Exploring PCDD/Fs and potentially toxic elements in sewage sludge during smouldering treatment

Potentially toxic elements (PTEs), persistent organic pollutants, and emerging contaminants make sewage sludge management challenging. There is significant interest in thermal treatment technologies that can destroy these compounds. The most common thermal treatment, incineration, poses risks due to formation and/or release of hazardous substances in process emissions such as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and PTEs. Smouldering has been introduced recently as a potential treatment for managing sewage sludge. Smouldering systems present several advantages over traditional incinerators; however, there are still uncertainties regarding process by-products. This key question was investigated in three laboratory-scale tests (0.08 m radius) and five oil drum-scale tests (0.3 m radius) that were evaluated for PCDD/Fs and PTEs in the mixture before and after treatment as well as in process emissions. Volatile organic compounds (VOCs) were also measured. These experiments represent a broad spectrum of conditions to evaluate process emissions, from robust self-sustaining to extinction of smouldering. Robust smouldering had negligible PCDD/Fs in process emissions. Weak smouldering had low levels of PCDD/Fs (emissions factor: 3.3 ± 0.3 μg TEQ/Mg dried sludge destroyed), levels less than uncontrolled emissions from commercial incinerators. Overall, smouldering acted as a sink for PCDD/Fs, as only 0-3% of the PCDD/Fs originally present in the sludge were released in the emissions, and >99% of the remainder were destroyed with <1% remaining in post-treatment ash. No evidence was found to support de novo synthesis or precursor reactions forming new PCDD/Fs. In addition, 94-100% of all the PTEs analyzed were retained in the post-smouldered material. These results indicate that only minimal emissions treatment for PTEs, PCDD/Fs, and VOCs may be necessary for future sewage sludge smouldering systems. These low emissions risks combined with its unique ability to handle high moisture content waste, indicate that smouldering has significant potential as a valuable waste management technique.