Hydrothermal liquefaction (HTL) of microalgae has rarely been commercialized. Click image to enlarge.

Wenguang Zhou writes in Sciencetrends.com about work recently published in the journal Bioresource Technology regarding why Hydrothermal Liquefaction (HTL) of microalgae — a bio-oil technology which does not require dewatering and drying of algal biomass — up to now, has rarely been commercialized.

One of the problems impeding its industrialization is the production of wastewater (the aqueous phase product) during HTL. More than 20% of the carbon and 50% of the algae is transformed to this wastewater. The cost for wastewater treatment by traditional methods would very high and the nutrients would be wasted.

Alternatively, this wastewater may be treated as a nutrients source for algae cultivation. During the last decade, researchers have been trying to make this possible. The wastewater may be used as carbon, nitrogen, phosphorus or even the sole nutrient source to sustain the normal algae culture. But it has to be diluted by a hundred times or even higher because the concentrations of nutrients are too high, and most importantly, the high concentrations of inhibitory substances such as phenolics, heavy metals, and cyclic nitrogenous compounds.

Inhibitory effects from these compounds may be alleviated by tuning the wastewater to a desirable level by adjusting HTL process parameters (e.g., changing reaction temperature), pre-treatment of wastewater to remove or reduce the inhibitory compounds, or balancing nutrients contents in wastewater. On the other hand, isolation and selection of robust algae species or species obtained from genetic engineering and metabolic evolution would be very effective to decrease the inhibitory effects.

Another promising way is to co-cultivate multiple algae species or cultivate algae with other microorganisms, such as bacteria and fungi. It could be more advantageous when the microorganisms could settle by itself because of bio-flocculation. Additionally, cultivation of multiple species (polyculture) can support the stable ecosystem of the wastewater treatment system because of enhanced biodiversity.

The work was conducted by Lijian Leng, Jun Li, Zhiyou Wen, and Wenguang Zhou from Nanchang University.

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