Wastewater treatment plants (WWTPs) play an important role within the urbanwater cycle in protecting receiving waters from untreated discharges. However, WWTPs processes also affect the environment. Life cycle assessment has traditionally been used to assess the impact of direct discharges from WWTPs and indirect emissions related to energy or chemical production. The water footprint (WF) can provide complementary information to evaluate the impact of a WWTP regarding the use of freshwater. This paper presents the adoption of the Water Footprint Assessment methodology to assess the consumption of water resources in WWTPs by considering both blue and grey WFs. The usefulness of the proposed methodology in assessing the environmental impact and the benefits from WWTP discharge to a river is illustrated with an actual WWTP, which treats 4,000 m3$d 1, using three scenarios: no treatment, secondary treatment and phosphorus removal. A reduction of the water footprint by 51.5% and 72.4% was achieved using secondary treatment and chemical phosphorus removal, respectively, to fulfill the legal limits. These results indicate that when treating wastewater, there is a large decrease in the grey water footprint compared with the no-treatment scenario; however, there is a small blue water footprint

The authors would like to thank the Spanish Ministry of Science and Innovation (PCIN-2013-074) and the European Union (Marie Curie Career Integration Grant PCIG9-GA-2011-293535 and SANITAS ITN Project agreement 289193). Serni Morera was awarded an FI scholarship from the Catalan Government (2015FI_B2 00071). Lluís Corominas received the Career Integration Grant (PCIG9-GA-2011-293535) from the EU and the Ramon y Cajal Grant (RYC-2013-14595) from Spanish Government. LEQUIA has been recognized as a consolidated research group by the Catalan Government with code2014-SGR-1168. The authors also acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group (2014-SGR-291) e Catalan Institute for Water Research