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West Hills Water Treatment Plant

Project Summary

The West Hills Water Treatment Plant was constructed in 2017 off of Union Road on the western side of Hollister. Similar to the Lessalt Water Treatment Plant, West Hills treats imported water from San Luis Reservoir to potable drinking water standards. The water is then delivered to residential and commercial customers in the Hollister area. This plant is a key component of the Hollister Urban Area Water & Wastewater Master Plan (HUAMP). The imported surface water is much lower in hardness and salinity than the groundwater that had previously been the only source of drinking water for the region.

While San Benito County Water District served as the lead agency, West Hills showcases the power of cooperation between agencies as Sunnyslope, the City of Hollister, and San Benito County Water District all participated jointly for the overall good of the community.


HDR Inc. was contracted for the design of the West Hills plant. After significant water quality testing and analysis, it was decided to use the ActiFlow water treatment technology in combination with Powder Activated Carbon to clean the water. A major consideration for this was the rapid change in water quality that would be experienced as source water would alternate between San Luis Reservoir and San Justo Reservoir. The location of the plant was also chosen to allow for water to gravity flow into Hollister's water distribution system. The ActiFlow technology allows for a much smaller treatment plant footprint which was important because a significant portion of the site had to be dedicated as California Tiger Salamander habitat. A booster pump station along Union Road was required to pump water up the hill to the plant. The plant was designed to be doubled in capacity at a future time.


The project went out to bid and Auburn Constructors LLC was awarded as the lowest responsible bidder. They graded the main site, booster station site, and the access road. Special care was given not to disturb the environmentally sensitive Tiger Salamander habitat. The 24" raw water supply line was tied into San Benito County Water District's pipeline in Union Road and ran up to the plant. Auburn built the water ActiFlow treatment structure and the concrete basins for each treatment stage, followed by the gravity sand filters. A 500,000 gallon tension wire wrapped concrete tank for the treated water was constructed and about 90% buried. The administration building holding the plant laboratory, control room, bathroom, office, and conference room was built of reinforced concrete block while the shop is a steel frame metal sided building. The chemical storage area and PAC silo were constructed west of the treatment structure. The backwash settling basins were lined with thick plastic sheeting and shot-creted. The used PAC and sludge drying beds were located on the northern side of the site. Computerized controls and monitoring systems were programmed and installed so that remote operation of the plant is possible. The 20" treated water pipeline leaving West Hills was installed down Riverside Road and up Nash Road to two of the City's well sites. Blending stations to mix the lower quality well water with the high quality water from West Hill were constructed at each well site.