Integrated Route of Fast Hydrothermal Liquefaction of Microalgae and Sludge by Recycling the Waste Aqueous Phase for Microalgal Growth

In this study, waste aqueous phase after hydrothermal of sludge was used to cultivate the green microalga Chlorella sp. Results showed that waste aqueous phase can be used as a medium for Chlorella sp, with nitrogen and phosphorus removal efficiency of more than 90 %. At the same time, it was found that the introduction of CO2 during microalgal cultivation can effectively promote the growth of Chlorella sp and increase the lipid content respectively, comparing to the control. Further, based on response surface methodology, the optimal cultivation conditions of Chlorella sp were 5 % CO2, BG-11/SAP = 1:1 mixed medium and growth for 8 days. Chlorella sp was cultivated under the optimal aforementioned conditions followed by hydrothermal liquefaction, then the differences between the bio-oil and aqueous phase obtained by fast hydrothermal liquefaction and hydrothermal liquefaction were compared. This study showed that Chlorella sp can effectively remove N and P in waste aqueous phase after hydrothermal of sludge. At the same time, the addition of CO2 was beneficial to increase the unit biomass of Chlorella sp and promote the accumulation of lipids. Further comparison of different hydrothermal liquefaction processes showed that fast hydrothermal liquefaction can obtain bio-oil with higher quality in a shorter reaction time.