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REGULATORY ISSUES ASSOCIATED WITH DISCHARGING MEMBRANE WASTE STREAM: A CASE STUDY

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The Sweetwater Authority is a joint powers public water agency serving a population of approximately 175,500 consumers through 33,600 connections within the City of National City, and portions of the City of Chula Vista and Bonita. The Richard A. Reynolds Groundwater Demineralization Facility (Reynolds Facility) is located in Chula Vista in southern San Diego County. It was designed to help Sweetwater Authority reduce its reliance on imported water by utilizing local water resources.

The Reynolds Facility is a fully automated 4.0 million gallon per day (MGD) reverse osmosis treatment plant. It is designed to reduce total dissolved solids and remove particulate matter (including bacteria and viruses) from four alluvial aquifer wells (shallow brackish groundwater under the influence of surface water) and six San Diego Formation wells (semi-brackish groundwater). The San Diego Formation is a deep groundwater aquifer and is not under the influence of surface water. All shallow groundwater (alluvial well water) is directed through the reverse osmosis system. The deeper groundwater (San Diego Formation water) is able to bypass the treatment process. The Reynolds Facility consists of three treatment trains each with 26 pressure vessels in a two-stage array (17 in the first stage and 9 in the second stage).

Pretreatment of the groundwater includes scale inhibitor and sulfuric acid feed systems along with cartridge filters. Feed pumps pressurize water for passage through the reverse osmosis membranes. Membrane product water (permeate) is blended with water from the San Diego Formation. This blended water is then post-treated before it enters the distribution system. Post treatment incorporates air stripping, pH adjustment, and chloramination. A cleaning system is provided to clean the membranes as needed.

Approximately 0.8 MGD of brackish concentrate (membrane process residual brine) is produced as a byproduct of the demineralization process. The TDS of the concentrate ranges between 7000 to 9000 mg/L. The brine is discharged to the Sweetwater River which drains to the San Diego Bay. The TDS of San Diego Bay is approximately 40,000 mg/L and seawater is generally 35,000 mg/L.

Discharge of the brine involved approval from the following regulatory agencies: Bureau of Reclamation, the U.S. Fish and Wildlife Service, and the California Regional Water Quality Control Board. In addition to the usual NPDES permit requirements, Sweetwater Authority has implemented a mitigation and monitoring program to ensure that the project does not adversely affect biological resources. This program includes an Alluvial Basin Management and Monitoring Plan to predict, detect and mitigate the potential risk of adverse impacts to the vegetation communities along the Sweetwater River and a Concentrate Monitoring Plan to monitor the salinity, nitrogen, nitrate, and chlorophyll A levels in the Sweetwater River.

This program includes upstream monitoring (alluvial groundwater water quality, soil moisture, stream gages, and vegetation monitoring) and downstream monitoring (water quality sampling along the Sweetwater River channel to the San Diego Bay, vegetation monitoring, and nitrogen mass loading modeling). Other monitoring includes surveys for the endangered light-footed clapper rail. This bird typically breeds in salt marshes as well as fresh and brackish water habitats. The monitoring program is designed to determine if operation of the Demineralization Facility adversely affects the clapper populations along the Sweetwater River.
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Document Type: Research Article

Publication date: 2001-01-01

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