Waste Management & Research 2014, Vol. 32(7) 586–600 © The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X14538303 wmr.sagepub.com Introduction Sludge is a term used to define the liquid or semi-solid residue of wastewater treatment from domestic, industrial, or commercial sources. It contains 25–120 kg m −3 by weight of total suspended solid (TSS), with a high fraction of organic compounds. Sludge also contains microorganisms, including pathogens, parasites, and other microbiological pollutants. Direct exposure to sludge is considered detrimental to public health because of the possible contact with vectors infectious to humans. In addition, the pres- ence of other toxic pollutants contained in sludge such as heavy metals (e.g. zinc, lead, copper, chromium, nickel, cadmium, mer- cury, etc.) and difficult-to-treat organic compounds (e.g. poly- chlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dioxins, pesticides, endocrine disruptors, etc.) makes sludge a potentially harmful substance to the environment. Sludge is also a valuable source of organic carbon, nitrogen, phosphorous, as well as some inorganic compounds such as sili- cates, aluminates, etc., which can be recycled or further utilized for industrial or agricultural purposes (Dentel, 2004; Fytili and Zabaniotou, 2008; Rulkens, 2004; Rulkens and Bien, 2004). Moreover, it is a potential renewable energy source with a stored chemical energy content of 9–29 MJ kg −1 of TSS (Tchobanoglous et al., 2003), or approximately 0.2–3.5 GJ m −3 of sludge. As with other types of biomass, this energy was originally captured through photosynthesis, and can be recovered when the chemical bonds between adjacent oxygen, carbon, and hydrogen atoms are broken by various biological and thermo-chemical processes (Fytili and Zabaniotou, 2008; Tchobanoglous et al., 2003). Sludge management and treatment is a capital-intensive pro- cess that accounts for up to 50% of the total cost of wastewater treatment (Campbell, 2000), and contributes approximately 40% of total greenhouse gas (GHG) emissions associated with the wastewater treatment process (Liu et al., 2013). Contemporary sludge treatment approaches aim to achieve: (1) reductions in total weight and volume to facilitate transportation and provide Overview of current biological and thermo-chemical treatment technologies for sustainable sludge management Linghong Zhang 1,2 , Chunbao (Charles) Xu 3 , Pascale Champagne 1,4 and Warren Mabee 2,5 Abstract Sludge is a semi-solid residue produced from wastewater treatment processes. It contains biodegradable and recalcitrant organic compounds, as well as pathogens, heavy metals, and other inorganic constituents. Sludge can also be considered a source of nutrients and energy, which could be recovered using economically viable approaches. In the present paper, several commonly used sludge treatment processes including land application, composting, landfilling, anaerobic digestion, and combustion are reviewed, along with their potentials for energy and product recovery. In addition, some innovative thermo-chemical techniques in pyrolysis, gasification, liquefaction, and wet oxidation are briefly introduced. Finally, a brief summary of selected published works on the life cycle assessment of a variety of sludge treatment and end-use scenarios is presented in order to better understand the overall energy balance and environmental burdens associated with each sludge treatment pathway. In all scenarios investigated, the reuse of bioenergy and by-products has been shown to be of crucial importance in enhancing the overall energy efficiency and reducing the carbon footprint. Keywords Sludge, biosolid, energy recovery, bioenergy, environmental impact, life cycle assessment 1 Department of Civil Engineering, Queen’s University, Kingston, ON, Canada 2 Department of Geography, Queen’s University, Kingston, ON, Canada 3 Department of Chemical & Biochemical Engineering, Western University, London, ON, Canada 4 Department of Chemical Engineering, Queen’s University, Kingston, ON, Canada 5 School of Policy Studies, Queen’s University, Kingston, ON, Canada Corresponding author: Chunbao (Charles) Xu, Department of Chemical & Biochemical Engineering, Western University, London, ON, N6A 5B9, Canada. Email: cxu6@uwo.ca 538303WMR 0 0 10.1177/0734242X14538303Waste Management & ResearchZhang et al. research-article 2014 Review Article