Quenching a Coastal Community’s Thirst
Related search terms from www.waterinfolink.com: desalination, brackish water, RO
While Sand City, Calif., is home to only a few hundred residents, this Monterey County coastal community is a popular shopping and tourist destination, seeing as many as 40,000 people in a day and employing more than 4,000. Faced with a diminishing water supply, the city selected CDM to design-build a desalination facility to provide drinking water and facilitate city-wide redevelopment.
The plant—which uses reverse osmosis (RO) and ultraviolet (UV) disinfection to treat highly brackish seawater—produces up to 600,000 gal per day of potable water. Source water is drawn from four subsurface beach wells, which were selected to avoid adverse environmental effects and to provide source water at a reduced level of salinity.
Desalination Permitting
Permitting for seawater desalination facilities has been a complex process in California, delaying the implementation of nearly all of the currently planned facilities. The Sand City desalination facility is the first full-scale, municipal desalination plant in California to receive permitting under the new surface water treatment regulations. To execute the project, several permits were obtained to protect groundwater supplies, marine species, sensitive habitat areas and public safety.
To minimize challenges and ensure that permitting would be secured, a conservative approach was taken on the project. Efforts included the use of subsurface intakes rather than drawing from an open intake source; the use of subsurface brine discharge, which limits the salinity of brine to below seawater salinity; and providing filtration and disinfection for Giardia and Cryptosporidium viruses—achieving removal requirements determined by the California Department of Public Health and the U.S. Environmental Protection Agency Long Term 2 Enhanced Surface Water Treatment Rule (LT2).
Treatment Process
The subsurface intake wells supplying the facility’s source water are located 2,500 ft from the plant, in a coastal region. In addition to protecting the environment and public health, five years of testing have demonstrated that the beach well sources have reduced salinity in comparison to seawater, making treatment less energy-intensive and less costly. Total dissolved solids levels vary seasonally between 17 and 28 g/L, while the salinity of Monterey Bay seawater is 35 to 36 g/L.
The water passes through cartridge filters followed by single-stage seawater RO membranes for primary treatment. RO permeate is treated with UV disinfection and then chlorinated and stabilized using carbon dioxide, a calcite contactor, caustic soda and blending with existing distribution system water. Concentrate from the desalination process is sent to two redundant subsurface discharge wells. To simplify permitting, the city opted to ensure that concentrate does not exceed the salinity of Monterey Bay seawater, eliminating the risk of localized salinity increases in the bay.
Resource & Cost Preservation
With seawater desalination often criticized for being too expensive and energy-intensive for practical implementation, it was important that effective measures were taken to reduce energy and operations costs for the Sand City desalination facility.
The design-build approach allowed for close collaboration between Sand City, the design team and the constructors, enabling significant changes to be made during design and after the start of construction. Making adjustments during these stages of project execution increased efficiency, improved reliability of the facility and simplified future operation.
In addition, unique design and construction approaches and the use of a 50% Proposition 50 grant administered by the Department of Water Resources will allow the facility to produce water at an expected average cost of less than $3 per 1,000 gal.
These primary factors are responsible for the facility’s significant cost savings:
- A reduced RO recovery rate of 33% (50% is a typical seawater RO recovery rate) results in lower feed pressures and improved product water quality, while also avoiding the need to dilute waste concentrate under most operating conditions.
- Energy recovery devices reduced the size of the feed pumps by more than 60%, which improves plant efficiency and reduces long-term operating costs.
- A single-rack RO skid design with four isolatable columns of pressure vessels reduces the required footprint and interconnection piping compared with using four individual skids.
- Chemical systems were simplified by eliminating acid, using calcite contac-tors rather than slaked lime and using post-treatment blending to assure that the product water is compatible with the highly variable system water.
- The plant will only operate during off-peak electrical usage hours, resulting in significant cost savings through a low power demand reduced-rate structure offered by California Edison.
As California’s population continues to grow, drinking water sources become scarce and seawater desalination becomes an increasingly common supply alternative, it is hoped that the Sand City desalination plant will be a model for other coastal cities.