Urban waste water treatment in European "Big Cities" in 2016

Figure 3 shows the proportion of urban waste water collected, and the level of treatment applied, as a percentage of the population equivalent in cities reported to have more than 150 000 inhabitants. Sewage not collected nor treated is shown in red; sewage treated in individual appropriate systems is shown in dark grey; sewage collected but not treated is orange; primary treatment, such as settling is yellow; secondary treatment to reduce dissolved and suspended organic compounds, such as those using biological methods, is shown in light green; more stringent 'tertiary' treatment to reduce phosphorus and/or nitrogen is dark green; and other more stringent treatment (e.g. disinfection) is dark blue.