Step 3: Primary Settling
The
material, which will settle, but at a slower rate than step two, is
taken out using large circular tanks called clarifiers. The settled
material, called primary sludge, is pumped off the bottom and the
wastewater exits the tank from the top. Floating debris such as grease
is skimmed off the top and sent with the settled material to digesters.
In this step, chemicals are also added to remove phosphorus.