9. Complementary and tertiary treatments

Tertiary treatments are complementary treatments carried out after primary physico-chemical treatment and secondary biological treatment of wastewater.

After secondary treatment, pollutants may remain that require more advanced treatment methods to be removed or reduced to acceptable levels. Tertiary treatments can include disinfection processes (for example, with chlorine or UV), nutrient removal (nitrification and denitrification), advanced filtration (such as microfiltration, ultrafiltration, sand or activated carbon), and adsorption or advanced treatment technologies (such as ozonation or advanced chemical oxidants).

The aim of tertiary treatment is to "polish" the treated water to meet the most stringent discharge or reuse criteria, and this may include the reduction of persistent substances, heavy metals, micropollutants or residual...