home
Water Industry News

Scarce water, decreasing costs push desalination to fore


December 19, 2004

As Californiaís population grows, more cities are considering drinking ocean water. Declining water supplies and advancing technology are pushing water districts all along the California coast to consider desalination.

The idea of making fresh water from a salty source is not new ó various countries in the Middle East use desalination, and plants exist in California, Florida and Texas. But in the United States, the amount of clean water produced by desalination is tiny compared with plans for the future.

Nine desalination plants are proposed on the shores of the Monterey Bay National Marine Sanctuary. Monterey County is considering proposals for one that would produce about three times as much clean water as is produced now by all the plants in the state combined, according to a 2004 California Coastal Commission report.

Two are being studied close to home ó one that would be built by the city of Santa Cruz for use during drought years and another in Moss Landing.

While planning is under way here, the Marin Municipal Water District is taking a more active approach. The district will begin building a small pilot desalination plant this month.

This plant, which uses a process called reverse osmosis, will test the feasibility of producing clean drinking water from the salty, sediment-filled water of San Francisco Bay.

Marinís decision was one of necessity. Recent dry winters have not replaced water the district has been drawing from its local reservoir, said Rob Theisen, a general manager of the Marin district.

"We find ourselves now in a bit of a deficit," he said.

The district had intended to buy extra water from Sonoma County Water Agency, using $37.5 million in bonds approved by Marin voters in 1992 to build a new pipeline to carry the water south to Marin.

But "about two years ago, it became obvious that SCWA could not provide enough," Theisen said.

Cost concerns
Marin officials had considered desalination more than a decade ago. In 1990, the district built and ran a small plant to test its feasibility as a water source. The plant provided clean, good-tasting water, but it cost much more than importing water from Sonoma.

Almost 15 years of technological advances have made the prices much more comparable.

"With more and more individuals, communities, even countries getting involved in desalination, the costs have been improving," Theisen said.

Energy is the largest desalination cost, making up one-third to one-half the price of the cleaned water, according to the Coastal Commission report. But thatís a significant improvement over 10 years ago. Now, the membranes used to filter water need much less energy, and techniques have been developed to recycle some of the energy used in filtering.

In the past decade, because the price of buying fresh water has increased and the price of cleaning salty water has decreased, desalinated water has gone from costing about 300 times as much as imported water to just 1.5 to two times as much.

The smaller increase in price over imported water is something that Marin, and many other water districts, are willing to consider to be assured a consistent, reliable water source.

For its second pilot plant, Marin approved $1.2 million to build on a 40-by-80-foot plot near the Richmond-San Rafael Bridge. The plant will run for nine months beginning next year, testing the technology and providing the district a small amount of clean water.

A lengthy process
Making bay water clean enough to drink is a multi-step process.

First, the water must pass through a filtering system to remove any small floating particles. These bits of dirt and debris must be removed from the water before it gets to the finer levels of purification, otherwise "itís kind of like taking a water treatment process and putting golf balls in it," Theisen said.

Next comes reverse osmosis, which cleans water on a molecular level. The water is pumped up to a very high pressure, "probably 20 times what you have in your pipes at home," said Mike Armstrong, general manager of the Marina Coast Water District, which maintained its own small desalination plant until the main motor broke last year. The pressure needed for reverse osmosis is about the same amount of force you would feel if you tried to balance an adult elephant on the palm on one hand.

At such a high pressure, water can be pushed through a membrane with holes so tiny only water molecules can fit. About half the water gets forced though, and the other half is used to wash away anything too big to get across the membrane. At the Marin plant, some of the newly cleaned water will be pushed through the reverse osmosis system a second time to remove any tiny amounts salt that managed to get through the first time.

Reverse osmosis "produces some of the cleanest, best water youíve ever seen, just because it removes just about anything and everything," Armstrong said. It actually removes so much that the water becomes slightly acidic, making it taste "pretty flat," he said. That is corrected by adding lime and carbon dioxide, and chlorine to disinfect the final product.

About 90 percent of people who taste-tested the desalinated water from the Marin plant preferred it over fresh water from a local reservoir, Theisen said.

Safety issues
As good as the water might taste, some people have raised questions about the health and environmental impacts of the proposed plant.

The state Office of Environmental Health Hazard Assessment has advised against frequent consumption of fish from San Francisco Bay because of pollutants such as mercury and DDT in the water. This raises concerns that desalinated water also will be polluted. But the plant does not draw its water from the bottom, where most pollutants are concentrated, Theisen said, and the filtering processes would remove any pollutants that did get drawn into the plant.

Another concern is the plant produces wastewater twice as salty as normal bay water, which some fear would kill life in the bay.

But the Marin district has worked out a plan to mix desalination wastewater, which is too salty, with effluent from a nearby sewage treatment plant, which is too fresh. The combination will be released into the bay a mile from shore.

In fact, Marin water officials hope that a full-size desalination plant would actually help protect the environment by preserving the water level of Lagunitas Creek, which Theisen calls one of the best nurseries for salmon on the West Coast.

A drawback to desalination is the cost. A dollarís worth of water from a freshwater reservoir would probably cost about $1.50 to extract from the bay, said Theisen. But if desalinated water accounts for only one-tenth the total supply, as planned in Marin, customers would see an increase of only about five cents for every dollar of their bill.

At least some customers would be willing to accept that increase.

"Iíd be willing to pay a little more money for good-quality water," said Marin real-estate agent Barry Crotty, "and Iíd be less inclined to waste it if it cost me more."

Success at Marinís trial plant would pave the way for a much larger plant that ideally would be designed, approved and built by 2008, Theisen said. The plant would provide as much as 10 million gallons a day, enough to keep 11,000 households happily drinking, showering, and watering their lawns for as long as the Pacific Ocean stays in existence.

 

Contact Elise Kleeman at llabarth@santacruzsentinel.com