A portion of the RAS is wasted each day to keep the activated sludge levels at the proper level. This sludge, known as waste activated sludge, is collected in the gravity thickener which provides additional time for the solids to concentrate and separate from the clear water. The clear water flows over the weirs at the tank periphery and back to the wet well as recycled water. A scrapper arm collects concentrated sludge at the bottom of the tank and directs it to a draw-off hopper. A pump located in the pump room of the Blower Building pumps the sludge from the gravity thickener to either the aerobic digester ( the outer ring around the gravity thickener) for further stabilization, or a sludge holding tank in the Sludge Handling Building for pressing.
The aerobic digester is equipped with course bubble diffusion equipment for further sludge stabilization. A second pump conveys sludge from the aerobic digester to the sludge holding tank in the Sludge Handling Building.
In the Sludge Handling Building, sludge is dewatered using a belt filter press. After this process, sludge, although still wet, is in a cake like state and can be handled as a solid. A truck transports the sludge cake to the Sludge Storage Building for storage. The sludge is stored on a series on concrete beds with narrow drainage slots for drainage of water. Lastly, an outside disposal service incorporates the sludge into a DNR approved farm field where it is used as a fertilizer and soil conditioner.