Northwest Wastewater Treatment Plant
Treatment Process

Follow the flow of wastewater through the treatment process by clicking in the following areas on the Northwest Wastewater Treatment Plant diagram below:

schematic, Northwest Wastewater Treatment Plant
raw wastewater pump station Selector basin Blower building aeration basin Clarifier Clarifier Clarifier Disinfection Post aeration basin Mixed liquor return pump Effluent pump station Effluent structure Return sludge pump station Sludge thickener Thickened sludge pump station Sludge storage Sludge storage Thickened or dewatered sludge building

Raw Wastewater Pump Station (to diagram)

Raw Wastewater Pump Station Headworks

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 (to diagram)

Selector Basin

This portion of the treatment process is called, Bio Nutrient Removal (BNR). Used for removal of phosphorus and nitrogen from domestic wastewater.

The two stage process:

Stage 1, phosphorus uptake: A mixed anaerobic zone, with raw wastewater and return activated sludge from the Clarifiers.

Stage 2, De-Nitrification: A mixed anoxic zone, adds a large portion of the flow with nitrite/nitrate back from the last stages of the aeration basin. Where it is converted to nitrogen gas, which is released to the atmosphere.

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Blowers (to diagram)

Blower Building

The activated sludge process requires a dissolved oxygen level edequate to maintain the biological treatment process. compressed air is piped to the Aeration Tank, Selector Basin and Post Air, from the blower building.

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Aeration Basin (to diagram)

Aeration Basin

The Aeration Basin is divided into three 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 first stage, a natural biological treatment process is used to remove the largest portion of carbonaceous material and to convert ammonia to less toxic nitrates. The next two 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 also occurs in the last two stages, reducing organic nitrogen to nitrites & nitrite. A large portion of the nitrite/nitrate rich wastewater stream is returned to the Selector, through the Mixed Liquor Return Pumps, for de-Nitrification.

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Mixed Liquor Recirculation Pump Station (MLRPS) (to diagram)

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 de-Nitrification (conversion to nitrogen gas).

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Clarifiers (to diagram)


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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Disinfection (to diagram)


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.

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The UV basin consist four channel with forty 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 four channels as needed to meet the 4.25 MGD per channel. The UV effect is nearly instantaneous, requires little contact time. The system is a High-output, low-pressure UV system. The System:

  • Destroys bacteria (E. coli) viruses (hepatitis and polio), and other water-borne pathogens
  • Destroys Giardia and Cryptosporidium which are resistant to chlorine.
  • Reduces harmful micropollutants (such as herbicides and pesticides).
  • Works at least 20 times faster than chlorine.
  • Is environmentally friendly, reduiced libility -- is safe to handle, no toxic chemicals,no chlorine leaks.
  • Produces no known disinfection by-products, unlike chlorine.
  • Is more cost-effective than chlorine systems.

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Post Aeration Basin (to diagram)

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 (to diagram)

Normally, wastewater from the Post Aeration Basin flows over the basin effluent weir, as in the Post Aeration Basin image above, and drains 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 (to diagram)

Plant Effluent

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

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Return Sludge Pump Station (to diagram)

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 Sludge Thickener and transferring scum collected in the Clarifiers to the Sludge Thickener. 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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Sludge Thickener (to diagram)

The Sludge Thickener receives waste activated sludge and concentrates or thickens the solids by gravity settling. Thickening the solids reduces the volume of sludge to be handled by subsequent sludge treatment processes. The Sludge Thickener also removes scum and other floating material which may be in the waste activated sludge.

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Thickened Sludge Pump Station (to diagram)

The Thickener Pump Station has three pumping systems to support the operation of the Sludge Thickener. The Thickener Recirculation Pumps continuously recirculate thickener supernatant to the thickener to keep the thickener contents fresh and aerobic. The excess thickener Supernatant returns back Raw Wastewater Pump Station for treatment. The Sludge and Scum Transfer Pumps remove thickened sludge and scum from the thickener and pump it into the Sludge Storage Basins.

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Sludge Storage Basins (to diagram)

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.

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Thickened or Dewatered Sludge Building (to diagram)

The sludge stored in the existing sludge storage basin is hauled to the south plant for digestion. If needed the thickened sludge can be dewatered to eliminate as much of the water content in the sludge and thus reduce its volume. Sludge dewatering is accomplished with two centrifuges. The liquid separated from the solids flows by gravity back to the Raw Wastewater Pump Station.

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