Aquifer thermal energy storage (ATES) is being increasingly employed in the Netherlands and elsewhere as a means of saving energy. The groundwater used in these systems often contains dissolved iron; and the oxidation of the iron can cause clogging in the extraction and infiltration sources. Subsurface iron removal can prevent these problems. This technique has for my years been successfully used in the production of drinking water, but it also offers prospects for other sectors that face problems in the use of ferrous groundwater. In this project we are studying the possibility of applying it in ATES systems.
ATES geothermal energy systems can be faced with the clogging of their groundwater sources, which can impact their proper functioning. Chemical well clogging, caused by iron oxidation and precipitation, can occur when groundwater of different compositions is mixed during extraction operations. By removing the iron from the groundwater before it is pumped up, one can prevent well-clogging problems caused by iron precipitation. This can be done relatively simply by means of subsurface iron removal: a technique in which the first step involves the injection of oxygenated (aerated) water into a groundwater well. The oxygen reacts with the iron in the groundwater, which precipitates in the soil in the form of rust. The iron-free groundwater can then be pumped up, usually through the same well. The volume of groundwater is many times larger than was the case prior to the injection of the aerated water. The injection-extraction cycle can then be recommenced. This technique has been successfully applied in drinking water production for many years. Subsurface iron removal also offers prospects for other sectors that encounter problems in the use of groundwater that is rich in iron or manganese.
This project’s challenge is to find a sustainable solution for existing ATES systems that face problems with chemical clogging caused by iron precipitation. To this end, a pilot is being carried out on the ATES system of the Universitair Ziekenhuis (UZ) Leuven (Belgium). The system was installed over five years ago following a successful trial with two test wells. But since then it has never worked properly. Right from the beginning, the formation of ferric hydroxide precipitation led to clogging of the infiltration sources. In this project we intend to study whether subsurface iron removal can prevent the well-clogging problems.
To achieve the desirable solution, we must take into account the fact that an ATES system is rather different from a drinking water abstraction system. This means that the implementation of subsurface iron removal has to be specifically incorporated into the framework of ATES systems. On the one hand this raises substantive knowledge questions about differences in water temperature and the chemical composition of the groundwater. And, on the other, it involves more operational questions related to the cyclical extraction and infiltration of water and their control. A key area of concern is the long-term storage of water in the subsurface. And then there is the requirement that the extracted groundwater not contain iron at any time, since additional iron removal at the surface – which is common at drinking water utilities – is not an option in ATES systems.
This project focusses on determining how the process of subsurface iron removal can work in ATES systems in practice, and on how it can be optimally controlled to ensure a long-term stable situation. It is anticipated that the technology will offer possibilities for reducing or preventing well-clogging problems in existing ATES systems. In addition, subsurface iron removal could make it possible to realise ATES systems in places that have to date been avoided because of the risk of well clogging. It therefore constitutes an opportunity to contribute positively to the energy transition. A successful pilot at UZ Leuven will mean that the ATES system there can be equipped with a definitive subsurface iron removal installation, after which more heat and cold sources can be put in place.