Production of acid-free bio-oil through improved co-HTL of sludge and microalgae: Experiment and life cycle assessment

Sludge and microalgae have been reported as the main residues of microalgal-bacterial symbiosis wastewater treatment, characterized by high moisture content and large output. Effective utilization is still a challenging task. Aiming to find out the most optimized and applicable process, eight co-hydrothermal conversion schemes were investigated in this study. It was found that using hydrothermal carbonization as pretreatment of sludge, co -hydrothermal liquefaction of two raw materials could have 45.65% bio-oil, 5.07% more than conventional co -hydrothermal liquefaction with simple mixing of sludge and microalgae in the reactor. The acid content of bio-oil was reduced by 14.4% at the same time. In another improved process, all acid of bio-oil was eliminated when the co-hydrothermal liquefaction occurred between microalgae and hydrochar obtained by sludge hy-drothermal carbonization. This is more conducive to equipment material selection and reduces the difficulty of upgrading. Furthermore, the mechanism of acid transformation was explored by the hydrothermal experiment of model compounds. Based on experiment data, three optimal co-hydrothermal liquefaction systems were evalu-ated through conception process construction and life cycle assessment. The Global Warming Potential of co -hydrothermal liquefaction of sludge and microalgae was 39.97 kg CO2eq, while the Global Warming Potential of co-hydrothermal liquefaction of microalgae with hydrochar obtained by sludge hydrothermal carbonization was only 25.49 kg CO2eq, which showed that sludge hydrothermal carbonization exhibited the largest envi-ronmental benefits. And sensitivity analysis showed that combining hydrothermal carbonization with co -hydrothermal liquefaction could reduce the influence of various environmental impact categories and improve environmental stability.

Automated summary

This study aimed to find the most optimized and applicable co-hydrothermal conversion process. The results showed that the combination of hydrothermal carbonization as a pretreatment of sludge and co-hydrothermal liquefaction of sludge and microalgae can achieve higher bio-oil yield and lower acid content. Furthermore, combining hydrothermal carbonization with co-hydrothermal liquefaction can reduce environmental impacts and improve environmental stability.