A tertiary treatment is sometimes necessary donwstream to comply with the most stringent parameters (suspendid solids,  COD, BOD, nitrogen,  total phosphorus) and eliminate micropollutants and endocrine disruptors. This is achieved by physical and/or physical-chemical processes. This step also aims to achieve water quality to optimize downstream disinfection (chlorination, UV), when it is needed.
This step can generate "tertiary" sludge.