Desalination and Water Purification Research Program Results
Significant accomplishments from the DWPR.
Industry standard 16-inch elements: In 2003, DWPR funded a consortium of membrane manufacturers located in California and Minnesota to evaluate and establish a “standard” diameter for large reverse osmosis elements. This would be added to the 4-inch and 8-inch diameter elements which have been in common usage even in large capacity plants today. The market recognized that it would be best to have a common diameter for replaceability. This 16-inch standard is being adopted for large capacity plants such as Singapore's 2.6 million gallon per day Power Seraya project installed in 2007, and the new 50+ MGD Sorek, Israel, desalination plant scheduled to open in 2013. The membrane elements are being supplied by Hydranautics Water Systems.
Testing of slant wells for seawater intake in Orange County, California: This novel approach to seawater intake under the seafloor avoids environmental issues like impingement and entrapment and is planned for use in a new seawater desalination plant.
Membrane bioreactor systems (MBRs): Evaluation of MBRs was carried out in San Diego, California. MBRs are a newly developed membrane systems used for treatment of wastewaters that are simpler and more compact than conventional activated sludge plants or oxidation ditches. The testing funded by DWPR appears to have given potential users the confidence to invest in MBRs, essentially creating a new industry. Technologies demonstrated include those from eight major manufacturers: (Zenon Municipal Systems, Mitsubishi Rayon Corporation, Kubota Corporation, Lyonnaise des Eaux/Degremont, Koch Membrane Systems, Huber Technology, Kruger Inc. and Parkson Corporation).
Membrane Fouling: Excellent scientific work has been performed at Yale University in New Haven, Connecticut. This has defined operating conditions to minimize fouling and cleaning conditions for removal of fouling layers.
Concentrate Disposal: Eastern Municipal Water District in Perris, California, carried out a landmark comparative study of how to dispose of concentrate, the salt remainder from an inland desalting plant. In a separate effort, Mickley and Associates developed the background literature for disposal of concentrate in the United States, which is widely used and frequently referred to.
Zero discharge desalination (ZDD): Concentrate disposal is being made more effective by work at the University of Texas at El Paso. The ZDD process is a response to the problem of disposal of concentrate from desalting plants. This is a complex process in which salts are manipulated in a manner to precipitate out salts of low solubility in a fairly pure form for possible reuse. This process has been proof tested at Panoche in California and is currently being demonstrated at El Paso, Texas, and to be demonstrated at Alamogordo, New Mexico. This process appears to be of interest to the city of Alamogordo.
Direct contact membrane distillation (DCMD)
In DCMD, currently being developed by New Jersey Institute of Technology, hot brine flows on one side of a gas-filled porous, hydrophobic hollow fiber membrane and cold distillate flows on the other side of the membrane; water vapor evaporated from hot brine is recovered in the cold distillate stream by condensation. This is shown in figure 4. Appropriate high performance hollow fiber membranes have been developed for this process. A small module was proof tested in 2004 and 2005. Some commercial interest has been shown in this process. What needs to be done is to convert the membrane containing element into a reasonable commercial item.
Membrane filtration for pretreatment of seawater
A DWPR project in Corpus Christi, Texas, was among the first to demonstrate that membrane filtration provided more reliable pretreatment for a seawater reverse osmosis plant than conventional clarification and media (sand) filtration. Membrane filtration (ultrafiltration and microfiltration) avoids many of the difficulties experienced by conventional systems when faced with filtering seawater of varying composition and turbidity resulting from heavy weather events and natural seawater seasonal variations. This process is now used throughout the world when pretreating challenging seawater.
The Natural Freeze-Thaw process: This process was originally developed in North Dakota. Originally proposed to be a means of extracting water from saline groundwater using the natural temperature cycle as the driving force, it is currently commercialized as a process to reduce the quantities of impaired water produced by coal bed methane production. Developed with North Dakota winters in mind, it has been shown to be successful as far south as Northern New Mexico. It is currently being commercialized as the Natural Freezing Process by BC Technologies. See the map of areas where this process might be appropriate.
Appropriate Areas for the Natural Freezing Process Shown in Blue
Wind Powered Desalination: A study of the relationship between windpower and water desalination carried out using data from Hull, Massachusetts, indicated that seawater desalination powered by an offshore wind farm would be economically practical.
Other Accomplishments: It is not always possible to determine the impact of a project. A distillation process called “dewvaporation” was developed to the point where it is now a commercial product. A spray cleaning process was developed that appears to have had use in cleaning membrane elements used in whey treatment, but not yet in water plants. Two separate projects relating to wind-driven seawater desalination make economic sense, but no plants have yet been built. Similarly, a small photovoltaic driven unit was developed to desalt brackish water for small isolated communities. The very promising VARI-RO™ Direct Drive Engine project was taken to, but not through, the demonstration stage. A cross-flow hollow fiber membrane module suitable for mounting in an 8-inch pressure vessel was developed. The figure below shows a view of a 30-card packet from this development. A project in which dendrimers were periodically injected into the feed of a reverse osmosis system demonstrated that boron rejection could be substantially increased; this implies that materials could be developed to improve rejection of other specific problem components without requiring development of new membranes.
Packet showing hollow fiber membrane mounting system