Exploring hydrothermal liquefaction (HTL) of digested sewage sludge (DSS) at 5.3 L and 0.025 L bench scale using experimental design

A common perspective within the prospect of a greener future is utilising our waste materials. One waste material of which the world has abundant resources, and where we will keep having resources, is sewage sludge. This waste material is getting an increased focus, and is commonly utilised by anaerobic digestion processes for methane production. This leaves a bioresidue of digested sewage sludge (DSS). In this study, DSS is submitted to hydrothermal liquefaction (HTL) to produce bio-oil. The studied process includes upscaling as well as considering the effects of temperature, reaction medium of water or ethanol, degree of reactor filling and stirring rate. Promising results are found as high oil yields are obtained also after upscaling. The results reported here show that stirring reduces the need of high temperatures during HTL, providing energy savings that are promising for further upscaling. In addition, a total of 18 compounds are identified and semi-quantified, showing an abundance of fatty acids and fatty acid derivatives within the oil, encouraging further studies towards separation of said fatty acids for use as biodiesel.