Pyrolysis as an economical and ecological treatment option for municipal sewage sludge

Abstract

Pyrolysis of sewage sludge was selected as a potential disposal method that provides biochar and bio-oil. An intrinsic problem of sewage sludge is the high concentration of heavy metals, which is retained in the char. This study aims to compare both slow and fast pyrolysis char and their leachability of heavy metals for agricultural land use, as defined by the U.S. EPA regulation. Slow pyrolysis biochar yields decreased from 51.6 wt% at 300 °C to 32 wt% at 500 °C. Similarly, fast pyrolysis biochar yields also decreased from 30 wt% at 400 °C to 26 wt% at 500 °C. The higher heating value reduced for the chars produced at high temperatures because of greater ash content. The maximum higher heating value on ash free basis was 34.6 MJ kg−1 for the fast pyrolysis char at 500 °C. We developed a new method for the calculation of the enthalpy of pyrolysis (2.2 MJ kg−1), which was used to create a complete energy balance for the pyrolysis of sewage sludge. We performed an economic analysis for a typical sewage sludge pyrolysis plant processing 2.1 ton of dry solids per hour. For such a scenario we found that the annual total capital investment and annual expenditure were 13.5 million CAD and 1.32 million CAD, respectively. An environmental life-cycle assessment determined that pyrolysis of sewage sludge with use of the biochar as a substitute fuel in a cement kiln had the least impacts on global warming potential and fresh water ecotoxicity.