The Shelbyville Water Resource Recovery Facility is designed to treat a flow of 8.0 MGD (Million Gallons Day) with a 16.0 MGD peak flow.
The wastewater enters the plant from two main pumping stations. One pump station handles the flow from the north side of the Big Blue River, including the Northridge Industrial Park. This lift station was relocated and constructed in 1988 and upgraded in 2001. Three smaller pump stations pump into it. The other pump station is Conrey Street Pump Station. All domestic and industrial wastewater from south of Big Blue River flow through this pumping station before entering the plant. Before entering this pump station, the wastewater may come from over seventy-five miles of sewers and five other smaller lift stations.
At the Conrey Pump Station, there is an inline double drum channel monster, which grinds waste material before it has a chance to get to the pumps. The wastewater is then pumped by four dry pit submersible Fairbanks-Morse pumps that move the wastewater through a 24-inch force main under the Big Blue River and into the treatment plant headworks.
As the wastewater enters the plant, it goes through a Parshall flume, which is used to measure the flow. It then enters two grit chambers, at a 60/40 split, which slows the flow down so that the sand, gravel, seeds, and heavy inorganic particles may settle out. The grit is then pumped by two Wemco Model C grit pumps to a Westech Grit Mitt classifier and stored in a dumpster until transported to the landfill.
The wastewater flows into three circular primary settling tanks, which slow the flow down even more. This is so we will have liquid and solid separation. The primary tanks are to remove floatables, such as grease and oil, and settleables, such as organic solids.
After the wastewater leaves the primary settling tanks, it enters the recirculation pump station and is pumped by four variable speed driven Fairbanks-Morse pumps up onto three high rate trickling filters. The filters are fourteen feet tall and are filled with plastic crossflow media.
From there, the wastewater flows into two aeration basins. After a single pass through the two aeration basins, which are supplied air by one or more of five Hoffman blowers, the water moves on to the three large secondary clarifiers. The sludge from these clarifiers are returned back to the aeration basins and mixed with the wastewater to allow solids contact within the aeration basins.
The water then flows through an UV tank for disinfection and flow measurement. The flow-measuring device is an ultrasonic metering element calibrated to a 36-inch Parshall flume. The treated flow is then discharged to Big Blue River.
The solids and floatables are separated and removed from the primary settling tanks. The solids are pumped from the bottom of the clarifiers into two primary digesters. The floatables are skimmed into collection boxes and removed via the Vactron machine to the scum drying beds for holding until transportation to the landfill.
The sludge that is pumped to two primary digesters is heated at approximately 95 degrees and mixed by two Perth gas mixers then through six gas bubble boxes at the bottom of each tank. The sludge is then transferred to the secondary digester where it is stored for 7–14 days for liquid solid separation. This digester is not heated or mixed. We let the natural decomposition occur, which causes more liquid-solid separation. The water off the secondary digester is put back through the treatment system after receiving treatment in a pre-aeration tank. The solids from the secondary digester is mixed with polymer and pressed between two belts on a belt filter press.
Our facility involves anaerobic digestion where, in the absence of oxygen, bacteria digest residual solids and create methane gas as a byproduct. We have installed a Dystor® gas holder system design that uses a dome-shaped, engineered membrane system to store methane gas, provide sludge storage, and prevent odors. We use this gas as a substitute for natural gas to heat our primary digesters.
If methane is released directly into the atmosphere, it is a potent greenhouse gas. In fact, its global-warming potential is 21 times greater than that of carbon dioxide. Using it to generate energy encourages more efficient collection and thereby reduces emissions into the atmosphere. For this reason, energy recovery from methane, where economically viable, is of considerable benefit to the environment.
The solids are dewatered using a belt filter press; this process squeezes the sludge between two porous belts that thickens the sludge by removing a majority of the water. The solids enter the belt press at around 4% to 6% solids and leave at 24% to 27% solids. The solids are then stored on-site and land applied to farm ground.