Abstract
Commercially available heat transfer fluids used in borehole heat exchangers were investigated for their composition, their biodegradability as well as their ecotoxicity. The main components of the fluids are organic compounds (often glycols) for freezing protection. Biodegradation of the fluids in laboratory studies caused high oxygen depletion as well as nitrate/iron(III) reduction under anaerobic conditions. Additives such as benzotriazoles for corrosion protection were persistent. Ecotoxicity data show that the commercially available fluids caused much higher ecotoxicity than their main organic constituents. Consequently, with regard to groundwater protection pure water as heat transfer medium is recommended. The second best choice is the usage of glycols without any additives. Effects on groundwater quality should be considered during ecological-economical cost-benefit-analyses of further geothermal energy strategies. The protection of groundwater as the most important drinking water resource must take priority over the energy gain from aquifers.
Zusammenfassung
Handelsübliche Geothermie-Wärmeträgerfluide wurden anhand ihrer chemischen Zusammensetzung, ihrer biologischen Abbaubarkeit sowie ihrer Ökotoxizität im Hinblick auf eine mögliche Grundwassergefährdung beurteilt. Hauptbestandteil der Fluide sind organische Substanzen wie z. B. Glykole als Frostschutzmittel. Der biologische Abbau der organischen Bestandteile der Fluide führte in Laborversuchen zu einer hohen Sauerstoff-Zehrung sowie unter anaeroben Bedingungen zu Nitrat-/Eisen-Reduktion. In den Fluiden enthaltene Additive wie Benzotriazole waren persistent. Die Ökotoxizität der Formulierungen im Leuchtbakterienhemmtest und im Fischeitest war gegenüber den organischen Grundsubstanzen deutlich erhöht. Aus Sicht des vorsorgenden Grundwasserschutzes ist die Verwendung von Wasser als Wämeträgerfluid zu empfehlen. Die nächstbeste Alternative ist der Einsatz reiner Glykole ohne Additive. Bei der weiteren Nutzung geothermischer Energie sollten mögliche Auswirkungen auf die Grundwasserqualität in die ökologisch-ökonomische Kosten-Nutzen-Analyse sowie in die behördlichen Entscheidungen über die Zulassung einer Erdwärmenutzung einbezogen werden. Die Trinkwasserversorgung muss immer Vorrang vor der Nutzung des Grundwassers als Energiequelle haben.
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Acknowledgments
We gratefully acknowledge financial support from the DVGW (Deutscher Verein des Gas- und Wasserfaches e.V., German Technical and Scientific Association for Gas and Water) and the Innovationsfonds Klima- und Wasserschutz of the badenova AG & Co. KG.
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An erratum to this article is available at http://dx.doi.org/10.1007/s00767-017-0360-5.
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Schmidt, K.R., Körner, B., Sacher, F. et al. Biodegradability and ecotoxicity of commercially available geothermal heat transfer fluids. Grundwasser 21, 59–67 (2016). https://doi.org/10.1007/s00767-015-0311-y
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DOI: https://doi.org/10.1007/s00767-015-0311-y