The aim of this work is to quantify the relative contribution to the overall environmental impact of the construction phase compared to the operational phase for a large conventional activated sludge wastewater treatment plant (WWTP). To estimate these environmental impacts, a systematic procedure was designed to obtain the detailed Life Cycle Inventories (LCI) for civil works and equipment, taking as starting point the construction project budget and the list of equipment installed at the Girona WWTP, which are the most reliable information sources of materials and resources used during the construction phase. A detailed inventory is conducted by including 45 materials for civil works and 1,240 devices for the equipment. For most of the impact categories and different life spans of the WWTP, the contribution of the construction phase to the overall burden is higher than 5% and, especially for metal depletion, the impact of construction reaches 63%. When comparing to the WWTP inventories available in Ecoinvent the share of construction obtained in this work is about 3 times smaller for climate change and twice higher for metal depletion. Concrete and reinforcing steel are the materials with the highest contribution to the civil works phase and motors, pumps and mobile and transport equipment are also key equipment to consider during life cycle inventories of WWTPs. Additional robust inventories for similar WWTP can leverage this work by applying the factors (kg of materials and energy per m 3 of treated water) and guidance provided

The authors would like to thank Trargisa S.A. for providing the data and feedback on the WWTP of Girona, Voltes S.L.U. for the technical support, Banc BEDEC, the Ministry of Economy and competitiveness for the Ramon and Cajal grant from Lluís Corominas (RYC-2013-14595) and the REaCH project (CTM2015-66892-R). Serni Morera’s FI scholarship (2015FI_B2 00071), STSMscholarship from EU-Cost Action Water2020 (ECOST-STSM ES1202-110614-044095). LEQUIA and ICRA were recognized as consolidated research groups by the Catalan Government with codes 2014-SGR-1168 and 2014-SGR-291,respectively