Strategies for Treating Variable Source Water
Project ID: 9316
Principal Investigator: Michelle Chapman
Research Topic: Desalination and Water Treatment
Priority Area Assignments: 2011 (Advanced Water Treatment), 2012 (Advanced Water Treatment), 2013 (Advanced Water Treatment)
Funded Fiscal Years: 2010, 2011, 2012 and 2013
Desalination of locally available saline waters offers the possibility of increased water supply from otherwise unusable sources. A number of places exist in the United States where both brackish water and sea water are available. A water supply developer will normally choose to desalt brackish water over sea water because the unit cost of product water will be substantially lower, typically about half that of desalted sea water. For a brackish water plant, the construction and energy cost is lower, and the fraction of water recovered from a given feed source is higher. Therefore, sea water desalting plants are a more expensive water source.
However, the reliability of the water source must be considered. Situations exist, particularly along the Gulf Coast of Texas and the Pacific Coast of California, where the brackish water source may be available only for part of the year. For such a location, a brackish water desalting plant would prove to be an unreliable water source and would need to be supplemented by sea water, a reliable source of water.
Current treatment processes are typically designed for a specific character of water. A plant designed for brackish water cannot be used to desalt sea water and vice versa.
The question is: "Can a process be devised to adapt a desalting facility to highly variable sources of water that would combine the economy of a brackish water plant with the reliability of a sea water plant?"
Need and Benefit
The Texas 2007 State Water Plan has identified a shortage of 8 million acre-feet annually by 2060. The State Water Plan also states that 800,000 acre-feet of this shortfall will be provided by desalination and another 1.6 million acre-feet provided by water reuse. As the variability in the climate increases, it is critical that treatment of new sources of water be developed.
Variable source water occurs in the following situations:
- Locations where brackish ground or surface water can be obtained during much of the year but where only sea water is available during dry periods.
- Communities that could use water from a variety of sources - either multiple wells of different composition or combinations of surface water, storm water drainage, municipal wastewater, and sea water.
- Coastal communities that could treat estuarial water where salinity varies widely with the tides. This occurs frequently in the gulf coast region.
Building a desalination unit to treat brackish water and sea water alternatively presents several challenges:
1) A typical sea water system contains only one stage because only 40-50 percent recovery is practical. It may also contain a full or second pass to further reduce concentrations of certain components to meet product water standards. By contrast, a typical brackish water unit contains two or possibly three stages depending on scaling limitations, discussed below. This comparison indicates that the flexible plant will differ from conventional sea water or brackish water plants.
2) Operationally, there is a great benefit from having the plant use all of the membrane elements regardless of whether the plant is in sea water or brackish water mode. This avoids the work involved in element removal and storage.
3) The pumping system for the plant will be more complex than that for either a sea water or a brackish water plant. Typically sea water plants use an inlet pressure of 600 to 900 pounds per square inch (psi) while brackish plants use 150 to 300 psi. Details of this will be worked out during Phase I.
4) Most conventional sea water plants are now built with energy recovery devices. The energy recovery device for any system is designed for a certain flow at a certain pressure, the energy from which is used to lift the feed water to high pressure. For the flexible unit, selection of equipment and operating conditions may not be optimal for either mode, but should be optimized for overall plant operation.
5) Sea water is generally not of a scaling nature. The salt is predominantly sodium chloride, which is exceptionally soluble. When ionic strength is as high as in sea water concentrate, the most stable form for ions is in solution and not as solids. Brackish sources that are not derived from sea water, however, tend to be high in calcium, carbonates, and sulfates that do form scale. The system would include an anti-scalant feed system that would be used when scaling brackish water is treated.
The study referenced above includes a diagram showing how a two-stage, high recovery system for desalting brackish feed water can be converted to a two-pass, medium recovery system for desalting a seawater feed. Although not shown in the diagram, the same energy recovery device can be used in both operating modes to provide energy conservation benefits. The best pumps for this unit would probably be variable speed drive pumps. Alternatives may be pumps with variable staging so that their optimum efficiency point can be changed from high pressure, low capacity for sea water to lower pressure, higher capacity for brackish sources.
Water savings are obtained by operating the system in sea water mode at a recovery of 40-50 percent and using a recovery of 75-90 percent for brackish water.
Oklahoma-Texas Area Office, Great Plains Region
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This information was last updated on December 6, 2013
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