Water, Design-Build, North America, United States, Government, Membranes
Hold the Salt
Innovative Seawater Desalination in California
For years, the state of California, USA, has struggled with drought conditions. Traditional water supply sources are inadequate to keep up with growing populations. Southern California relies on water that is pumped hundreds of miles from the Colorado River and northern California. It is a limited supply—costly and energy-intensive to deliver—that is at risk of being cut off entirely by a major earthquake.
To make seawater desalination a viable solution ... we can help our clients develop the right, specific treatment approach for each situation.
With the ocean being California’s only unlimited water supply, seawater desalination plants are increasingly being evaluated and developed throughout the state. However, seawater desalination has its challenges—stringent permitting requirements, and public concern over environmental impacts and costs, have delayed many of the proposed facilities.
The Sand City desalination plant uses reverse osmosis and ultraviolet light to treat a highly brackish seawater blend.
CDM Smith has been at the forefront of making seawater desalination a viable and sustainable option in California. The city of Sand City selected the firm to design-build a 0.6-million-gallon-per-day (mgd) desalination plant. With the first full-scale desalination plant in California to receive permitting approval under new regulations—and the only municipal seawater desalination project that operates continuously—Sand City has set a new standard.
A unique solution allowed CDM Smith and Sand City to obtain the required permits from 20 different agencies, and complete the project for almost half the price of the original design proposal. The plant uses reverse osmosis (RO) and ultraviolet light to treat a highly brackish seawater blend drawn from four vertical beach wells. The city selected this intake approach to avoid adverse environmental effects and provide a reduced level of salinity than pure seawater, making treatment more energy efficient and less costly.
Other cost savings resulted from several energy-efficient sources, including recovery devices that reduced the size of feed pumps by more than 70 percent; simplified chemical systems to minimize pretreatment; elimination of a lime feed system by using an operator-friendly limestone contactor; and the use of 90-percent efficient positive displacement pumps that can treat a wide range of salinities without variable frequency drives.
According to Greg Wetterau, CDM Smith associate, “California’s permitting process for a new desalination plant is complicated, but these strategies can make them realistic and sustainable options. Sand City is the first in a new wave of plants to be built in California. It has performed better than anticipated and is considered by many in the environmental community to be the right way to do seawater desalination.”
A Public Approach
Community outreach programs for the Santa Cruz desalination facility educate visitors on desalination, energy issues and the local water shortage.
In Santa Cruz, CDM Smith executed the design-build-operation of a pilot plant to evaluate treatment options and energy-saving measures for a seawater desalination facility fed by an open ocean intake. The pilot included four different pretreatment processes in parallel, upstream of four different RO systems, to determine the best pretreatment approach to minimize RO fouling.
After a year of pilot operation, CDM Smith is designing a full-scale, 2.5-mgd facility. According to Heidi Luckenbach, Santa Cruz desalination program coordinator, “The pilot project clearly demonstrated that the technologies can successfully meet and exceed state and federal water quality standards to produce a reliable supplemental supply for our community.”
An outreach program grounded in successful pilot results has been vital to engaging and informing the community on the critical need for a new water supply. The program included daily tours of the pilot plant to educate visitors on desalination, energy issues and the local water shortage. Residents and special interest groups also attended informational meetings to learn about the project and ask questions of agency staff and consultants.
In Dana Point, the first slant well desalination project is being developed by the Municipal Water District of Orange County (MWDOC) and other local agencies. Unlike Sand City’s vertical beach wells, the South Orange coastal ocean desalination project uses a slant well—pulling water through a sand and gravel aquifer under the ocean floor. These wells have the potential to draw much higher volumes of water than vertical wells, with minimal impacts to onshore aquifers and no risk of entrainment or impingement of marine life.
CDM Smith is coordinating the 18-month operation of a pilot facility, treating a portion of the water from the 2,200-gallon-per-minute demonstration slant well and leading the evaluation of pretreatment approaches to address the water’s unexpectedly high iron and manganese concentrations. Testing results have been positive, offering MWDOC an approach that is environmentally sensitive, reliable and less costly than a traditional seawater treatment approach.
Case by Case
Coastal communities in California and throughout the world are seeking alternative water supply options as they face drought, population increases and growing environmental challenges. “To make seawater desalination a viable solution, we have to overcome concerns that it is unreliable and unaffordable. We want to help clients develop new, sustainable supplies,” says Wetterau. “By gathering enough information and being creative in our solutions, we can help our clients develop the right, specific treatment approach for each situation.”