General equation for the water footprint calculation of a WWTP.
2.1. Blue water footprint (WFblue)
In WWTPs, the blue water footprint accounts for the water that evaporates during wastewater treatment and the water used for all processes related to the different WWTP unit operations (chemicals, energy consumption, residue management, transportation and sludge treatment) that is incorporated into the final product. For example, the consumption of chemicals and energy has an associated blue water footprint due to the water incorporated during the production of chemicals and energy. However, the lost return flow, which is considered in the blue water footprint, of other processes or products will be zero when the treated WWTP water is discharged into the same catchment. In certain cases, it can be interesting to consider the route of blue water, particularly in processes or products from agriculture (distinction of the water based on if it comes from the surface, groundwater or another source). Water recycled back to the process or used for other applications (e.g., WWTPs that have tertiary treatment and produce reclaimed water) should also be accounted (as avoided water) because it reduces the blue water footprint.
2.2. Green water footprint (WFgreen)
In conventional WWTPs, the green WF is not considered because it does not promote the evaporation of water from the soil or from vegetables and does not promote the incorporation of soil water with treated water.
2.3. Grey water footprint (WFgrey)
The proposed calculation for the grey water footprint in the WFA manual (Hoekstra et al., 2011) has been adapted to the specific domain of WWTPs. The new equation is based on a mass balance at the WWTP discharge point (see Equations (2) and (3) and Fig. 2). This mass balance-based approach considers that the grey WF is the minimum volume of water required to dilute the pollutant concentration from the WWTP effluent concentration to the maximum pollutant concentration allowed in the river.