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The City of West Branch, Ogemaw and West Branch Township’s sanitary sewer system consist of approximately 23 miles of 6 to 24 inch diameter collection and interceptor sewers.

With the establishment of the Water Pollution Control Act of 1972 and the Clean Water Act of 1977, Congress had established as goals the national preservation and restoration of the nations water.  Being located on the west branch of the Rifle River, and serving the City of West Branch, portions of Ogemaw, and West Branch Township’s the West Branch Area Wastewater Treatment Facility plays a key role in preserving the area and Michigan’s most valuable resource, its surface waters.

The Treatment Plant was completed and put into operation in March 1984.  The original plant was designed to treat a daily flow of .7 MGD with a maximum daily flow of 2.1 MGD.  From 2009 to 2012 a facility upgrade took place allowing the plant to treat a daily flow of .9 MGD with a maximum daily flow of 2.1 MGD and a 5.0 MGD peaks.


The following is a descriptive of the treatment facility as well as some pictures taken during the upgrade.
 

The Plant influent comes from a network of sanitary sewers, which collect domestic sewage from the City of West Branch, along with portions of Ogemaw and West Branch Townships.  The influent flow is continuously metered as it passes through a parshall flume located at the head of the plant.

After the flume the sewage can be directed into the automatic barscreen or the manual barscreen to separate debris larger than .75 inches from the waste stream.  The flow is then channeled into an aerated grit tank in which grit settles to the bottom and then is pumped into a grit washer dewatering screw.  Screenings and grit are deposited in dumpsters and disposed in a landfill.

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A divided wet well consisting of a raw side and an equalization side follows the aerated grit tank. The flow enters the raw wet well which consists of three variable speed, ten horsepower centrifugal pumps. The flow is then pumped to the primary settling tanks. 

If the influent flow exceeds maximum peak flow or plant flow set point, the wastewater will fill up the raw wet well and spill into the equalization wet well which consists of two variable speed, twenty horsepower centrifugal pumps. Once the equalization wet well level reaches a set level the wastewater will be pumped to two 100,000 gallon covered equalization basins equip with aeration and mechanical mixers which allow storage of the wastewater until the influent flow drops below the set point. When this happens, a flow controlled valve will allow the wastewater from the equalization basins to flow to the primary settling tanks.

If the influent flow does not drop below the set point and the two equalization basins fill up, the wastewater will overflow into the 2.4 million gallon equalization storage tank where it will be stored until the influent flow drops below the set point.  The stored wastewater flows by gravity back to the raw wet well. The rate of flow to the wet well is manually controlled by an auto valve. 

Alum is added at the head of the two, 60,300 gallon primary clarifiers.  As the flow passes through the two primary clarifiers, settable solids and floatable solids are removed.  Settable primary solids are automatically pumped to the primary anaerobic digester.  Effluent from the primary clarifiers flows by gravity to the trickling filter pump station.

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The secondary biological treatment process consists of two trickling filters, each 45 foot in diameter and 26 feet tall.  Primary effluent flows by gravity to the trickling filter pump station which consists of two wet wells connected by an isolation valve and four variable speed submersible pumps; two pumps in each wet well. This design allows us to operate the trickling filters in three modes; series, parallel, and parallel with recycle which we operate in.  The pumping rate is controlled by a wet well level set point. 

As the primary effluent flows into the trickling filter pump station both wet well levels are the same because the isolation valve is open. Wastewater is pumped to the top of both trickling filters at the same time and same rate.  This is the parallel process.  The wastewater passes through both filters. A portion of the trickling filter effluent may be channeled back to the trickling filter wet well to be recycled with the primary clarifier effluent and pumped back to the trickling filters for additional treatment.

The recycle flow attempts to maintain a constant flow through the trickling filters and is controlled by an automatic SCADA controlled modulating valve (recycle valve). If the influent flow decreases, the recycle valve opens and recycle flows increase. If the influent flow increases, the recycle valve closes and recycle flows decrease. 

The rest of the trickling filter effluent flows by gravity to a splitter box to be divided between three secondary clarifiers. Polymer is added at the splitter box to aid settling.  

In the secondary clarifiers the solids settle to the bottom of the tanks and clear water flows over weirs at the top of the tank.  The solids are pumped to the primary clarifiers to settle with the primary solids and then be pumped to the primary digester.  Effluent from the secondary clarifiers flows by gravity to the tertiary filters.

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The tertiary filters consist of four 9ft. x 9ft. gravity sand filters. The filter effluent flows by gravity to an open channel UV disinfection system. 

As the secondary effluent passes through the filters they become dirty and water levels rise. When they reach a set level they will automatically backwash. The water used for filter backwashing is pumped from the clear well using two twenty horsepower variable speed centrifugal pumps. The clear well is kept full by plant effluent flowing by gravity to the clear well after UV disinfection.  The backwash wastewater flows by gravity back to the equalization wet well. 

If the plant flow increases and the tertiary filters are unable to keep up they can be bypassed.  Also each filter has an over flow. If filters are bypassed or water reaches the overflows the secondary clarifier effluent flows by gravity directly to the UV disinfection system.
 
The UV disinfection system consists of two banks of UV lights operated in series and managed by a system control center. The UV intensity is controlled by a submersible UV sensor and a dose pacing sensor which is controlled by the effluent flow meter. The UV control center activates the cleaning system which wipes each UV quartz with a cleaning solution.  After leaving the UV’s the flow passes through a parshall flume flow meter and then discharged to the Rifle River. 

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Scum and sludges are pumped to the anaerobic digester from the scum boxes and primary settling tanks.  The primary anaerobic digester achieves solids reduction by being heated and mixed.  This process yields a stabilized biosolids, water, and methane gas.  The stabilized biosolids is stored in the secondary digester and biosolids storage tank.  The stabilized biosolids are applied to agricultural land as a soil and crop enhancer, the water is returned to the equalization wet well to be treated, and the methane is utilized to maintain a liquid temperature of 96 degrees in the primary digester.

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