A photo of gravity thickener is shown below. Gravity thickeners used dewatering the treatment sludge are designed on the basis of solids loading. Typical concentrations of unthickened and thickened sludges and solids loadings for gravity thickener are shown below. For operation, alternatively, sludge - blanket depth should be measured. Dissolved - air flotation is being widely preferred for especially waste activated sludge.
Air is introduced into a solution that is being held at an elevated pressure.
When the solution is depressurized, the dissolved air is released as finely divided bubbles carrying the sludge to the top, where it is removed. Photos of DAF are shown below. Higher loadings can be used with flotation thickeners than are permissible with gravity thickeners because of the rapid separation of solids from the wastewater. Typical solids loadings for DAF units are shown below.