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West Hills Water Treatment Plant


The West Hills Water Treatment Plant was constructed in 2017 through the joint partnership of San Benito County Water District, Sunnyslope Water, and the City of Hollister. Located on the western side of Hollister north of Union Road, this plant purifies surface water coming from San Luis Reservoir and San Justo Reservoir so that it can be delivered to customers in the Hollister area. Planning for this plant began as part of the Hollister Urban Area Water & Wastewater Master Plan in 2008 to address the long-term water quality and sustainablity needs of the region. 

San Benito County Water District owns the plant but contracts with Sunnyslope Water to conduct the daily operation and maintenance at West Hills. Treated water is sent to the City of Hollister and Sunnyslope Water for distribution to customers.

Water produced from West Hills is very high quality and significantly lower in both hardness and salinity than the local groundwater. Because of this improved water quality, City of Hollister customers no longer need water softeners and are encouraged to bypass or remove them. Water softeners make it hard to meet wastewater treatment requirements.

Treatment Process

Raw Water Pumps

Water treated at West Hills is delivered primarily from San Luis Reservoir through the 54" diameter Hollister Conduit. But at times, water also comes from the local San Justo Reservior located on the southwest side of Hollister. When receiving water from San Justo, a booster pump station is used to get the water to West Hills.

Chemical reactions remove non-toxic iron and manganese from the water at this booster station directly off Union Road. Water then travels along the access road up to West Hills.

Poweder Activated Carbon

West Hills utilizes powder activated carbon (PAC) to remove microscopic organic substances from the water. The PAC adsorbs the contaminants, trapping them in its porous structure with molecular forces. This is the same general technology as a Brita Carbon Filter though on a much larger scale. By removing these organinc substances, West Hills prevents toxic Disinfection By-Products that can form when chlorine interacts with organic molecules.


In order to reduce the footprint and environmental impact of the plant, designers utilized the ActiFlow treatment technology from Veolia. With this technology, the pre-filtration process is accelerated. A coagulant is added to the water along with a very fine sand, causing tiny particles suspended in the water to bind to one another and the sand. As the particles form bigger and bigger clumps, they settle to the bottom of the basin. The cleaner water at the top is skimmed off and sent to the filters. The dirty water at the bottom is removed and the sand is recycled. The concentrated sludge is sent to the settling basins.


West Hills uses conventional gravity sand filters to remove the last of the contaminants from the water. These basins are filled with different layers of sand which catch particles as the water passes through the pores between the sand grains. Clean water is drawn from the bottom of the filters as gravity pushes the water down through the sand beds. Various samples and measurements are taken continuously to ensure the cleanliness and safety of the water. 


The final step of the process is chemical disinfection to kill any pathogens that might have survived the treatment process. West Hills uses liquid chlorine to disinfect the water. The water pH is also ajusted to help prevent corrosion of water pipes in the streets and in customer's homes. Water then enters a 500,000 gallon tank where the chlorine has time to kill any pathogens. After that tank, water travels through a pipeline to Hollister and Sunnyslope well sites where it then enters the distribution system.

Sludge Removal

Dirty water pulled from the ActiFlow process and from filter backwashes is sent to the Settling Basin. Here the dirt particles and contaminants are given additional time to settle to the bottom. The cleaner water at the top is skimmed off and sent back to the start of the plant. The dirty water at the bottom is then pumped to the drying beds. These drying beds allow the water to evaporate leaving behind the dry sludge and used PAC. This dry sludge is then removed and taken to the landfill for disposal. 

Operation & Control

Sunnyslope Water staff conduct the daily operation and maintenance of West Hills. Highly qualified operators control and monitor every process of the plant. Various sensors and instruments constantly measure different water properties such as pH, oxidation/reduction potential, temperature, total organic carbon, and many more. These sensors are all connected to the SCADA electronic operating system through which operators can remotely monitor and control every aspect of the plant. Regular laboratory testing is used to verify the water quality and treatment process.