The Green Bay Facility was built in 1975. It was the first wastewater treatment facility in the country to treat both municipal and paper mill waste.
The Green Bay Facility is located on the mouth of the Fox River. On average the facility receives about 30 million gallons per day of wastewater from 18 surrounding communities.
Domestic wastewater arrives at the facility near the bottom of the Pump Station through a large (108") pipe called an interceptor. The domestic wastewater enters the treatment where we remove objects that are bigger than two and one-eighth inch in size, through one of two course bar screens. The Pump Station also contains four large pumps, which lifts the domestic wastewater to above ground level to the next step in the treatment process.
There are three Mill pumps within the Pump Station that handle wastewater from two nearby paper mills. The wastewater arrives through a separate interceptor (42 " pipe). The "Mill Waste" is pumped to the next step in the process and is then combined with the domestic wastewater.
Headworks is often considered the beginning of the treatment process at the Green Bay Facility. Within, step screens provide preliminary treatment, and degritting equipment separates and removes inorganic material (sand-like substances) from the waste streams. Incoming wastewater passes through two fine screen step screens, which trap debris. Once trapped, the solid objects are removed, then washed and compacted before being emptied into a bucket on the ground floor for removal to a landfill.
Once the wastewater has been screened in headworks the wastewater flows into the primary clarifiers. The wastewater is slowed and allows the solids that have been suspended in the water to settle out. The floating material (scum and grease) will float to the top and an arm skimmer circles the clarifier, collecting the material for further processing. Nearly 60% of the solids are removed before it moves on to the Aeration Basins.
In this step of the process, sometimes referred to as "Secondary" or "Biological" treatment, bacteria and micro-organisms growing in the aeration basins remove organic matter by consuming it as food.
The aeration systems utilize a process commonly known as the activated sludge process. The effluent from the primary clarifiers flow into any one of the six aeration basins. We provide both food (organic material) and air for the microorganisms to clean the wastewater.
The air is supplied from a compressor through
diffusers that lay near the bottom of the basin. Each basin contains 5500 diffusers.
The activated sludge process currently utilized by GBMSD is commonly known in the wastewater field as "single stage nitrification/denitrification with biological phosphorus removal". To properly operate the aeration systems, the operators at GBMSD must insure sufficient numbers of microorganisms are present to completely breakdown the influent waste, ammonia, and phosphorus.
After the wastewater leaves the aeration basins, it enters the final clarifiers where any solid material is settled out to the bottom of the basins.
The solids that settle out are either sent back to the aeration basins (return activated sludge) or are "wasted" to the Gravity Belt Thickeners (waste activated sludge).
The water that flows over the weirs is sent onto the disinfection process for the final step in the treatment process.
This is the final step of the liquids treatment process. As the wastewater prepares to leave the facility, a chemical is injected into the water called sodium hypochlorite (similar to household bleach) through a mixer at the very beginning of the disinfection chamber. The mixer is used to help distribute the chemical evenly in the water.
Next the water must flow through a series of four "passes" or channels to make sure that there has been sufficient time and contact to kill disease-causing bacteria. At the end of the last pass, another chemical is added. This chemical is sodium bisulfite. Sodium bisulfite is used to remove (neutralize) any residual chlorine.
The effluent chlorine residual is monitored and controlled through our computer control system. This system measures the residual chlorine just prior to the injection of sodium bisulfite. The equipment used to monitor the residual controls the amount of sodium bisulfite used.
The treatment plant operators conduct tests to verify that the residual chlorine has been removed from the effluent.
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.
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.