Methodology for water footprint assessment in WWTPs
To evaluate the water footprint of products and consumers, the Water Footprint Network (WFN) developed a methodology for water footprint assessment (WFA) to evaluate the impacts on water consumption caused by an activity (Hoekstra et al., 2011). The WFA methodology addresses freshwater resources appropriation using a four-step approach: (i) set the goals and scope; (ii) account for the water footprint of a process, product, producer or consumer as a
spatiotemporally explicit indicator of freshwater appropriation;
(iii) evaluate the sustainability of this water footprint and focus on a multi-faceted analysis of the environmental, economic and social aspects; and (iv) formulate strategies to improve the water footprint.
This section introduces the adoption of the WFN methodology for WWTP application and expands the WF accounting phase using a framework for the grey water footprint calculation. As shown in Fig. 1, the methodology consists of four phases, which is similar to those in an LCA analysis.
The first phase consists of defining the goal and scope of the assessment and includes the functional unit, the types of WF to be considered and the data sample. In the second phase, data are collected, and the water footprint is calculated. In the third phase, the water footprint is evaluated from a sustainability point of view, which considers the water availability in the analyzed region or period, and finally in the fourth phase, several recommendations are drawn to reduce the water footprint of the product or system analyzed.
The general equation to calculate the water footprint of a WWTP, which is the volume of water consumed during a period of time and includes the blue (WFblue), green (WFgreen) and grey (WFgrey) water footprints, is defined as the following: