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

Realization of quasi-steady-state piled smouldering using sequential operation chambers for industrial treatment of biomass wastes

Piled smouldering has great potential for treatment and utilization of biomass wastes. However, its unsteady-state nature limits its industrial utilization, as well as treatment of smoke. This article addresses this issue by proposing the sequential operation of numerous smouldering chambers to realize steady- or quasi-steady-state piled smouldering. The superposition characteristics of sequential unsteady-state curves were analysed theoretically, and a code was developed to determine an appropriate number of piled chambers at an allowance oscillation percentage. Smouldering experiments were performed on a single mini chamber (length × width × height: 340 × 140 × 140 mm3) containing piled wood pellets mixed with wood powder. The superposition of sequential burning rate curves was demonstrated using the code based on the mass loss data of experiments. Analysis shows that the perfect-steady state is possible given the superposition value of the burning rate curve is a constant in this proposed system. Experiments show that the molar ratio of CO/CO2 in smoke is almost a constant around 0.5 during densely piled smouldering, showing the great potential for self-sustained burning out the smoke. Based on the experimental results, the calculation results show that the relative oscillation range of burning rate (OSC) decreases from 75% to 3% while increasing the number of chambers from 2 to 7. This work provides a novel technology to enable quasi-steady-state smouldering for industrial utilization.

State of the art in self-sustaining smoldering for remediation of contaminated soil and disposal of organic waste

Smoldering combustion applications in energy and environmental fields have attracted increasing research attention in recent years. Smoldering has demonstrated considerable green advantages, such as having a low carbon footprint and being sustainable, for remediation of organic-contaminated soil and disposal of high-moisture, low-calorific value, slurry-type organic waste due to its self-sustaining reaction characteristic. This review aims to analyze and summarize studies on smoldering applications to refine the critical components of applied smoldering systems, key reaction characteristics, and corresponding influencing conditions that affect their effectiveness. Furthermore, the common characteristics and influencing factors of different smoldering application scenarios are compared to provide a comprehensive reference for commercial applications. Thus, this paper specifically includes an overview of the impact of inert porous media, combustible material, and oxidants in applied smoldering systems; a review of the research status of the three key reaction characteristics, including peak temperature, smoldering front propagation velocity, and self-sustainability; a summary of typical influencing factors, disposal material characteristics, and control conditions in the two mainstream application directions, which are remediation of contaminated soil and disposal of organic waste; and a comparative analysis of the common modes of applied smoldering beyond the lab scale. As a technically effective and energy-efficient emerging technology, the prospects of smoldering as a robust treatment process in environmental pollution cleanup are presented.

Smouldering to treat PFAS in sewage sludge

Wastewater treatment plants are accumulation points for per- and polyfluoroalkyl substances (PFAS), and are threfore important facilities for PFAS treatment. This study explored using smouldering combustion to treat PFAS in sewage sludge. Base case experiments at the laboratory scale (LAB) used dried sludge mixed with sand. High moisture content (MC) LAB tests, 75% MC sludge by mass, explored impacts of MC on treatment and supplemented with granular activated carbon (GAC) to achieve sufficient temperatures for thermal destruction of PFAS. Additional LAB tests explored using calcium oxide (CaO) to support fluorine mineralization. Further tests performed at an oil-drum scale (DRUM) assessed scale on PFAS removal. Pre-treatment sludge and post-treatment ash samples from all tests were analyzed for 12 PFAS (2C-8C). Additional emissions samples were collected from all LAB tests and analyzed for 12 PFAS and hydrogen fluoride. Smouldering removed all monitored PFAS from DRUM tests, and 4-8 carbon chain length PFAS from LAB tests. For base case tests, PFOS and PFOA were completely removed from sludge; however, high contents in the emissions (79-94% of total PFAS by mass) showed volatilization without degradation. Smouldering high MC sludge at ∼ 900 °C (30 g GAC/kg sand) improved PFAS degradation compared to treatment below 800 °C (<20 g GAC/kg sand). Addition of CaO before smouldering reduced PFAS content in emissions by 97-99% by mass; with minimal PFAS retained in the ash and minimal hydrofluoric acid (HF) production, as the fluorine from the PFAS was likely mineralized in the ash. Co-smouldering with CaO had dual benefits of removing PFAS while minimizing other hazardous emission by-products.