Green Bay Metropolitan Sewerage District

Liquids Processing

		Pump Station As the wastewater enters the facility, it passes through fine screening devices that are able to remove trash, rags, and other debris at least a 1/4" in size. The material collected is washed, compacted, and sent to a landfill for disposal.

		After the wastewater passes through the fine screening devices, the water is conveyed to the next treatment point by large centrifugal pumps. The wastewater is forced under pressure to the Preliminary Treatment Units for further processing.

		Preliminary Treatment Units (PTU) As wastewater flows through the preliminary treatment Units, the flow speed is slowed to one foot per second, allowing sand and other coarse material to settle to the bottom. The settled material is removed, washed, and disposed of in a landfill. The oil and grease that rises to the surface is skimmed off the top of the tank for further processing.

		Aeration Tanks After the wastewater passes through the PTUs, it flows through an anoxic zone for biological phosphorus removal. In this phase, the bacteria will consume the phosphorus before the wastewater enters the aeration tanks.

		From the anoxic zone, the wastewater moves to the aeration tanks where an abundant supply of oxygen is added through fine bubble diffusers. Now, the bacteria have an ideal environment to multiply and consume the organic material in the wastewater. The wastewater will remain in the aeration tanks for 6 to 8 hours.

		Intermediate Clarifiers Following aeration, the wastewater is again slowed to allow the bacteria to settle to the bottom and be captured and pumped back to the aeration basins to be reused (return activated sludge) or sent on to be either incinerated or landfilled (waste activated sludge). The De Pere Facility operates two intermediate clarifiers.

		Final Clarifiers The wastewater from the intermediate clarifiers flows to the final clarifiers where further settling occurs. At this point, about 85% to 90% of the pollutants are removed. The De Pere Facility has three final clarifiers, however only one is needed for operation at a time.

		Gravity Filter The wastewater from the final clarifiers goes through a gravity filter process where it passes through a filtering media, capturing the remaining suspended solids.

		Disinfection Before entering the environment, the wastewater goes through a disinfection process to kill remaining bacteria. The De Pere facility uses an ultraviolet light disinfection system that is capable of treating 15 mgd. The liquid chlorine treatment process that was used prior to 1997, is still retained and used during periods of high flow.

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