Soothing water woes with desalination
Growing demands. Diminishing supplies. A state mandate to find a remedy. That was the situation in Newport News, Va., where, in 1993, the city began looking for an additional water source for its 350,000 customers.
The city needed a fast-track solution, says Ronald Harris, chief of water resources development for Newport News. “We needed something operating within three to five years, and we needed it within our local jurisdiction,” he explains. By 1998, the city had tapped a brackish groundwater source and had begun treating the water at a quickly implemented, 6-mgd desalination plant.
Although it is not a long-term answer for Newport News, desalination gives the city time to discover a better alternative. “While treating brackish water is only an interim solution for us, it is introducing a feasible six- to eight-year window [in which we can find] a new source,” Harris says.
Like Newport News, many U.S. communities are pushing the limits of potable water supplies as they grapple with growing populations and extended dry seasons. However, with new technology that makes desalination more economical than it once was, some of those communities are turning to brackish water to remedy their shortages and open reserves.
Distillation vs. membranes
Desalination is not a new concept. It has been used extensively in Africa and in the Middle East, and it was used as early as the mid-1800s in the United States, where military ships employed the technology to provide drinking water for sailors. However, it has undergone notable changes – primarily with the introduction of membrane technology – that have made it more feasible for U.S. cities and counties to adopt.
The desalination process separates saline water into two streams: one with a low concentration of dissolved salts (freshwater) and another containing the remaining dissolved salts (concentrate). The oldest method of desalination is thermal distillation, during which seawater is boiled, releasing steam. Evaporation separates dissolved solids and minerals from the water, and the condensed vapor results in purified, salt-free water.
Internationally, distillation is used extensively to treat full-strength seawater. The first municipal desalination plant in the United States – built in the 1940s in Key West, Fla. – employed distillation, but the use of membrane filtration is more prevalent in modern U.S. facilities.
Reverse osmosis and electro-dialysis – based on separation rather than on distillation – make up the membrane filtration menu. Reverse osmosis forces water through a semi-permeable membrane that rejects a large percentage of dissolved salts and minerals, producing relatively pure water.
Electro-dialysis uses ion-specific membranes that are arrayed between anodes and cathodes to drive salt ions in controlled migrations to the electrodes, leaving freshwater behind.
Membrane filtration is relatively young and is still evolving, but its popularity is increasing steadily. (Brackish groundwater is abundant in the United States, and it has a low salt concentration: fewer than 10,000 parts per million [ppm] of total dissolved solids compared to 35,000 ppm of total dissolved solids for seawater. Membranes are effective in filtering the low level of solids in brackish water, and they are generally less expensive to operate than is distillation.)
Each desalination process has its advantages, and the key to a cost-effective and efficient system is to apply the appropriate technology based on the feedwater. Distillation’s advantage is that pretreatment requirements are lower than those for reverse osmosis, since coagulants are needed to settle out particles before water passes through membranes.
On the other hand, membranes have lower energy requirements than does distillation, and they remove contaminants that distillation alone does not. Additionally, while distillation relies on thermal processes, membranes do not, meaning the feedwater does not need to be heated.
When Tampa, Fla., began examining desalination in 1996, it considered distillation as well as membrane filtration. “We asked several private developers to look at the most innovative ways to provide quality water while minimizing the financial implications,” says Donald Lindeman, project manager for Tampa Bay Water. Because of the minimal pretreatment requirements and enhanced contaminant removal supplied by membranes, that technology was chosen to meet quality and supply needs, as well as permitting requirements, he explains. By the end of 2002, Tampa Bay Water will operate a reverse osmosis facility that will handle 25 mgd and supply 10 percent of the community’s water.
Tapping brackish sources
America’s population tends to concentrate in coastal areas, where the development of adequate water supplies often is complicated by: * the proximity of the community to tidal salt water; * the environmental impact of surface water withdrawals from sensitive river ecosystems; and * potential salt water intrusion in coastal groundwater due to excessive pumping.
All those concerns exist in Florida, along with the need to conserve groundwater for preservation of wetlands and lakes. As a result, several Florida communities – for example, Cape Coral, Hollywood, Sarasota County and Indian River County – have used desalination in an attempt to reduce their freshwater usage.
Tampa’s planned desalination facility (it will be the largest straight seawater plant in the country) is one of nine water resource projects that will incrementally reduce the city’s groundwater pumping from 158 mgd to 90 mgd by the end of 2007. The city also is studying the feasibility of developing small desalination plants to treat brackish groundwater and developing surface water enhancements – projects that could add 85 mgd to 111 mgd to its potable water supply. “We’re looking to diversify our sources of water,” Lindeman explains.
Similarly, in Fremont, Calif., the Alameda County Water District has proposed building a 5-mgd reverse osmosis plant to treat brackish water and supplement its water supply. For years, the district has withdrawn brackish groundwater and discharged it into San Francisco Bay to prevent salt water from intruding into fresh groundwater. That process will continue; however, if the plant goes on line as planned in 2002, the district will be able to reclaim the water that is now being pumped into the bay, and, after treating the water, use it to supplement freshwater supplies. “By recovering this water, we [will] have a new resource within our groundwater basin and within our control,” says Anna Lloyd, project manager for the district.
Not for everyone
Although many communities have implemented desalination to their advantage, others may find that it is not a feasible option for them. >From salt content of the feedwater to requirements for concentrate disposal, there are several factors that can affect cost-effectiveness.
Virginia Beach, Va., suffered water shortages for nearly 20 years. It often received inadequate amounts of water from its supplier, neighboring Norfolk.
>From 1978 to 1982, the city studied several alternatives, including >desalination, for establishing a reserve water source. A cost analysis >showed that desalination of seawater would be prohibitive for the city, >and the area’s supply of brackish groundwater was limited. “Mother Nature >seemed to be working against us,” says Thomas Leahy, water sources manager >for the Virginia Beach Department of Public Utilities.
Because of those limitations, the city rejected desalination and chose to implement a surface water project instead. By 1998, it had completed installation of a 76-mile, 60-inch diameter pipeline that transfers up to 60 mgd of water from existing hydroelectric and flood control impoundments to existing reservoirs owned by Norfolk.
In addition to affecting costs at the front end of desalination, salt levels also can present problems at the back end of the process, when cities must dispose of the concentrate. The concentrate must be discarded in an environmentally appropriate manner consistent with national pollutant discharge elimination system permits. “Aside from costs, concentrate disposal is one of the biggest hurdles to constructing large-scale [desalination] plants,” Lindeman says.
While facilities located near a saline body may encounter few troubles discharging salt into the sea, owners must pay attention to added constituents, dissolved oxygen and water temperature. Furthermore, municipalities cannot simply dispose of concentrate into receiving streams, the gulf or wetlands.
Disposal is an essential planning element. It may involve dilution, deep-well injection or transport to a suitable disposal site. Tampa Bay Water will blend concentrate with power plant cooling water and discharging the mixture into Tampa Bay. Meanwhile, Alameda County is in the process of obtaining permits to discharge its concentrate into flood control canals.
Cost dropping, efficiency rising
Over the last 20 years, the cost of desalination has dropped significantly. For example, in the late 70s, the cost of desalinating seawater was $8 per 1,000 gallons, compared to $3 per 1,000 gallons today. Similarly, it cost cities and counties $2 per 1,000 gallons to desalinate brackish water, compared to $1 per 1,000 gallons today. The changes are due primarily to the implementation of membranes and energy recovery devices.
Tampa expects to take advantage of those cost efficiencies when it builds its new seawater facility. By locating the plant at Tampa Electric’s Big Bend Power Station, the city will take advantage of existing operational resources to bring its desalination costs to a projected $2.08 per 1,000 gallons. The facility will use the power plant’s pretreated cooling water for its source water, and it will use the existing intake and discharge structures.
Because desalination relies primarily on the laws of physics to accomplish its work, technological breakthroughs are unlikely to be sweeping. However, significant changes are on the horizon. For example, membrane distillation (combining reverse osmosis with a thermal process) and continued evolution of membranes will further reduce the pressure – and energy – required for desalination. Additionally, turbines and work exchangers are being implemented to recover some energy by recycling concentrate pressure back to the head of the process.
Growing populations and changing weather patterns – along with today’s emphasis on aquifer management and ecosystem/marine life preservation – will ensure continued interest in freshwater conservation and alternative water sources. According to Harris, desalination can play an important role in expanding cities’ choices.
“There are such large amounts of unused water available. We need to get out of the mentality of living in a fresh groundwater veneer,” he says. “Freshwater lakes and rivers are merely the surface. There are large amounts of seawater and brackish groundwater than can be tapped.”
The authors work for Cambridge, Mass.-based Camp Dresser & McKee. William Suratt is a professional engineer in the Fort Lauderdale, Fla., office; Mark Maimone is an environmental engineer in the Woodbury, N.Y., office; and Thomas Missimer is a professional geologist in the Fort Myers, Fla., office.
