Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article


Doctor of Philosophy


Civil and Environmental Engineering


Gerhard, Jason I.

2nd Supervisor

Switzer, Christine


University of Strathclyde



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.

Summary for Lay Audience

The foods we eat contain many vitamins and minerals, which, if broken down to their most basic forms, consists of nutrients like phosphorus, and metals like cobalt and zinc. These nutrients and metals are present in small amounts in our faeces, however, during treatment at wastewater treatment plants, they become concentrated in sewage sludge. This is important because most of our phosphorus for fertilizers comes from mines which are being quickly depleted. Therefore, we need to recycle phosphorus and other valuable elements from other sources, including sewage sludge. However, just as valuable elements end up concentrated in sewage sludge, so can harmful compounds, such as PFAS, a group of human-made chemicals used for waterproof coatings and food containers. Therefore, before useful compounds in sewage sludge can be recycled (including phosphorus), the sewage sludge needs to be treated to remove harmful compounds. Incineration is a typical method for treating sewage sludge which consists of very high temperatures to burn the material. While effective, incineration is very expensive and requires a lot of energy.

An alternative method for treating sewage sludge is smouldering. Smouldering is a flameless, more energy efficient form of burning that is commonly seen in a barbecue. However, using smouldering to treat sewage sludge is relatively new so little is known about how well it removes harmful compounds, and if new ones are formed during the process, such as dioxins, a group of hazardous compounds formed during waste burning. This research looked at the types of harmful compounds present in and released during sewage sludge smouldering and tried to minimize them. Additionally, methods of recycling valuable compounds, especially phosphorus, were explored. Smouldering was able to treat the sewage sludge, forming an ash that contained almost no PFAS. Additionally, it is unlikely that smouldering sewage sludge produces any dioxins. In terms of element recycling, smouldering retained nearly all the valuable elements in the ash, making recycling simpler. This research demonstrated that smouldering is an effective treatment method for sewage sludge, removing most harmful compounds (while not forming any additional), and creating an ash that is rich in valuable elements, especially phosphorus.