There are two broad categories of wastewater treatment, the liquids and solids processes. The liquids process refers to the water portion of wastewater. Annually, the Appleton Wastewater Treatment Plant will treat over five billion gallons of wastewater. The solids process refers to waste (solids) removal and reduction. The end product of this process is a material called biosolids. Annually, the Appleton Wastewater Treatment Plant will recycle over 20,000 tons of biosolids to the agricultural community.
The Egg-Shaped Digesters of the Appleton Wastewater Treatment Plant
Wastewater is transported from homes and industry by a network of sewer pipes that is over three hundred miles in length. The wastewater can contain objects and debris that can damage a wastewater treatment plant’s equipment. Preliminary treatment processes are those that ‘get the wastewater ready’ for the downstream processes.
Bar Screens and Grinder
Wastewater enters either a bar screen or grinder. If the bar screen is on line it removes anything larger than three-quarters of an inch. If the grinder is in use it will grind any solid material entering the plant. Either of these processes ensures that large material won’t block pipes or jam equipment.
Bar Screen and Conveyor System
The next step in preliminary treatment is to remove grit. Grit contains material like sand which is abrasive to equipment and must be removed. This is accomplished by sending the wastewater to one of two grit vortex basins. Here the centrifugal force of the water pushes the heavy grit-like material outward, toward the wall of the basin. This material (sand, soil, etc.) then collects at the bottom of the basin and is removed. After grit has been removed the wastewater flow will enter the primary treatment process.
Secondary treatment is a biological process that utilizes thousands of pounds of microorganisms or microbes which, individually, are microscopic in size. This process is broken into two parts, aeration and final clarification. The aeration part allows for the microbes to first metabolize (eat) the waste (food). The final clarification part allows for removal of the microorganisms from the now treated water. The following describes how the two parts of this process work and how nutrients are removed.
The wastewater flow is mixed with the microbe population and then aerated. This is the ideal environment for the microbes to metabolize the waste in the wastewater. The mixture is aerated which means air is introduced into the bottoms of the tanks. Aerating the tanks keeps dissolved oxygen levels in the water high enough for the organisms to respirate (grow). With the use of a microscope, microbes are ‘checked on’ every day by plant operators. By controlling the microbe's environment in the aeration tanks the operators can select what type of microbes will best survive and be most productive.
The liquid mixture leaving the aeration tank enters the secondary clarifiers. It is in these large tanks that the microorganisms are separated from the treated water. The low velocities in these tanks allow for the microorganisms to cling together and then settle to the tank floor. The settled microorganisms are vacuumed off the tank bottom and pumped back to the aeration process where they will start the aeration/secondary clarification processes all over again. The clear water, or effluent, flows into the chlorine contact tank for disinfection.
Sodium hypochlorite is added in order to produce an effluent that essentially free of disease-causing organisms. This is accomplished in the chlorine contact tank that provides the necessary mixing through tank baffling.
Since chlorine compounds may have a toxic effect on aquatic life in the Fox River, Sodium bisulfite is added to neutralize the chlorine. The chemical reaction that results produces effluent that is free of harmful compounds. The Sodium bisulfite is added just prior to the effluent’s discharge into the Fox River.
Dissolved Air Flotation (DAF)
During the process of secondary treatment a portion of the microbes in the aeration tank must be removed in order to maintain that process’ effectiveness. The microbes that are removed are called waste activated sludge. This sludge is sent to the DAF. The purpose of the DAF is to dewater this waste activated sludge which will reduce the volume of sludge to other solids processes. DAF is accomplished by injecting air into a pressure vessel that contains water. After the water leaves the pressure vessel it enters the DAF tank and millions of tiny air bubbles come out of solution. These bubbles will cling to the microorganisms and ‘float’ them. The water remaining in the bottom of the tank is sent back to be processed in the plant.
Receiving Station Waste
The plant also accepts high strength liquid waste from industries that are outside of Appleton's sewage district. These wastes are prequalified and permits are issued by plant staff. This station allows the plant to generate revenue which reduces sewer user fees. Wastes of this type would present problems for our plant’s liquid processes, however, they are compatible with the anaerobic digester process. The liquid waste is pumped from the receiving station to the anaerobic digesters for treatment
One of the most interesting and visible characteristics of the plant is the presence of the egg-shaped anaerobic digesters. Anaerobic digestion is another biological process that utilizes microbes. These organisms stabilize the sludges from the DAF and primary treatment. Primary and waste activated sludges are pumped to a blending tank where they are heated. From the blending tank the blended sludge is pumped into the anaerobic digesters. These large egg-shaped digesters are maintained at a temperature of 95 degrees Fahrenheit and are void of oxygen. The products of anaerobic digestion are a reduced sludge volume, carbon dioxide, methane gas and water. The methane gas can be used for heating the sludge or other heating needs. The discharged sludge from the anaerobic digesters is then held in sludge blending tanks.
The belt filter press operation is a dewatering process that will reduce the volume of the sludge that will eventually be hauled from the plant and applied to farmers fields. In this operation anaerobically digested sludge from the storage tanks is pumped to the belt filter presses. Here the sludge is passed in between two porous belts. The water passes through the belts while the sludge is held between them. Additional water is removed by applying pressure to the belts.
Biosolids Storage Facility