Hydrothermal liquefaction of sewage sludge: a promising solution for circular bioeconomy

The rapid increase in sewage sludge production from wastewater treatment plants has become a pressing global environmental issue. Sewage sludge contains a variety of pollutants, including lignocellulose from plants and paper waste, microplastics, and pharmaceutical residues, posing significant risks to ecosystems and human health. Traditional waste management methods such as landfilling and incineration are often costly and environmentally harmful. In response, researchers are turning to hydrothermal liquefaction (HTL) as a sustainable solution. HTL converts sewage sludge into renewable biofuels under moderate temperature and pressure conditions, effectively recovering energy while bypassing the competition between bioenergy generation and food availability.

The study, led by Syed Comail Abbas from the University of Maine and colleagues, provides an in-depth analysis of current advancements in HTL technology. It examines the factors influencing bio-oil production and the transformation pathways of key pollutants during the HTL process. The research also explores advanced methods for upgrading sewage sludge, including chemical, mechanical, and in situ liquefaction techniques. Additionally, the study evaluates the potential applications of byproducts such as the aqueous phase, solid residues, and gases, further emphasizing the concept of a circular bioeconomy.

The findings reveal that HTL can effectively break down complex organic compounds in sewage sludge, producing bio-oil with high energy density. The study also highlights the potential for reducing the environmental impact of sewage sludge by converting pollutants like microplastics and pharmaceutical residues into less harmful substances. However, challenges remain in improving the quality and yield of bio-oil, as well as in the effective handling of byproducts. The researchers suggest that further research is needed to optimize HTL processes and explore alternative catalysts and solvents to enhance efficiency and reduce costs.

This study underscores the potential of HTL as a sustainable waste management solution and a valuable contributor to the circular bioeconomy. By transforming sewage sludge into renewable biofuels and valuable byproducts, HTL offers a promising pathway for addressing global waste challenges and meeting increasing energy demands.