Treatment Process

Raw Wastewater Pump Station
The Raw Wastewater Pump Station provides preliminary treatment, raw wastewater pumping and flow metering. Preliminary treatment includes a channel monster for reducing of large debris size, screening for removal of smaller solids and grit removal of abrasive materials such as sand, silt and gravel.

Preliminary treatment protects downstream process equipment from excessive wear. The raw wastewater pumps lift the wastewater to an elevation which permits gravity flow through the remainder of the treatment plant. Flow metering is provided for routine plant operation and control purposes.

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Selector Basin

The selector basin provides the right enviornment for Biological Nutrient Removal to occur. The raw wastewater and activated sludge bacteria are moved through an anaerobic zone to force the PAOs (Phosphate Accumilating Organisms) to release phosphorus from their cells. When these bacteria are moved into an aerobic environment, they take in more phosphorus than they released, removing the phosphorus from the wastewater stream.

The rest of the selector basin is in an anoxic condition. Facultative bacteria (can breath both dissolved and combined oxygen) are forced to breath only combined oxygen. They strip off the oxygen molecules from nitrite (NO2) and nitrate (NO3) releasing nitrogen gas into the atmoshere, thus removing nitrogen from the wastewater stream (denitrification).




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Photo of the Selector Basin of the treatment plant.


The activated sludge process requires a dissolved oxygen level adequate to maintain the biological treatment process. Compressed air from the blower building is piped to the following areas:
  • Aeration Basin
  • Post Air Basin
  • Selector Basin

Four 300 HP centrifugal blowers are available for providing the plant air.

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Control panels line the wall of the blower building.

Aeration Basin

The Aeration Basin is divided into 3 stages. This is the most important treatment process in the plant. The wastewater entering the plant is high in carbonaceous material and organic nitrogen, a large portion of which has been converted to ammonia on its way to the treatment plant. When properly operated, the activated sludge biological treatment process is very effective.
  • In the 1st stage, a natural biological treatment process is used to remove the largest portion of the carbonaceous, organic material and for phosphorus uptake to occur.
  • The next 2 stages also use the activated sludge biological treatment process to remove the remainder of the Biochemical Oxygen Demand (BOD) and Suspended Solids (SS) from the wastewater.
    • Biological Nitrification occurs in the last 2 stages, reducing ammonia to nitrites and nitrates.
    • A large portion of the nitrite / nitrate rich wastewater stream is returned to the Selector, through the Mixed Liquor Return Pumps, for denitrification.
Picture of the Aeration Basin, which sits outside.
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Mixed Liquor Recirculation Pump Station (MLRPS)

A portion of the flow from the last stage of the activated sludge process,is returned to the Selector, through the Mixed Liquor Return Pumps, for denitrification (conversion to nitrogen gas).

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The clarifiers receive wastewater along with suspended activated sludge solids from the Aeration Basin. The clarifiers provide a quiescent area which permits the solids to settle to the bottom. Clarified water drains over the weir to the next treatment process.

Floating materials in the wastewater entering the clarifiers, such as grease or scum, will float to the surface. This material is skimmed from the surface and collected in the clarifier scum box. Good clarifier performance is essential to the proper operation of the treatment plant.

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Wastewater sits in a califer, allowing the solids to settle at the bottom.
The Disinfection Building houses the controls for the UV system, required to disinfect the clarified wastewater following activated sludge treatment process. Disinfection is accomplished by treating the wastewater with sufficient UV to kill most of the bacteria remaining after biological treatment. The UV leaves no residual to be harmful to aquatic life in the receiving stream. The Disinfection Building includes the controls for the UV system and the effluent sampler.

The UV basin consists of 4 channels with 40 bulbs per channel. Each channel is rated for 4.25 MGD of wastewater from the clarifiers. The clarifier effluent flow is measured by an ultrasonic flow meter. Flow enters an influent box and splits between the 4 channels as needed to meet the 4.25 MGD per channel. The UV effect is nearly instantaneous, requiring little contact time.
The disinfection chamber seated over some treated wastewater.

UV System

 High-output, Low-pressure Ultra-Violet Light Disinfection System:
  • Destroys bacteria (E. coli) viruses (hepatitis and polio), and other water-borne pathogens
  • Destroys Giardia and Cryptosporidium which are resistant to chlorine
  • Is environmentally friendly, reduced liability - is safe to handle, no toxic chemicals, no chlorine leaks
  • Is more cost-effective than chlorine systems
  • Produces no known disinfection by-products, unlike chlorine
  • Reduces harmful micropollutants (such as herbicides and pesticides)
  • Works at least 20 times faster than chlorine
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Post Aeration Basin

The Post Aeration Basin is the final liquid treatment process. The purpose of the basin is to provide aeration of treated wastewater to raise the dissolved oxygen to a level meeting the discharge standard. Dissolved oxygen is necessary to sustain desirable forms of aquatic life in the receiving stream. Low dissolved oxygen levels in the plant effluent could degrade stream water quality.

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Effluent Pump Station

Normally, wastewater from the Post Aeration Basin flows over the basin effluent weirs and flows by gravity to the Little Sac River. When the river is at flood stage, however, water will back up into the plant to a level which will prevent gravity flow. Under these circumstances, the Effluent Pump Station must be put into operation to pump the flow into a high-water outlet chamber.

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Effluent Structure

Plant effluent is discharged into the Little Sac River from the Effluent Structure.

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Plant Effluent 2016

Return Sludge Pump Station

The Return Sludge Pump Station includes pumping systems for recirculating settled activated sludge from the clarifiers to the Aeration Basin, wasting activated sludge to the rotating drum thickeners and removing scum collected in the clarifiers. The pump station also includes equipment for non-potable water and compressed air. Finally, the pump station control room includes motor control centers for equipment in the pump station and the clarifiers.

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Rotating Drum Thickeners

 To control the treatment process, a portion of the activated sludge has to be removed daily. Waste activated sludge is pumped to two rotating drum thickeners were the sludge is concentrated to 5-7% total solids and pumped to the sludge storage basin.


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Sludge Storage Basins

Sludge Storage Basins store sludge prior to being hauled to the south plant for digestion. They provide a means to store sludge during weekends and holidays when crews are not available to haul the sludge. Sludge Storage Basin No. 2 is covered with a clear span, self-supporting geodesic aluminum dome to contain odorous air. A 32-inch diameter odor control air collection pipe conveys the air from the sludge storage basin to the Bio-filter and carbon filter, to remove odors. Since Sludge Storage Basin No. 1 is not covered, it is held in reserve for emergency or testing situations only.