Mocho PFAS Treatment Plant

The Project includes construction of the Mocho PFAS Treatment Plant at an existing well site, including water treatment vessels, piping, pumps, electrical, and other appurtenances. The water treatment facility or a portion of the facilities will either be screened with a wall, or all of the facilities will be enclosed within buildings. The booster pumps will be enclosed in a building for noise attenuation. Zone 7 is continuing to evaluate the screening options. The Project would also include the installation of a new water transmission pipeline beneath Stoneridge Drive to deliver the treated groundwater into the existing transmission system (Figure 3) at the Mocho Groundwater Demineralization Plant (MGDP). The treated groundwater connection would be installed on the existing influent water pipeline at the Mocho Groundwater Demineralization Plant. Treated groundwater would then be conveyed into the existing transmission system. In addition, the Project includes improvements at the Mocho Groundwater Demineralization Plant to increase demineralization treatment capacity and salt removal. The Mocho Groundwater Demineralization Plant began operating in 2009 and included space for additional reverse osmosis (RO) membrane filters. The improvements will allow the Mocho Groundwater Demineralization Plant to operate at its originally designed treatment capacity. The existing RO membrane filters have a total brine concentrate discharge of approximately 1,064 gallons per minute (gpm). With the additional RO membranes, the anticipated total concentrate would be approximately 1,310 gpm. The existing Dublin San Ramon Services District (DSRSD) Pretreatment Program Industrial Wastewater Discharge Permit, dated July 1, 2025, specifies that the rate of discharge shall not exceed an average of 1.6 million gallons per day (MGD). It is anticipated that the additional RO membranes would increase the total rate of brine concentrate discharge to an average of 2 MGD. The final RO membrane brine concentrate discharge rate will be determined during the detailed design phase and the discharge modifications are subject to the review and approval of DSRSD and East Bay Dischargers Authority. The installation of the membranes will likely require replacement of ancillary mechanical and electrical equipment, potentially including the pumps, variable frequency drives (VFDs), motor control center (MCC), switchgear, and other appurtenances. The Project also includes replacing the well pumps at Mocho Wells 2, 3, and 4 due to the pumps reaching the end of their useful lives and evaluating the use of higher pressure pumps and variable frequency drives for these pumps to meet the Mocho PFAS Treatment Plant hydraulic needs. Furthermore, as part of the Mocho PFAS Treatment Plant project, the “Wells and Mocho Groundwater Demineralization Plant Electrical Modernization Project” subtask includes replacing the existing switchgear and ancillary electrical equipment at Mocho Wells 3 and 4 due to the equipment exceeding their useful lives and will be sized to accommodate the Project’s electrical demands. The Project could be constructed to treat the entire Mocho wellfield capacity of approximately 16 MGD. This full wellfield PFAS treatment project is evaluated herein. However, Zone 7 is also continuing to evaluate smaller capacity PFAS treatment options at this wellfield that may result in reducing the footprint of facilities that are initially installed without meeting the Project’s secondary objective. The Mocho PFAS Treatment Plant Project would consist of the following key components (also see Figure 4 and pre-conceptual architectural renderings Figure 5a-5c): • Water transmission line – A new water transmission pipeline beneath Stoneridge Drive to deliver treated water to the Mocho Groundwater Demineralization Plant. • Pipeline – A pipeline at Mocho Well 3 to split water between the Mocho Groundwater Demineralization Plant and the new treatment plant. • PFAS treatment vessels – Vessels containing ion exchange resin (“media”) for removal of PFAS from the groundwater. • Cartridge filters – Pre-treatment cartridge filters to remove sediment from the water before it enters the PFAS treatment vessels. • Booster pump station – A booster pump station may be needed to increase water pressure needed to enable treatment. Booster pumps to be housed in a new building to attenuate noise. • Media truck driveway – A driveway for trucks to remove and replace the media approximately once every two years. • Switchgear – As part of the “Wells and Mocho Groundwater Demineralization Plant Electrical Modernization Project” subtask, the existing switchgear and ancillary electrical equipment at Mocho Wells 3 and 4 would be replaced due to the equipment exceeding their useful lives and would be sized to accommodate the Project’s electrical demands. • Electrical building – A separate building may be constructed to house electrical facilities. • Well pumps – The existing well pumps at Mocho Wells 2, 3, and 4 would be replaced due to the equipment reaching the end of their useful lives and to meet facility hydraulic needs. • Additional Mocho Groundwater Demineralization Plant Capacity – Additional reverse osmosis membrane filters would be installed within existing facilities originally designed and constructed to house them in order to increase treatment capacity and salt removal. The existing membrane filters have reached the end of their useful life and would be replaced. Ancillary equipment, such as the associated pumps, electrical equipment, and other mechanical equipment, would be upgraded as needed within the existing project facilities. • Site lighting – The site would be designed with lighting to facilitate safe operations and maintenance of the facilities. The lighting would be designed to minimize public impacts (e.g., motion activation switches, shields or hoods directing light downward, etc. to minimize light spillover, reduce glare, and prevent light trespass onto adjacent properties or into the night sky, in accordance with local lighting standards). • Site security - The facilities would be secured within walls or a building to secure the treatment vessels, electrical equipment, piping and other mechanical equipment. • Site finishes – The facilities would utilize paint colors and exterior finishes that are visually compatible with the surrounding built environment. • Vegetative plantings (e.g., trees, shrubs) and/or other amenities for visual screening of the treatment facilities to promote integration with the surrounding area. • Stormwater management features – Stormwater treatment facilities and site drainage structures to be constructed at Mocho Well 3 in accordance with the San Francisco Bay Regional Water Quality Control Board’s Municipal Regional Stormwater Permit requirements. The bioretention stormwater treatment facility, consisting of a swale with native drought-tolerant shrubs and plants, is anticipated to be approximately 1,200 square feet, with its location on the Mocho Well 3 site to be determined upon further existing utilities and soils investigations. • Trail restoration and tree replacement for any removal or damage.