The cost and effectiveness of reverse osmosis depends on the source of the water. For brackish ground water, which typically has about 10% the salt content of seawater, the pumps operate at the relatively low pressure of 100 to 200 pounds per square inch, and the cost can be on the order of $250 per acre-foot (325,000 gallons, or more than enough to provide water for two families for a year). By comparison, the cost of imported water in Southern California is about $230 per acre-foot.
The Orange County Water District operates a 15-million-gallons-per-day reverse osmosis plant in Fountain Valley that cleans waste water from sewage treatment plants. The water is injected into the ground to provide a barrier between underground reservoirs in the county and ocean water. That barrier prevents seawater from contaminating the reservoirs.
Much of the water injected into the ground flows inland and is pumped out again for use in the water system. Orange County plans six other desalination facilities that will bring the reclaimed water total to 75 million gallons per day, according to William R. Mills of the water district. All will treat waste water, not salt water.
For desalinating seawater, pumps must operate at pressures of 1,000 pounds per square inch or more and the cost can be four times as great as that of desalting brackish water, according to Randy Truby of Fluid Systems Corp., a membrane manufacturer in San Diego. Nonetheless, reverse osmosis plants often are desirable because they can be constructed in small modules that are rapidly assembled.
On San Nicolas, for example, the Navy installed a 12,000-gallon-per-day modular unit similar to those used by the Marines to purify water in the field, according to Lt. Cmdr. Gene Okamoto of Point Mugu. The unit was modified at Port Hueneme to provide around-the-clock operation. A second module of the same size will be installed later this year.
The San Nicolas units produce fresh water at a cost of $1,625 per acre-foot, Okamoto said. That is expensive, but it is substantially cheaper than the cost of barging water to the island, the only viable alternative. The two units will provide all the water needs for the 60 enlisted personnel and 165 civilians who track missiles from the Pacific Missile Test Center at Point Mugu.
Speed of construction was also a factor in choosing reverse osmosis for the installations at Santa Catalina and Santa Barbara. The 132,000-gallon-per-day unit on Catalina will service a new condominium development as well as provide about a quarter of the island's total water. The unit will be built by Hamilton Cove Associates, which is building the condominiums, but it will be owned and run by Southern California Edison, which operates all utilities on the island.
Like the water on San Nicolas, Catalina's is expected to cost about $1,625 per acre-foot. In this case, however, residents already are paying $2,600 per acre-foot for water from wells and catchment basins, so the new source will be cheaper.
Speed is especially important to Santa Barbara, where water resources have literally evaporated. After the permit process is completed within the next month or so, construction of the plant by Ionics Inc. of Watertown, Mass., is expected to take no more than 15 months. Water from the plant is predicted to cost about $1,900 per acre-foot and will increase the average Santa Barbara water bill, now about $20 per month, by 50%.
Other communities that have been exploring the possibility of using reverse osmosis to desalt seawater include San Luis Obispo, Monterey, San Diego, Morro Bay, Ventura and Oxnard.
For large quantities of water at a lower price, however, it almost certainly will be necessary to build distillation plants, probably in conjunction with an electric power plant. In this way, the heat remaining in steam after it has been used to generate electricity could be used to heat seawater for distillation, thereby lowering the cost of energy substantially. Engineers rate the efficiency of distillation plants according to the number of gallons of fresh water that can be produced from every 1,000 British thermal units (BTUs) of energy used.
The so-called multistage flash distillation plants in the Middle East use a variety of tricks developed in U.S. laboratories--including recapturing the heat given off when steam condenses and carrying out part of the evaporation at lowered pressures--to produce about eight or nine pounds of water per 1,000 BTUs.
The efficiency of such plants can be boosted to as much as 13 pounds of water per 1,000 BTUs, but the Saudis and other nations rich in natural gas have not seen the need to pursue high efficiencies.
"The Saudis are very conservative in design and operation," said Oak Ridge's Eissenberg. "They're not interested in improving the design, but in replicating a proven design."
