Cost-Effective SWRO Brine Disposal Using an Existing Effluent Outfall

For seawater desalting to produce potable water, brine disposal may be a paramount challenge. Some communities have a potentially valuable future opportunity because they have a wastewater effluent outfall with available capacity. Effluent discharges are already regulated through national pollutant discharge elimination system (NPDES) permits, typically set based on a dilution credit, resulting from diffuser performance during worst case conditions. To address the need for dependable water supply the City of Santa Cruz (City) and Soquel Creek Water District (District) have joined together to plan, permit, construct, and operate a seawater desalting facility using membrane technology (SWRO). This paper reports a technical analyses and planned facilities modifications to the City’s effluent outfall for brine discharge. The desalination plant, as described in the program-level Environmental Impact Report, would proceed with an initial production flow of 2.5 million gallons per day (mgd), with the potential to expand to 3.5 and 4.5 mgd, if required. The City and the District plan to combine brine with effluent from the City’s Water Pollution Control Facility (WPCF) before disposing it through the City’s existing ocean outfall. An evaluation determined how such a combined discharge will still achieve minimum initial dilution ratio (MIDR) (seawater to effluent) of 139:1 required by the City’s National Polluti on Discharge Elimination System (NPDES) permit for the WPCF and how the brine discharge must be modulated to achieve continual permit compliance.

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