Municipal wastewater continuously carries vast quantities of microplastics, generated through everyday activities in homes and businesses, to wastewater treatment plants. At these facilities, a large proportion of these microplastics – alongside valuable nutrients such as nitrogen and phosphorus – ends up in sewage sludge, the mud-like liquid residue remaining after wastewater treatment. Analysing microplastics in sewage sludge is challenging and lacks a harmonised method, despite the approaching requirements to monitor the presence of microplastics in sewage sludge imposed through the EU’s revised Urban Wastewater Treatment Directive.
In her doctoral research, Yli-Rantala optimised a method for extracting microplastics from sewage sludge for subsequent analysis. Her results showed that the biogas process commonly used in the treatment of sewage sludge has only a minor, if any, effect on microplastics. Therefore, the biogas process cannot be considered an effective means of reducing the microplastic content in sewage sludge. In addition, Yli-Rantala discovered that during the solid-liquid separation of treated sludge, approximately 99% of microplastics accumulated in the solid fraction, which is typically destined for land application. Her findings highlight the urgent need to develop and adopt sludge treatment technologies that enable the safe return of nutrients and organic matter to the soil, while effectively eliminating microplastics and other contaminants to prevent environmental pollution.
“Seeing the amount of microplastics in just half a gram of dried sludge under the microscope was eye-opening – it really made tangible the enormous quantity of microplastics we are currently allowing to enter the soil,” Yli-Rantala says.
Yli-Rantala’s research demonstrated that only about 1% of microplastics is diverted to the liquid fraction in the solid-liquid separation process, along with approximately 35–65% of nitrogen. This suggests that the liquid fraction could serve as a viable source of nitrogen for nutrient recycling with a relatively low microplastic content. More widespread recycling of nitrogen from the liquid fraction could therefore support the production of cleaner sludge-based fertilisers and contribute to greater self-sufficiency in fertiliser production.
Public defence of a doctoral dissertation on Friday 26 September
The doctoral dissertation of MSc (Tech) Elina Yli-Rantala in the field of environmental engineering, titled Microplastics in Wastewater Sludges – Extraction, Occurrence and Implications for the Circular Economy, will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University in room Pieni Sali 1 (FA032) in the Festia building on the Hervanta campus (address: Korkeakoulunkatu 8, Tampere) on Friday 26 September 2025 at 12:00.
The opponent will be PhD Salla Selonen from the Finnish Environment Institute. The Custos will be Professor Marika Kokko from the Faculty of Engineering and Natural Sciences.