In Finland, municipal wastewater treatment generates nearly 850,000 tons of sludge annually. Typically, biogas is produced from sludge at treatment plants, which can be used for heat, electricity, or biofuel production. Sludge, including after the biogas process, contains nutrients and organic carbon that could be utilized for soil improvement and fertilization. However, it may also contain problematic components such as microplastics or pharmaceuticals. Moreover, sludge is highly moist, which reduces its economic feasibility and practicality due to increasing transportation costs.
In her thesis, Hämäläinen focused on the treatment of wastewater sludge using hydrothermal carbonization (HTC) and pyrolysis, methods that can produce solid hydrochar or biochar and a liquid fraction from treated sludge. Among these, HTC is considered a promising option for integration into wastewater treatment plants since it doesn't require pre-drying of sludge.
The thesis experimentally examined the suitability of HTC for two different types of wastewater sludge: digested municipal wastewater sludge and pulp-and-paper mill wastewater sludge. The effects of HTC conditions were also investigated. Additionally, the study assessed the impact of implementing HTC on the nutrient and energy balances of a full-scale biogas plant handling wastewater sludge.
The pulp-and-paper industry is increasingly interested in HTC of sludge as the industry moves towards carbon neutrality and seeks alternative sludge treatment solutions that are more sustainable and lower in emissions.
“Hydrochar produced from pulp-and-paper mill sludge through HTC could be used for carbon sequestration in soil, reducing carbon dioxide emissions,” says Hämäläinen.
Production of emission-free biogas should be increased in Finland
In her research, Hämäläinen demonstrated that HTC produces hydrochar with a volume of less than half of the untreated wastewater sludge volume. Hydrochar also has higher concentrations of phosphorus, minerals, and carbon. Furthermore, the hydrochar from the pulp-and-paper industry sludge had a higher calorific value. It was observed that higher treatment temperatures increased calorific value and ash content while more efficiently reducing volume. However, nutrient recovery suffered as more nutrients ended up in the gas phase as the temperature increased from 210 degrees Celsius to 250 degrees Celsius. Heavy metal concentrations and pharmaceutical residues in hydrochar were also examined, and the results indicated that hydrochar could be safely used in soil as a source of nutrients and carbon.
The quantity and characteristics of the liquid fraction generated from thermochemical treatments of sludge depend on the process and conditions. The liquid fraction may contain recoverable nutrients, but it could also contain harmful compounds. In line with circular economy principles, an efficient utilization method must be found for the liquid fraction.
The study found that the liquid fraction from HTC could be utilized in the biogas reactor of a wastewater treatment plant, thereby increasing biogas production. Even though a biogas reactor is not typically found in connection with a pulp mill, the liquid fraction from pulp-and-paper mill wastewater sludge also has biogas production potential. Laboratory experiments showed that the liquid fraction from pyrolysis could be introduced into the biogas reactor along with wastewater sludge, although in its pure form, it inhibited microbial activity.
“Based on scale-up calculations conducted in my thesis, utilizing the biogas process for the treatment of the liquid fraction increases nutrient recovery and enables the conversion of carbon into methane. Increasing biogas production would be beneficial throughout Finland since it represents waste-derived and emissions-free energy that can be used similarly to natural gas,” Hämäläinen emphasizes.
Anna Hämäläinen is originally from Espoo, Finland. Currently, she works as a researcher at Viikki in University of Helsinki, where she is developing wood-based stabilizing agents for food products.
Public defense on Friday 20 October
The doctoral dissertation of M.Sc. (Tech) Anna Hämäläinen in the field of Environmental Engineering titled Increasing sludge valorization by combining thermochemical treatment with anaerobic digestion: Focus on hydrothermal carbonization and pyrolysis will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University at 12:5 PM on Friday 20th of October 2023 in Konetalo Lecture Hall K1702, Hervanta Campus, Korkeakoulunkatu 8, 33720 Tampere. The Opponent will be Associate Professor Dr. Luca Fiori from the University of Trento (Università degli Studi di Trento). The Custos will be Professor Jukka Rintala from the Faculty of Engineering and Natural Sciences.
The dissertation is available online.
The dissertation can be followed remotely via remote connection (Panopto).
Photo: Peetu Tuominen