Exploring Valuable and Potentially Harmful By-Products Formed and/or Released from Smouldering Treatment of Sewage Sludge

The presence of potentially toxic elements (PTEs) and emerging contaminants (e.g., per- and polyfluoroalkyl substances (PFAS)) makes sewage sludge management challenging. Due to their hazards, there is significant interest in thermal treatment technologies that can destroy these compounds, like incineration. However, incineration still poses several risks due to forming and/or releasing hazardous emissions (e.g., polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) and PTEs). More recently, the use of smouldering has been introduced as a potential treatment technique for managing sewage sludge. Smouldering presents several advantages over traditional incineration due to its lower energy and pre-treatment requirements and potential for beneficial by-products; however, little is known about the process by-products. This question was investigated during smouldering tests conducted at the laboratory reactor scale and oil drum reactor scale. Tests were evaluated for key compounds of interest – PCDD/Fs, PTEs, and PFAS – before and after treatment as well as in process emissions. For the PFAS experiments, adjustments were made to the tests to improve PFAS degradation. The USEPA Leaching Environmental Assessment Framework (LEAF) was then used on the post-treatment ash to evaluate phosphorus and PTE release and extraction potential. This study found negligible PCDD/Fs in process emissions during robust smouldering and low levels of PCDD/Fs during weak smouldering. Overall, smouldering acts as a sink for PCDD/Fs. In addition, 94-100% of all the PTEs analyzed were retained in the post-treatment ash following smouldering treatment, not released in the emissions. Smouldering completely removed all PFAS from 3C-8C from the sludge under all laboratory conditions, where much of the PFAS was likely volatilized into the emissions requiring further treatment. Supplementing the sewage sludge with granular activated carbon increased the energy of the system and improved PFAS degradation for high moisture content sludge. When a calcium amendment was added, the PFAS content in the emissions was 97 – 99% lower than all other conditions. Smouldered sewage sludge ash contains higher quantities of inorganic phosphorus than the parent sludge and releases lower initial and total PTEs. Furthermore, 72% of the phosphorus is recoverable. With low emissions risks, high potential for PFAS treatment, and phosphorus reuse opportunities for land application and direct recovery, smouldering has significant potential as a valuable waste management technique

Applied smouldering for co-waste management: Benefits and trade-offs

Smouldering combustion is emerging as a valuable application for many environmentally beneficial purposes, including waste-to-energy. While many applied smouldering systems rely on small fractions of fuel mixed within inert porous media (IPM) like coarse grain silica sand, there is also an opportunity to pursue co-waste management with much higher fuel loadings. This study explores these co-waste systems by blending non-smoulderable wastes (e.g., sewage sludge) with smoulderable wastes (e.g., construction waste woodchips) to form porous solid fuels (PSFs). Key differences between these IPM and PSF systems were identified by contrasting the temperature profiles in space and time, specific mass loss rates, and emissions profiles across experiments in multiple reactors (with 0.054, 0.080, and 0.300 m radii). For example, the PSF systems exhibited higher throughputs, a more straightforward path towards continuous operation, improved scalability, and higher production of potentially useful by-products (e.g., CH4, H2) than the IPM systems. However, the PSF systems were more sensitive to extinction and exhibited lower fuel moisture content limitations than the IPM systems. Altogether, this study illuminates the benefits and trade-offs of co-waste smouldering

Scaling up self-sustained smouldering of sewage sludge for waste-to-energy

Self-sustained smouldering combustion presents strong potential as a green waste-to-energy technique for a range of wastes, especially those with high moisture content like wastewater sewage sludge. While well-demonstrated in laboratory experiments, there is little known about scaling up this process to larger, commercial reactors. This paper addresses this knowledge gap by systematically conducting and analyzing experiments in a variety of reactors extending beyond the laboratory scale. This work reveals a robust treatment regime; however, it also identifies potential complications associated with perimeter heat losses at scale. Two key impacts, on the smouldering reactions and the air flow patterns, are shown to potentially degrade treatment if not properly understood and managed. Altogether, this study provides novel insight and guidance for scaling up smouldering science into practical, waste-to-energy systems