Richfield Water Treatment Plant

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Water Treatment Plant

 

The Richfield Water Treatment Plant (WTP) was originally constructed in 1964.  The building has undergone numerous upgrades throughout the years to continually produce high quality water to the residents of Richfield.  The treatment process is described below.

Raw Water

Richfield draws its drinking water from underground sources called aquifers.  An aquifer is a soil, gravel or rock formation that contains and allows movement of water.  Rock, which seemingly is solid, can actually have very small voids or pore spaces that can hold a significant amount of water.

Richfield has six wells located in the Prairie du Chien/Jordan aquifers and one well located in the Mt. Simon Aquifer.  Raw water from the wells is pumped from the well houses to the water treatment plant.

Lime/Soda Softening

Water within the aquifers comes in contact with minerals which dissolve into the water.  Some of these minerals, like calcium and magnesium, add “hardness” to the water.  Hard water can lead to mineral deposits on plumbing fixtures, spots on glasses and dishware, and requires more soap to get things clean.  Hard water can also lead to deposits in hot water heaters leading to more maintenance or more frequent replacement.

Back in the 1960’s the Richfield City Council decided to provide softened water to the residents of Richfield.  The WTP uses the lime/soda softening process to achieve this.

Raw water from the wells enters a reactor with a large mixer.  Lime, soda ash and a polymer are added to react with the calcium and magnesium and raise the pH of the water to around 11.3 to 11.5.  The high pH leads to the formation of insoluble particles called “floc”.  The water then moves to a clarifier where these particles are allowed to precipitate, or settle, by gravity while the clear, softened water rises to the top of the clarifier and is taken to the next step in the process. 

Recarbonation

The high pH from the softening process is undesirable for consumption and can continue to precipitate calcium and magnesium in the distribution system.  In order to stabilize the water, carbon dioxide (CO2) is added through a process called recarbonation.  Liquid CO2 is injected into the water and results in a lowering of the pH to around 8.3 to 8.5.  This pH is more stable and allows a very small amount of calcium to precipitate and coat the pipes in the distribution system.  This thin coating protects the pipes from internal corrosion.  This point in the process is where chlorine is added for disinfection, as well as fluoride.

Filtration

After recarbonation the water is sent through six gravity filters.  The filters consist of silica sand media, backwash troughs, and piping to convey water to and from the filters.  Water moves downward through the filter and any remaining impurities are removed.  The measure of filtration performance is turbidity.  Turbidity is a measure of the clarity of the water.  Generally, the influent turbidity entering the filter is between 1 and 2 NTU, while the effluent turbidity leaving the filter is around 0.05 NTU.  

As the filters do their work removing impurities they will begin to plug up and restrict the flow of water.  When this happens a filter backwash is initiated.  The backwash process takes clean water and runs it in reverse through the filter.  The backwash water is collected by the backwash troughs and sent to a backwash retention tank where the particles are settled out clean water is return to the plant.  This eliminates wasting of water.

After filtration the water is stored in a 2.5 million gallon clearwell.  The water is pumped from the clearwell to the distribution system by four 250 horsepower high service pumps.

Water Plant Residuals

During the lime/soda softening process the floc that settles in the clarifiers is collected and sent through a filter press.  The filter press forces the solids/water mixture through a cloth media and separates the water and the solids.  The water is recycled back to the plant while the solids are loaded into trucks and hauled away to be land applied on agricultural fields for soil stabilization.