On a typical day, more than three million gallons of wastewater are transported to the wastewater treatment plant through nearly 138 miles of collection system pipes. During heavy rain events or snow melt, the facilities can treat up to a rate of 28 million gallons per day (MGD). Following are brief descriptions of the main treatment processes.
When the wastewater reaches the plant, it is raised 30 feet by three, 15 million gallons per day (MGD) screw pumps. Raising the wastewater allows it to travel by gravity through the rest of the treatment processes. The influent pumping station is designed to accommodate a fourth pump in the future.
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 they trap debris on metal bars 1/8 inch apart and do not allow them to be carried any further by the water. With these fine screens, more inorganic solids are removed at this process so they need not be re-handled in later processes. Debris is compressed to dry and is then automatically bagged and land filled.
From the fine screens, wastewater travels to a diversion/splitter box where it is conveyed to an oxidation ditch. In the oxidation ditch, air is introduced into the water by 125 hp aerators at both ends 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 microorganisms remain in suspension and continue to consume organics in the wastewater for a designated period of time.
High flows caused by groundwater or rain water leaking into the system are handled well with the design of the long oxidation ditches and the reserve capacity built into the facility. In 2016 the plant was switched over to biological phosphorus removal. Here the natural bacteria are conditioned to take out more of the phosphorus and nitrogen, without the use of chemicals. Besides the cost savings, the plant effluent quality has improved and there is less impact on Mill Creek and the waters downstream.
After being treated in the oxidation ditch, the wastewater flows to clarifier tanks. In the clarifiers, the microorganisms in the wastewater are allowed to be taken out of suspension and they then settle to the bottom of the tank. From here, they are pumped back to the oxidation ditches to mix with the incoming wastewater and continue their work to remove organic material. Periodically, a small portion of this return flow is pumped to the gravity belt thickener for thickening.
The gravity belt thickener removes water from the return flow microorganism solids coming from the final clarifiers. The liquid removed is then sent back to the oxidation ditch for further treatment.
The thickened solids from the gravity belt are called biosolids. These biosolids are stored in two storage tanks with a combined capacity of more than 4 million gallons. Biosolids from these tanks are pumped to tanker trucks and hauled away to be land spread on farm fields during spring and fall.
Treated water from the clarifiers flows through a flume and over a cascade aerator where it aerates the water. It then travels along a 1/4 mile vegetated swale to Mill Creek. The vegetated swale has been carefully designed and constructed to blend in with the natural surroundings. Approximately 15 species of native grasses were planted along critical areas of the swale side slopes for stabilization. Another 8 species of aquatic plants were planted along the swale bottom. Riprap has been included to prevent erosion of the stream bank and stones have been placed throughout the swale length to create a more natural environment of aquatic life.