Subsurface iron removal with ATES geothermal energy systems

Technology

ATES geothermal energy systems do not function well when their groundwater sources are clogged. This clogging can occur for instance when groundwater of different compositions is mixed during abstraction, leading to iron oxidation and precipition. By removing the iron from the groundwater before it is pumped up, one can control 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. 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-abstraction cycle is then repeated. 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.

Challenge

This project’s challenge was to find a sustainable solution for existing ATES systems that face problems with chemical clogging caused by iron precipitation. To this end, a pilot was carried out on the ATES system of the Universitair Ziekenhuis (UZ) Leuven (Belgium). This system was installed over six years ago following a successful trial with two test wells. However, the system never worked properly following the test phase. Right after the installation the formation of ferric hydroxide precipitation already began to cause clogging of the infiltration sources. In this project we studied whether subsurface iron removal can control the well-clogging problems.

To produce the desired solutions, one must take into account the fact that an ATES system is quite different from a drinking water abstraction system. This means that the implementation of subsurface iron removal has to be set specifically within the framework of an ATES system. On the one hand this raises substantive knowledge questions regarding differences in the chemical composition of the groundwater. And, on the other, it also involves more operational questions related to the cyclical abstraction and infiltration of water and their control. A key area of concern is the long-term storage of water in the subsurface. There is moreover the requirement that the abstracted 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.

Solutions

This project focussed on the operation of subsurface iron removal in ATES systems in practice, and on how it can be optimally controlled to ensure a long-term stable situation. A pilot was set up and conducted to determine the possibility of pumping up the iron-free groundwater from the cold sources of the ATES system. This was done by means of a temporary Fermanox installation, after which the groundwater was infiltrated for a lengthy period into the heat sources. The pilot clearly demonstrated that with subsurface iron removal, iron-free groundwater can without any problem be pumped up, with no deleterious effects on groundwater quality. On the basis of these results, the UZ Leuven decided to install a permanent subsurface iron removal system. The hospital’s ATES system might subsequently be equipped with more heat and cold sources.

Successful cleaning of backbone with Ice-Pigging

The revitalisation of the ATES system requires that the transport pipe, or ‘backbone’, be cleaned. During the conduct of the pilot, it was apparent that the backbone was fouled by past iron precipitation to the point where the use of simple water flushing was not enough to clean it.  After exploring the various backbone cleaning options, the TKI parties decided to use Ice-Pigging. This is an innovative cleaning technique in which ice slurry is pumped through a pipe, removing sediment and other undesirable fouling. It is estimated that the use of Ice-Pigging resulted in the removal of about 130 kg of sediment from the backbone. Following the Ice-Pigging cleaning, onsite turbidity and MFI measurements during the pipe’s water flushing showed that the water remained clean. Ice-Pigging is thus a suitable method to clean complex pipe networks of this kind.

Well-clogging risk map

A second objective of the project was to assess whether subsurface iron removal could contribute to the realisation of ATES systems in areas where they were usually not considered appropriate, because of the occurrence of chemical well clogging with iron. To this end, on the basis of groundwater quality data from Flemish monitoring networks, an initial risk map was made of the occurrence of iron and/or manganese oxidation in water abstracted from the phreatic aquifer. This mapping indicates that well clogging could occur in large areas of Flanders due to the mixing of nitrate-containing and ferrous groundwater. However, in these areas subsurface iron removal nonetheless offers the possibility of realising ATES systems.