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Willow Creek Surface Water Treatment Plant

 

The Willow Creek surface water treatment plant was constructed in 1984 with a design capacity of 500 GPM.  Compliance with the SWTR has resulted in the plant capacity being reduced to 325 GPM, which is equivalent to a filter rate of 3.0 GPM/ft2.  BLWC has been able to obtain temporary approval from CDPH to operate the plant at 400 GPM during peak demand periods since 2007.  The operation of the plant is controlled by the water level in the Pines Tank, which is located at the highest elevation in the distribution system except for the chlorine contact (CT) tank.  A valve located at the plant opens when the Pines Tank telemetry calls for water based on the tank level.

 

The Company’s Willow Creek water rights date back to 1923 and are for the diversion of 0.92 cfs of water for domestic purposes and cannot exceed 355 acre-feet (115,669,224 gallons) per year.  In recent years, the total water consumed from the Willow Creek source averaged 95 million gallons; therefore, there are additional unused rights of approximately 20% for future demand.  The Willow Creek intake is located approximately 1,600 feet upstream from the plant and almost 200 feet higher in elevation than the plant location.  This elevation difference provides a static pressure of approximately 84 psi at the head of the treatment plant. 

 

The intake structure consists of a 16-inch-diameter intake lateral located in a portion of the stream that has been artificially built-up to promote submergence of the half-inch slots located in the top of the lateral. 

 

During periods when the water level in the creek is particularly low, the intake lateral end-cap is opened, allowing unstrained water to enter the intake.  Water is conveyed from the lateral to a two-stage redwood sedimentation box located a short distance away.  The box has a manual sluice gate to allow removal of settled material at the bottom of the box.

 

Water leaving the sedimentation box is carried to the treatment plant through an intake pipeline that is over 60 years old.  The unlined steel pipeline is mostly aboveground, but there are a few locations that are covered.  The pipeline traverses extremely rugged terrain, follows a very indirect route, and is in poor condition.

 

At the treatment plant, the arriving raw water from Willow Creek is injected with a cationic polymer ahead of a static mixer and 1,800 gallon contact vessel.  Following the contact vessel, filtration is accomplished by one Rescue Engineers four-cell horizontal pressure filter approximately 6 feet in diameter and 18 feet in length.  The filter media consists of 6 inches of anthracite coal on top of 18 inches of filter sand that is supported by 16 inches of gravel.  Water leaving the filter is chlorinated.  Chlorination is by a 12.5-percent sodium hypochlorite feed system.

 

Back wash cycles, which take almost a full hour to perform, occur every one or two days, depending on the rise in differential pressure across the filter.  Backwashing of the filter is accomplished using filtrate from three filter cells to backwash the fourth.  Each filter cell is backwashed for a period of 10 minutes.  After all four cells are backwashed, the entire filter is flushed to waste for 5 minutes.  Backwash water is discharged to a seasonal stream adjacent to the plant that connects with lower Willow Creek approximately 100 yards downstream.

 

In 1997, in response to requirements of the SWTR, a baffled 195,100-gallon chlorine contact tank was added near the treatment plant.  Chlorinated filtrate flows by gravity head or is pumped from the treatment plant to the CT tank, which is the highest point in the system.  The CT tank must provide a CT of at least 101 assuming a 1.0-mg/L chlorine residual.  This equates to a required contact time of 101 minutes.  However, excess capacity was built in to the design to accommodate a future expanded flow rate of 650 GPM.  The required CT was calculated based on a 2-log credit given for the treatment plant filters for giardia and 1-log credit for virus.  The remaining deactivation credits of 1 log for giardia and 3 logs for virus are provided by the CT tank.  The calculations are based on a low water temperature of 2 C° and pH of 7 to 8.  From the CT tank, the water flows by gravity to the storage tanks and reservoir and subsequently into the distribution system.

 

The Company is in the final permitting stage for the construction of a new Surface Water Treatment Plant which will comply with all current drinking water standards for the treatment of surface water.  The anticipated start date is the fall of 2015 or early 2016.

 

Groundwater Sources

 

BLWC uses groundwater sources to supplement the Willow Creek supply during the high-demand summer months and during storm conditions when turbidity is high in the creek.  The groundwater sources consist of three active wells with a capacity of 190 GPM and one standby well with a capacity of 30 GPM.

 

Uranium Treatment Facility for School Road Well

 

Due to high concentrations of uranium, the School Road well was placed in a standby status in 1998.  The water company evaluated the possibility of blending this water with its treated surface water, but abandoned this option due to cost and still having residual amounts of uranium present.  In 2006, the Water Company undertook the design and construction of wellhead treatment facilities to remove the uranium from the water.

 

The uranium treatment plant was installed on the School Road well source in the spring of 2007.  It was tested and became operational June 21, 2007. A one page handout was distributed at a public meeting conducted by BLWC to explain the facility.  

 

This equipment allowed the School Road well to be placed in an active status.  As such, BLWC has allocated 64 GPM of its 125 GPM capacity for the proposed Pines Townhome Project (previously 35 GPM was allocated for the 112 unit Ridgeline Subdivision).  It is anticipated only 13 GPM average year-round and 25 GPM peak-month demand will be needed to supply that project thus allowing for a large reserve of capacity.

 

Advantages to current customers of the system are:

  • The cost of $123,359.38 for the design, engineering, equipment and installation for the treatment facility was paid by the Pines Townhome Project, and

  • The entire 125 GPM is now available to the system unrestricted by the former limitation of 15 days of annual use (and not over 5 consecutive days) as a standby source.

  • The water is now treated and meets EPA and CDPH safe drinking water standards

 

Storage

 

The Current storage facilities consist of six (6) metal tanks and one concrete reservoir with a total storage capacity of 1,519,350 gallons.  All are inter-connected within the system and provide gravity flow to the majority of the system.

 

Supply and Storage Requirements for New Projects

 

New Subdivisions within the water company’s service area have the requirement to provide both a source of supply and adequate storage to satisfy their project so service to existing customers is not diminished.  This has been accomplished through main line extension agreements whereby these improvements are brought into and become part of the water company’s facilities.  This has been done for North Shore Estates, Ridgeline subdivision, Slide Creek Estates, Worldmark, and Bass Lake Estates.  Existing customers have benefited by these subdivisions since they added ground sources to the system not previously available.

 

A.  Supply

For the Ridgeline subdivision, in the 1980’s, 35 gallons per minute (GPM) of the School Road well capacity was allocated for that 112 unit project or, .31 GPM per unit.  In order to provide for a greater reserve, we have allocated 64 GPM of the well production to the Pines Townhome project, or .95 GPM per unit.  Based on current consumption rates for residential customers, the Pines Townhome project is estimated to add year-round average demand of 13 GPM and peak month demand of 25 GPM.

 

B.  Storage

In 2003, 83% of the water consumption was for residential use by 896 residential connections.  The corresponding storage for that use was 737,376 gallons.  This represents 823 gallons of storage/residential connection and is the requirement placed on the Pines Townhome project.  This equates to 55,000 gallons of storage and will be provided by utilizing a portion of the 300,000 gallons the Pines Resort already has in place.