Effect of compaction on bisulfide diffusive transport through MX-80 bentonite.

Canada's deep geological repository (DGR) design includes an engineered barrier system where highly compacted bentonite (HCB) surrounds the copper-coated used fuel containers (UFCs). Microbial-influenced corrosion is a potential threat to long-term integrity of UFC as bisulfide (HS-) may be produced by microbial activities under anaerobic conditions and transported via diffusion through the HCB to reach the UFC surface, resulting in corrosion of copper. Therefore, understanding HS- transport mechanisms through HCB is critical for accurate prediction of copper corrosion allowance. This study investigated HS- transport behaviour through MX-80 bentonite at dry densities 1070-1615 kg m-3 by performing through-diffusion experiments. Following HS- diffusion, bromide (Br-) diffusion and Raman spectroscopy analyses were performed to explore possible physical or mineralogical alterations of bentonite caused by interacting with HS-. In addition, accessible porosity ε was estimated using extended Archie's law. Effective diffusion coefficient of HS- was found 2.5 × 10-12 m2 s-1 and 5.0× 10-12 m2 s-1 for dry densities 1330 and 1070 kg m-3, respectively. No HS- breakthrough was observed for highly compacted bentonite (1535-1615 kg m-3) over the experimental timeframe (170 days). Raman spectroscopy results revealed that HS- reacted with iron in bentonite and precipitated as mackinawite and, therefore, it was immobilized. Finally, results of this study imply that HS- transport towards UFC will be highly controlled by the available iron content and dry density of the buffer material.

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 âˆ¼ 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.

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.

Phosphorus recovery and reuse potential from smouldered sewage sludge ash.

Smouldering treatment of sewage sludge - and recapturing phosphorus - provides important steps towards a circular economy. This study reveals that bulking sludge with sand or another organic waste, e.g., woodchips, created a material that was readily converted to ash by self-sustained smouldering. Simultaneous phosphorus and regulated potentially toxic element releases from ash were evaluated using leaching methods from the USEPA Leaching Environmental Assessment Framework (LEAF). Extraction potentials were also determined to evaluate direct recovery as an alternative to land application. Compared to the parent sludge, post-treatment ash from smouldering sludge with sand contained higher quantities of inorganic phosphorus in sorbed and mineral phases, which can provide beneficial slow phosphorus release to plants and avoid early phosphorus washout during land application. Ash also released lower initial and total quantities of potentially toxic elements than virgin sludge. As an alternative to land application, approximately 42% of retained phosphorus can be recovered directly using acidic extraction, and an additional 30% from emissions. In contrast, co-smouldering sludge with woodchips was more suited for direct recovery with 78% of phosphorus potentially recoverable via emissions capture and yield increasing to 99% with acidic extraction of resulting ash. Co-smouldering also produces a single post-treatment ash and can be readily operated continuously, which aligns with current incinerator configurations at wastewater treatment plants and makes adaptation highly feasible. With phosphorus reuse opportunities for land application and direct recovery, smouldering sewage sludge creates an important opportunity for a phosphorus circular economy as part of wastewater treatment sludge management.