The first part of the paper analyzes the degradation of the water of a river in terms of its exergy while passing through the urban zone of a city. This analysis describes and compares the degradation process of the river from point to point, so it can be used to unify all the water quality measurements into the same exergy units [kJ/kg]. The second part introduces an exergy efficiency for a wastewater treatment plant that proposes to unify the means to measure the efficiency of the water treatment plants and their sub-systems. This particular case of study concerns the Lerma River in Mexico, which runs through Salamanca City, by determining its exergy at five sampling points.