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 from the Middle Big Creek drainage basin, a sub-district of the Little
Blue Valley Sewer District, is pumped to the Little Blue River drainage basin through
a series of three pumping stations. Wastewater enters the raw wastewater pump station
through trash rakes at elevation 683.0 and is pumped to the Preliminary Process 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
Preliminary Process Building the wastewater is screened through three bar screens with
1/4 inch openings. The bar screens remove material such as rocks, rags, 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 so it
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 land fill, while the solids containing human waste is pumped to the solids treatment
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 converts the solids, both suspended and dissolved,
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 flows by gravity, or is pumped, to the Missouri River. The
wastewater treatment process removes approximately 90 to 95% of the pollutional material
contained in the wastewater.
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 heated to approximately 400 degrees Fahrenheit. The heated solids
are treated with coagulating chemicals and dewatered on belt filter presses. The dewatered
solids are pumped to an incinerator for burning and the ash remaining after burning
is hauled to a land fill.