suitable for further processing. The water phase was analysed for all major nutrients, TOC and TN to determine the suitability of the recycled aqueous phase for algae cultivation. Growth trials were performed for each algae strain in a standard growth medium and compared to the growth rates in a series of dilutions of the recycled process water phase. Growth was determined by cell count and chlorophyll a absorbance. Growth occurred in heavy dilutions where the amount of growth inhibitors was not too high. The results show that the closed loop system using the recovered aqueous phase offers a promising route for sustainable oil production and nutrient management for microalgae.