Treatment Process

On a typical day, more than 3 million gallons of wastewater are transported to the wastewater treatment plant through the city's nearly 80 miles of sewer system. During storm events or heavy snow melt, the facilities can treat up to 22 million gallons per day. Following are brief descriptions of the main treatment processes.

dix-treat-1.jpg When the wastewater reaches the plant, it is raised 40 feet by two, 18 mgd screw pumps. The screw pumps are 75" long and 9" in diameter. They are the longest screw pumps in North America. Raising the wastewater allows it to travel by gravity through the rest of the plant. The influent pumping station is designed to accommodate a thrid pump in the future.
dix-treat-2.jpg Wastewater flows from the top of the screw pumps to the "fine" step screens where inorganic solids are removed. These screens are unique in that the opening between the bars on the screens is only 1/4 inch. With these fine screens, more inorganics are removed at this early stage in the treatment process so they need not be rehandled in later processes. Debris is compresssed to dry and is then automatically bagged and landfilled.

From the fine screens, wastewater travels to a diversion/splitter box where it is conveyed to oxidation ditches. In the oxidation ditches, the wastewater has air added to it by 150 horsepower aerators at the end of each tank. Air is introduced to sustain the microorganisms in the wastewater that consume organics. The aerators at the ends of the tank also keep the wastewater in the tank flowing, so the microorganisms remain in suspension and continue to consume organics in the wastewater for a designated amount of time.

dix-treatment-domes-thumb.jpg After the oxidation ditches, the wastewater flows to one of the two final clarifiers. In the clarifier, the microorganisms in the wastewater are allowed to settle out of suspension and go to the bottom of the tank. Then they are pumped back to the oxidation ditches to mix wth the incoming wastewater and continue to remove organic material.
dix-treat-4.jpg Periodically, a small portion of return flow is pumped to the gravity belt thickener for thickening. The gravity belt thickener removes water from the return flow microorganisms from the final clarifiers. The liquid removed is then sent back to the oxidation ditches for further treatment.
dix-treat-5.jpg The thickened solids from the gravity belt thickener are called biosolids. These biosolids are stored in three storage tanks with a combined capacity of more than one million gallons. Biosolids from these tanks are pumped to the beltfilter presses to be dryed and hauled to the Lee County Landfill and used for daily cover.
dix-treat-6.jpg The City of Dixon is required to disinfect the effluent water leaving the plant between May and October 31. This is done by ultraviolet light, when the effluent water passes by the light it inactivates any disease causing bacteria in the water. Treated water from the clarifiers flows through a flume and over a cascade aerator allowing for further aeration of the water. The effluent water flows into the Rock River.


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