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News Title Wastewater Treatment Process

The facility begins treatment by automatic screening and grit removal. Wastewater flowing into the treatment plant will occasionally contain pieces of wood, roots, rags, and other debris. To protect equipment and reduce interference with in-plant flow, the debris and trash are removed by a bar screen. The bar screen consists of parallel bars placed at an angle in the influent channel. Trash collects on the bars and is automatically removed by mechanical means. Grit is the heavy material present in wastewater, such as sand, gravel, cinders, eggshells, etc. The grit collects in the hopper of the aerated grit chamber where it is air lifted to a grit washer. An auger transports the washed grit to a dumpster for disposal at the landfill.

The next step in the treatment process is sedimentation or primary treatment. In this process the wastewater flows into two large tanks called primary clarifiers. Here the flow velocity is reduced to allow settleable solids to fall to the bottom of the tank and lighter material to float to the surface. The solids that collect at the bottom of the tank, called sludge, are pumped to the anaerobic digester for stabilization and then applied to farmland. The floatable material is skimmed from the surface and is also pumped to the digester.

Following primary clarification the wastewater is piped to the oxidation ditch. Rotary brushes in the oxidation ditch supply mixing and oxygen to the wastewater. Continuous mixing and aeration of wastewater produces a microbial mass called activated sludge. These microorganisms are able to metabolize contaminates in the wastewater.

The effluent from the oxidation ditch, called mixed liquor, flows to two large secondary clarifiers. Here, the flow velocity is reduced allowing the mixed liquor to settle and form return activated sludge (RAS). The RAS is continuously pumped back into the oxidation ditch to metabolize more contaminates. In order to maintain a proper balance of microorganisms, it is necessary to remove a portion of the RAS. This is referred to as waste activated sludge (WAS). The WAS is dewatered by means of a filter belt press and disposed of at the landfill.

The clear effluent from the secondary clarifiers passes through sand media filters to remove the remaining suspended matter in the treated wastewater. After filtration, chlorine is added to the treated wastewater to kill pathogenic (disease-causing) bacteria. Sulfur dioxide is then added to remove the chlorine and the treated wastewater is discharged to the receiving stream.