Sewage sludge-to-energy approaches based on anaerobic digestion and pyrolysis: Brief overview and energy efficiency assessment

Energy recovery from sewage sludge offers an opportunity for sustainable management of sewage sludge and energy. Anaerobic digestion and pyrolysis are among the most promising processes applicable for sewage sludge-to-energy conversion. Anaerobic digestion of sewage sludge forms methane-rich biogas, which can be utilized as fuel to offset heat and electricity consumption of the wastewater treatment sector. However, the digestion process has the limitation that it cannot sufficiently extract the energy in sewage sludge. The digested sludge is still energy profitable in that it contains considerable organic matter, but poor in biodegradability. Sludge pyrolysis is an innovative process that can convert both raw and digested sludge into useful bioenergy in the form of oil and gas, forming biochar as a byproduct that is environmentally resistant and holds potential for carbon sequestration and soil conditioning. It is expectable that sludge pyrolysis would step into practical deployment in the near future.