Suspension mixed lagoons flow through activated sludge systems where the effluent has the same composition as the mixed liquor in the lagoon. Typically the sludge will have a residence time or sludge age of 1 to 5 days. This means that the COD removed is relatively little and the effluent is therefore unacceptable for discharge into receiving waters.[4] The objective of the lagoon is therefore to act as a biologically assisted flocculator which