Diablo Grande’s Comprehensive Water Treatment System

The community of Diablo Grande is currently being served by an existing 1 million gallon per day water treatment plant, Public Water System No. 5010039. The permit for the plant, 03-10-04P-004, was issued to the Western Hills Water District on August 24, 2004. The existing system was described to the CDPH in an August 2003 “Application for Domestic Water Supply Permit”.

A permit amendment for the addition of one Actifloc water treatment unit with a treatment capacity of 2 million gallons per day (MGD) was sent to the CDPH in May 2006 but was placed on hold pending resolution of construction issues with Veolia Water. The construction of this 2 MGD unit is referred to as the Phase 1 Expansion (Phase 1). Some modifications to the existing plant were made to accommodate the implementation of the Phase 1 Expansion; some of these modifications are currently in use at the plant.

Diablo Grande is a master planned residential-and-resort community of approximately 30,000 acres in western Stanislaus County, about 10 miles southwest of Patterson, California. The community will be developed in four phases extending over a 25-year period and will ultimately consist of 5,000 dwelling units. The system will have year-round occupancy as the development is designed as primarily residential housing. The access road into the development was completed in 1997.

A water treatment plant was constructed to serve the needs of the community. That plant consists of two 1 MGD Actifloc water treatment systems and performs full, conventional treatment for contaminant removal and disinfection. Treated water from that plant is pumped via the Zone 3 booster pump station into a 1 million gallon (mg) storage tank on a hill above the residential area. The base elevation at the tank site is 1,484 feet and will serve the developments Zone 3 area.

The Phase 1 expansion added a new 2 MGD Actifloc Filter unit and associate plant upgraded to accommodate this system. Several improvements were also made to enhance plant performance.

The water treatment plant provides service to the community twelve months per year. Water service connections will be both residential and commercial. Water usage includes those typically associated with private residences (such as household use and irrigation). Commercial uses will include water for the Hotel, Shopping Center, Spa, Golf Club House, Golf Maintenance Center, Resort Condominiums, and the Swim and Tennis Clubs. With the exception of a winery, no heavy commercial development is planned for Diablo Grande.

The Diablo Grande water treatment plant currently provides water to approximately 400 active service connections.

The T2300 unit connected into the existing plant systems. Those systems include the Raw Water tank, a clearwell, a waste equalization tank, a solids storage tank, and the recycle clarifier. The existing system was originally designed for a 2 MGD flow rate and provide sufficient capacity for the Phase 1 water treatment plant.

During Phase 1 construction, the existing clearwell was modified to improve the baffling factor, and thus improve water disinfection. Curtain walls were constructed within the clearwell. The clearwell, as originally constructed, was prone to short-circuiting, a condition that contributed to poor disinfection.

At the beginning of this year the WHWD placed in service Chloramination System Improvements. They consisted of an ammonia injection system with a 800 gallon storage tank, two (2) chemical feed pumps, clearwell piping modifications, a dedicated clearwell recirculation pump, modifications to the Zone 3 Pump Station piping to include a surge tank, instrumentation and SCADA Control.

The Chloramination System is sized for a maximum plant flow rate of 4 MGD with a nominal flow rate of 2 MGD and a minimal flow rate of 0.5 MGD.

The Actifloc process uses a microsand ballast that functions as a seed, enhancing floc formation and settling. Raw water from the California Aqueduct will enter the Raw Water tank located at the treatment plant. That water will be treated with a coagulant, in this case, alum. Alum being injected into the mixer will be completely mixed with the raw water. The coagulant will destabilize the suspended solids, making them easier to remove in the Actifloc system.

The raw water enters at the Coagulation tank. From there, the water flows into the Injection tank, where polymer and microsand are added. The contents in the injection tank are continuously mixed providing contact between the destabilized solids, polymer and microsand. In the Maturation tank, the floc particles, polymer, and sand adsorb to one another form larger particles that rapidly settle out of solution in the settling tank. The settled solids are removed from the bottom of the tank. The ballasted floc is recycled to a hydro cyclone that separates the microsand from the attached particles. The microsand is recycled back into the Injection tank.

The water then rises slowly through lamellar settling tubes, arranged at an angle to the tank sidewalls before entering the filters. The reclined settling tubes allow smaller particles to fall a short distance in the rising water column, simplifying removal.

When the Phase 1 improvements are completed, the WTP will consist of a new 2.0 MGD Actifloc module and two existing, 1.0 MGD Actifloc modules. The rated plant capacity will be 2.0 MGD. Since the 2 MGD unit was never commissioned and placed in service the water treatment plant capacity remains at 1.0 MGD.

There are five existing pump stations in the WHWD system. These consist of 4 large raw water pump stations and one treated water pump station. The first of the raw water pump stations is located at the raw water intake at the intersection of Lower Oak Flat Road and the California Aqueduct. The other three are located in series along the pipeline from the California Aqueduct to Diablo Grande. A fifth pump station (treated water) the Zone 3 storage tank pump, is located at the water treatment plant site.

A 155,000-gallon clearwell, located at the water treatment plant, holds treated water prior to the water being pumped to the Zone 3 storage tank. The clearwell is configured to maintain a minimum of 146,795 gallons for disinfection purposes.

The clearwell tank is a ground level, cylindrical steel tank. The tank is 50 feet in diameter with a 16-foot height and a steel roof.