Recent advances in energy recovery from wastewater sludge

Sludge management is one of the most difficult and challenging tasks of wastewater treatment plants due to its high water content and poor dewaterability and strict regulation for sludge reuse or disposal. One of the recent goals of wastewater treatment plants is to develop more environmentally friendly processes to reduce the volume of sludge for disposal and to convert sludge into bioenergy. Energy recovery of the sludge generally includes the conversion of the sludge into biogas, syngas, and bio-oil which can be further converted into electricity, mechanical energy, and heat. In this chapter, methods for energy recovery from wastewater sludge in the forms of bio-oil and biogas are reviewed. An overview of different sludge types and the common methods for measuring their characteristics and bio-oil production from sludge through hydrothermal treatments are discussed. Hydrothermal treatment reaction pathways and the effects of various operating parameters such as reaction temperature and time, the presence of a catalyst, feedstock type, water-to-biomass ratio, and water at sub−/supercritical conditions are discussed. In addition, some available hydrothermal processes in pilot or demonstration scales are introduced. Biogas production from sludge through anaerobic digestion is also reviewed, and effects of various pretreatment methods such as thermal, chemical, thermochemical, and mechanical treatments on the anaerobic digestion performance and the final biogas yield are discussed. Furthermore, the established and emerging technologies for each pretreatment method are introduced. © 2018 Elsevier Ltd. All rights reserved.