How The Treatment Process Works
Wastewater collected in the Little Blue River drainage basin flows by gravity through a series of gravity sewers ranging in size from 24 to 120 inch in diameter to the Atherton Wastewater Treatment Plant located approximately 2 miles south of the Missouri River. The plant is designed for an average dry weather flow of 52 million gallons per day (mgd). Wastewater enters the raw wastewater pump station through trash rakes at elevation 683.0 and is pumped to the Headworks Building to elevation 735.0, a height of 52 feet. The pump station houses 8 pumps ranging in size from 35 to 80 mgd capacity. The station has a firm pumping capacity of 420 mgd. In the Headworks Building, the wastewater is screened through three mechanical screens with ¼ - ½ inch openings. The fine screens remove material such as rocks, rags, “flushable” wipes, sticks, and undissolvable material that could harm downstream processes and equipment. Flow from the preliminary treatment process then flows by gravity to four primary clarifiers.
Primary clarifiers settle out solids material after approximately 3 hours detention at average design flow. Solids removed in the clarifiers account for approximately 35% of the strength of the wastewater and 60% of the suspended solids. The solids contain grit which is too small to be removed with the bar screens, therefore the solids are pumped to cyclonic degriters where the solids and grit are separated. The grit is collected and hauled to a landfill, while the solids containing human waste is pumped to the solids treatment process.
After primary clarification, the wastewater flows to the secondary treatment process consisting of two aeration basins and five final clarifiers. The aeration process is designed for approximately 5.5 hours detention and provides biological treatment, where the organic material in the wastewater is used by bacteria for food, and converted to a more stable form. The stabilized wastewater flows to five final clarifiers for removal of the stabilized solids and bacteria. The stabilized solids and bacteria removed in the final clarifiers are either returned to the aeration basins for seed or wasted to the solids treatment process. The effluent from the final clarifiers is either pumped to Ultraviolet (UV) Disinfection or sent to the Missouri River depending upon seasonal treatment requirements.
In the UV Disinfection process, the treated wastewater is exposed to high doses of the germicidal wavelengths of UV light, causing bacteria, viruses and protozoa to be rendered incapable of reproducing and infecting. This disinfected wastewater is then sent to the Missouri River.
The solids treatment process treats the degrited solids from the primary clarifiers and the waste sludge from the secondary treatment process. The primary solids are allowed to settle in gravity thickeners while the waste secondary solids are thickened on gravity belt thickeners after coagulating chemicals are added to enhance water/solids separation. Both gravity thickened primary solids and chemically thickened secondary solids are mixed together and further dewatered using a centrifuge or belt filter presses. The dewatered solids are pumped to an incinerator for burning and the ash remaining after burning is recycled as an agricultural fertilizer or landfilled.
Jeff Shook, Executive Director
Little Blue Valley Sewer District