press release, 12. June 2024

Two Birds with one Stone: Groundwater treatment and thermal storage

A UFZ team is field-testing a system for the underground storage of thermal energy that simultaneously purifies contaminated groundwater

Using thermal energy in aquifers can provide an important contribution to CO₂-free heat management. However, groundwater aquifers in cities and industrial areas are frequently contaminated, especially those close to the surface. They therefore cannot be directly used for seasonal heat storage. Researchers from the Helmholtz Centre for Environmental Research (UFZ), the Christian-Albrecht University of Kiel and Eneotech Environment Ltd. have now started up a pilot plant at the UFZ site in Leipzig as part of the KONATES research project. It is intended to serve as a heat exchanger while also removing pollutants from the groundwater. This could provide a blueprint for low-CO₂, energy-efficient management of contaminated aquifers in general.

Aquifers are important because they are a supply of drinking water or process water in many areas. However, they can also provide another important service by being used for seasonal thermal storage. UFZ researchers in Leipzig are focussing on this: They are using an aquifer up to 5 meters thick at a depth of about 12 meters beneath the UFZ site, in which the groundwater flows through the sandy gravel at a constant temperature of 14 degrees Celsius and a speed of roughly one meter per day. "The water can be taken from the aquifer in the summer, extracting the cold from it through heat exchangers. This can be used to cool buildings and the heated water can be pumped back underground. By pumping the heated water fed into the system back out of the aquifer in the winter, the heat recovered can be used to heat buildings," explains Prof. Holger Weiß, who is coordinating the KONATES research project. What's special about the project: The researchers want to use an ATES (Aquifer Thermal Energy Storage) plant not only to generate heating and cooling for a future CO₂-free science park, but also to purify groundwater contaminated with chlorinated hydrocarbons, contamination that can be found in many urban and industrial areas. 

The team headed by geologist Holger Weiß has therefore constructed a pilot plant in the Leipzig Science Park. In this plant, the groundwater is pumped up from the aquifer via a well and is routed to two containers. The water is heated up and in successive cycles reinfiltrated. "Our modelling with the UFZ open source modelling platform OpenGeoSys has shown that a heat recovery of 25 percent is even possible for the shallow aquifer under the specific hydraulic conditions. A higher efficiency level is not currently intended in the trial phase (proof-of-concept) and on this scale," says Holger Weiß. Second, field experiments are investigating which microorganisms occur in groundwater at different temperatures and whether these break down the pollutants in the groundwater. "Based on earlier studies on a similar aquifer near Wittstock (the test field at Kiel University), we know, for example, that the composition of the microbial community changes drastically between water temperatures of 45 and 60 degrees Celsius," reports the UFZ researcher. However, it is unclear whether the microorganisms surviving at these high temperatures break down precisely those chlorinated hydrocarbons polluting the aquifer beneath the UFZ site. It is known from preliminary investigations that this area hosts especially widespread populations of microorganisms of genus Dehalococcoides, which can break down chlorine-containing compounds. The researchers will therefore heat the extracted water to up to 80 degrees Celsius and then determine the temperature at which the bacteria break down pollutants most efficiently. 

They will also investigate the biogeochemical effects of microbial breakdown on the technical equipment. For example, the microorganisms may form biofilms at elevated temperatures, blocking the small filter slits in the well. Corrosion of plant parts and handling problems with the extremely hard groundwater in Leipzig can also arise if heating causes lime in particular but also iron to precipitate. The researchers will also test new purification methods. The current state of the art is to absorb the chlorinated hydrocarbons with activated carbon, with the drawback that this subsequently has to be regenerated in an elaborate process or disposed of. New zeolite adsorbers developed at the UFZ are to be tested in the plant as a more sustainable alternative. "These filter materials can be regenerated on site and then reused," says Holger Weiß. 

The extracted and treated groundwater from both containers is then pumped back into the aquifer via a second well. There it cools as it continues flowing to the boundary of the UFZ site. In a third well located downstream, the groundwater is monitored to ensure that it does not exceed the specified temperature limit of 16 degrees Celsius as it leaves the site. A total of twelve measuring points distributed around the plant also provide information on the temperature and pollutant level history in the aquifer and on which bacterial strains break down the pollutants at which temperatures. 

The researchers in the KONATES project now have time until mid-2025 for the experiments - time they need to determine the performance limits of the ATES pilot plant. They will then prepare a guideline with recommendations for the ideal combination of seasonal heat storage with groundwater remediation measures. "The goal is to first develop low-CO₂ energy management systems for the Leipzig Science Park", says Holger Weiß. He sees a large potential for the method throughout Germany. "Many industrial and urban areas are contaminated by chemicals such as chlorinated hydrocarbons. Because contaminated groundwater can be cleaned when using geothermal energy, this climate-neutral heat management method could be used to kill two birds with one stone."

The KONATES ("Contaminated ATES") runs to 07/2025. It is operated with the support of the Federal Ministry of Education and Research (BMBF) in its Geosciences for Sustainability" (GEO:N) research programme in the BMBF initiative "Possibilities and limits of thermal energy storage in aquifers."

Further information

Prof. Dr. Holger Weiß
UFZ Department of Environmental Informatics
holger.weiss@ufz.de

UFZ press office

Susanne Hufe
Phone: +49 341 235-1630
presse@ufz.de