Project Type

Indirect Potable Reuse Program Support


Padre Dam Municipal Water District


Santee, CA

Current Status

Ongoing since 2013

Key Features

  • Process design of 100,000 gpd demonstration facility
  • High-recovery reverse osmosis design and pilot testing
  • Pathogen reduction crediting with free chlorine disinfection
  • Regulatory and permitting support
  • Process optimization
  • Operations

Project Overview

Padre Dam Municipal Water District is pursuing the expansion of its recycled water program through the East County Advanced Water Purification Program to produce at least 30% of the East County’s potable supply. Trussell has been involved with Padre Dam since 2013, with treatment process design of the 100,000 gpd capacity demonstration facility. This project is the first to seek and receive approval from the California Division of Drinking Water for minimum retention times in the groundwater aquifer (2 months). To justify shorter aquifer storage times, Padre Dam and Trussell demonstrated increased pathogen removal through the treatment process. Their novel, cost-effective strategy includes seeking DDW approval for free chlorine disinfection of the recycled water used as feedwater. The team coordinated with both an Independent Advisory Panel (IAP) and DDW to gain approval for the demonstration testing plan, and to update the group with project results during and at the conclusion of the year-long testing.

The demonstration facility consists of full advanced treatment (membrane filtration, reverse osmosis, and advanced oxidation) in addition to free chlorine pre-treatment. The demo testing included pathogen and chemical challenge testing to gain insight into the reliability of the treatment train. Trussell and Padre Dam also conducted a USBR-funded study to investigate methods for achieving high recovery through the RO system to reduce the RO concentrate waste generated during treatment, a priority for this inland utility. Trussell tested both the Desalitech Closed Circuit RO system and a conventional 3rd+4th stage RO system, receiving feed water from the primary 2-stage RO concentrate. The testing demonstrated 95% overall recovery with the CCRO system, and 92.5% overall recovery with the conventional 3rd+4th stage RO. Trussell performed an economic analysis to enable Padre Dam to select the most cost-effective alternative for full-scale design, based on a 20-year lifecycle cost analysis, and this will be using the CCRO as a 3rd stage RO system. The full USBR report on the testing may be found here:

Trussell continues to support Padre Dam with testing and treatment process optimization for both the existing wastewater facility and future design of the full-scale 12.5 mgd production capacity wastewater and advanced treated water facility.

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