Electronic Thesis and Dissertation Repository


Doctor of Philosophy


Chemical and Biochemical Engineering


Dr. Charles (Chunbao) Xu

2nd Supervisor

Dr. Madhumita (Mita) B. Ray

Joint Supervisor


Growing interest in renewable energies due to shrinking reserves of fossil fuels and climate change concerns have led to extensive research towards gaseous and liquid fuels production from renewable energy resources such as biomass and wastes. Energy generation from municipal and industrial wastes such as wastewater sludge is also environmental friendly way to deal with large volume of waste disposal with the additional advantage of eliminating part of the indirect greenhouse gas emissions from energy crops-derived biofuels.

In this thesis, a novel process for co-production of biogas and bio-crude oil from high-water-content wastewater sludge through hydrothermal liquefaction (HTL) treatments is developed. Hydrothermal liquefaction is a thermochemical process where raw sludge can be heat treated directly in the absence of oxygen and in the presence of water as the reaction medium mostly in subcritical or near critical conditions (T < 374 oC and P < 22.1 MPa). This eliminates the need to dewater/dry biomass which can be a major energy input for biofuel production via other processes such as pyrolysis or gasification. Since hydrothermal liquefaction is a promising technology for conversion of high-water-content biomass without the need of costly sludge dewatering, it could replace the conventional sludge treatment by making valuable energy products out of a waste material.

Wastewater sludge was treated by two scenarios, operating at temperatures in a lower range (40-80 oC) and a higher range (200-350 oC), respectively. The low-temperature treatment was considered as sludge pre-treatment before anaerobic digestion, aiming to examine the possible relationship between increased solubilisation of the sludge as a result of the pre-treatment and its digestibility for biogas production. The high-temperature treatment scenario was performed to produce value-added products such as bio-crude oil from co-processing of wastewater sludge (more than 90% water content) with another type of lignocellulosic biomass to adjust substrate concentration to a higher level with better economics of the process, and to increase the bio-oil yield and quality. The main by-product from the high-temperature process (water-soluble product) was used as a potential feedstock for biogas production through anaerobic digestion.