Review of the Desalination and Water Purification Technology Roadmap (2004) / Chapter Skim
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3 Key Technological and Scientific Issues for Desalination
3 Key Technological and Scientific Issues for Desalination
Pages 24-53
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From page 24... ...
The impacts of energy use will need to be examined for desalination plants to become more widely used. While research and technological developments continue to reduce the costs of desalinated water by optimizing performance, additional cost reductions may be more difficult to achieve, especially as many current systems are already operating at high efficiencies.
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From page 25... ...
Membrane technologies have the potential to contribute to water supplies through their use in treating degraded waters in reuse or recycling applications since membrane technology can remove microorganisms and many organic contaminants from feed water. Compared to thermal distillation processes, membrane technologies generally have lower capital costs and require less energy, contributing to lower operating costs.
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From page 26... ...
. These cost reductions have occurred through economies of scale and improvements in membrane technology (e.g., increased salt 5Scaling is the deposition of mineral deposits on the interior surfaces of process equipment or water lines as a result of heating or other physical or chemical changes.
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From page 27... ...
. Continued improvements in membrane materials, permeability, and energy recovery devices could generate additional cost reductions.
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From page 28... ...
, and Erec is the energy recovered through an energy recovery turbine. The required feed pressure was calculated with the above stated parameters for a multi-element membrane unit using the software package IMS by Hydranautics, which assumes the performance of commercial seawater membranes.
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From page 29... ...
The Roadmap identifies a significant portion of the research areas critical to improving membrane technologies in desalination. However, there are some areas that are not included in the Roadmap, and some of the existing topics should be expanded.
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From page 30... ...
Therefore, the development of fouling-resistant membrane surfaces and elements would be beneficial, leading to longer membrane life spans and reduced operating costs from both cleaning and pretreatment to reduce fouling. Given widely different feed water qualities and membrane configurations, it would be difficult to develop a membrane surface that is completely resistant to all types of fouling; thus, module restoration will also be necessary.
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From page 31... ...
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From page 32... ...
THERMAL TECHNOLOGIES Approximately one-half of the world's installed desalination capacity uses a thermal distillation process to produce fresh water from seawater. Thermal processes are the primary desalination technologies used throughout the Middle East because these technologies can produce high purity (low TDS)
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From page 33... ...
As noted in the Roadmap, thermal seawater distillation processes employed in the Middle East are mature technologies that may not have broad application in the United States. While thermal desalination is not expected to displace membrane-based desalination as the predominate desalination technology in the United States, thermal technologies have substantial potential and should be considered more seriously than they have been to date.
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From page 34... ...
In the case of MSF and MED processes, the concentration factor for thermal and membrane seawater desalination is very similar, but the overall thermal desalination plant discharge may be diluted because a significant amount of cooling water may also be discharged with the concentrate. The thermal technology research areas and projects identified in the Roadmap are generally appropriate but could be expanded and in some cases revised.
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From page 35... ...
The use of alternative energy sources, which is discussed later in this chapter under "Cross-Cutting Technology-Related Research," is a potential area for future research which could result in improved desalination economics and broader application of desalination. Cooling Water Alternative Most thermal seawater desalination processes require large amounts of cooling water and have significantly greater seawater intake flow rates than comparably sized membrane systems.
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From page 36... ...
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From page 37... ...
Research Topics to be Deleted Three of the research topics proposed in the Roadmap -- renewable energy sources, solar ponds and forward osmosis -- should be deleted from the section on thermal technologies (see Table 3-2)
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From page 38... ...
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From page 39... ...
and indirect potable reuse applications. Thus, this section on water recycling and reuse is not focused on the purification technologies themselves, since improvements to membrane technologies are addressed in a previous section of this report.
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From page 40... ...
only endorsed indirect potable reuse under certain conditions, since a number of questions remain regarding the risks. The decision to adopt indirect potable water reuse cannot be made by researchers but must be made locally, influenced by public and political support and informed by risk management and water management considerations, including a full assessment of the community's alternative water resources.
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From page 41... ...
Several additional research topics are also suggested below, and Table 3-4 provides a summary of the recommended changes. Identification and Quantification of Contaminants More complete identification of the contaminants present in treated wastewaters is needed so that the risks of indirect potable reuse can be more clearly understood.
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From page 42... ...
Membrane Bioreactors The submersible membrane bioreactor (MBR) is an emerging membrane technology that has the potential to revolutionize industrial and wastewater treatment.
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From page 43... ...
Lessons Learned from Successes and Failures in Reuse Projects The Roadmap cites the lack of public and regulatory acceptance for indirect potable reuse projects as a key factor limiting the ability to expand water reclamation and reuse, stating that recycling and reuse suffers from an "unfair stigma." However, the Roadmap does not appear to address this controversial issue. Several states have demonstrated regulatory acceptance of recycled water for a variety of applications from landscape irrigation to indirect potable reuse, and Arizona, California, and Florida have identified indirect potable reuse projects.
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From page 44... ...
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From page 45... ...
Various options are available for inland desalination concentrate management, including: deep well injection, pond evaporation, near zero liquid discharge (n-ZLD) and ZLD, solar energy ponds, shallow aquifer storage for future use, or return to a saline water body via pipeline.
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From page 46... ...
. · Solar Energy Ponds utilize concentrate from desalination plants to capture solar radiation and convert it into useable energy.
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From page 47... ...
Currently very little information is available for use by engineering design firms conducting feasibility analysis of the commercial use options. Review of Research Directions The Roadmap contains a rather disjointed list of items related to concentrate management technologies that are limited to chemicals associated with desalination plants and do not include research on disposal techniques for waste from reuse/recycling facilities.
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From page 48... ...
Reject water from a reverse osmosis desalination plant is not sufficiently concentrated and must be further concentrated -- most likely in an evaporation pond-before it can be utilized in a solar power pond. Wider use of solar energy ponds may depend on research to improve salt concentration technologies and studies to evaluate using life-cycle economics.
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From page 49... ...
However, the water from the desalination plants producing the lowest concentration will usually exceed the salinity that can be used to irrigate the most salt-tolerant agricultural crops. An additional constraint to using desalination concentrate to irrigate crops is that even halophytes have an upper limit to the salt concentration tolerated in the root zone.
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From page 50... ...
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From page 51... ...
Priorities Among the concentrate management research topics identified in the Roadmap and those additional topics suggested by this committee, several have been included among the highest priority. These topics were identified as those most likely of contributing substantially to the objectives set by the Roadmapping Team.
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From page 52... ...
Complete life-cycle analyses for proposed desalination and water reuse facilities are needed to understand the future costs of increased use of desalination (compared to other new fresh water sources) , including costs of water treatment, transportation, storage, and security, and all associated energy costs, including social and environmental costs.
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From page 53... ...
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