Final settling tanks follow the aeration tanks and are the last step in the activated sludge process. Final settling at Norwalk WPCF occurs in large circular settling tanks that allow the aerated solids from the secondary treatment process to separate from the water. A portion of the solids are returned to the secondary treatment system as seed bugs (known as “Return Activated Sludge” or RAS), the rest is removed (known as “Waste Activated Sludge” or WAS). Clean water exits the tanks for further treatment.