Gravity Thickening

There are four gravity thickener basins. The basins receive sludge and scum (stuff floating on top of the water in each basin) flow from the four primary clarifiers along with the scum from the ten final clarifiers.

The gravity thickeners provide a quiescent condition where the primary sludge can settle to the bottom of the basin further thickening the sludge prior to pumping to the sludge holding tanks (SHT) in the solids processing building. The quiescent condition in the gravity thickeners also allows the scum and grease to float to the surface of the basin.

A rotating collector mechanism scrapes the bottom sludge toward a center sump for removal and skims the floating material into a scum manhole. Scum pumps transfer the scum from the manholes to the scum concentrator in the solids building. This scum is then further thickened and then pumped to the Incinerator as a fuel source.

Gravity Belt Thickeners

The Gravity Belt Thickeners (GBT) are designed to thicken the waste activated sludge which comes from the final clarifiers. To aid in de-watering the sludge, a chemical called "polymer" is added to the sludge at the front of the GBT. The sludge and the polymer become bonded to each other thus causing "flocculation," where the solids start to separate from the liquid. The sludge and polymer flow onto a moving belt where the water and sludge start to separate. The water falls through the porous weave of the belt leaving the sludge behind. The water drains to a pan below the belt and is pumped back to the beginning of the treatment process. The sludge moves with the belt and is turned continuously by plow-like devices called chicanes. The chicanes move the sludge back and forth allowing the water to fall through the belt to the drain. Near the end of the belt a ramp contacts the belt at its leading edge, forcing the sludge to go up and over the ramp. This action causes the sludge to roll back on itself. This rolling motion cases further dewatering of the sludge. The ramp is adjustable to further enhance the thickening of the sludge. The thickened sludge is sent next to the Belt Filter Presses.

Belt Filter Presses

The solid materials (thickened Primary and Waste Activated Sludges, and Septage) that are removed from the wastewater and collected throughout the treatment process need to be dewatered, or thickened, prior to their disposal.

The sludges are combined in Sludge Holding Tanks and mixed to keep them well blended. The blended sludges are conditioned with polymer before being fed onto the Belt Filter Presses (BFP). Only two of the four BFPs are in operation at a time, with the remaining two in stand-by.

The sludge is moved along the BFPs on a porous cloth belt, which allows the water to drain through. The solids remain on top of the belt, quickly concentrating as the water drains away. The BFPs have a second belt at the end of the first where the solids are squeezed between the two. The belts move through a series of rollers applying more than 500 pounds of pressure, to squeeze out as much water as possible. The solids coming off of the BFPs are referred to as dewatered "cake" and are conveyed to the incinerators.

Incineration

Prior to disposal in local sanitary landfills, the dewatered belt filter press sludge is burned to an ash within multiple hearth incinerators. Burning the sludge cake reduces the weight and volume, resulting in less expensive landfill hauling fees.

Incineration at the Green Bay Facility takes place on a nearly continuous basis. Typically, one incinerator operates for a three-month period, while the other unit receives any required maintenance. Then it sits idle until the rotation occurs once again. Control and monitoring of the incineration systems takes place from within the solids building control room.