Multi-criteria Decision Analysis is Useful for Determining Optimal De-salination Design

While desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> processes can generally be applied interchangeably to different situations, the different methods vary in cost effectiveness depending on external conditions. Two categories of desalination methods are included in this study—thermal and membrane. Thermal desalination technologies include multistage flash (MSF) distillation, multiple-effect distillation (MED), and vapor compression (VC). Membrane technologies include reverse osmosis (RO)<!–[if supportFields]>XE “reverse osmosis (RO)” <![endif]–><!–[if supportFields]><![endif]–> and electro-dialysis (ED). For any of these technologies, optimization of plant design is based primarily on the salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–> of the water (usually categorized into low salinity brackish water and high salinity seawater), quality of water product, and volume of production.  For this study, however, other factors such as environmental impact and political preference were also taken into consideration. Afify (2010) looked at five different location types including three aquifers (brackish water) and two coastal settings (seawater). Each location differed in the type of water usage, ranging from small scale use along the desert fringes of Egypt to coastal tourism and resorts. Afify used Multi-criteria Decision Analysis (MCDA) to determine the best technology for each location. He found that, independent of water usage, ED is preferred for brackish water treatment, though RO would be better for larger desalination volumes, while MED is slightly preferred for seawater followed closely by MSF and RO. For any of the technologies, a higher water usage is preferred as it is more cost effective over small scale plants. —Erin Partlan
Afify, A., 2010. Prioritizing desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> strategies using multi-criteria decision analysis. Desalination.250, 928–935.

Afify used the method of MDCA to evaluate the selected desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> alternatives for a range of location scenarios in Egypt. MDCA uses weighted numerical values for each evaluation criterion so that criteria of varying importance can be used altogether. Furthermore, numerical valuations of qualitative aspects allow them to be included in this method. However, there is no set method for determining the weighting values, and as a result, the interpretation of importance or quality as numerical values can be fairly arbitrary.
 Afify looked at five different water sources—low-usage desert fringe aquifers, the moderate-usage Nile<!–[if supportFields]> XE “Nile” <![endif]–><!–[if supportFields]><![endif]–> aquifer, low-usage coastal aquifer, seawater for tourism, and seawater for coastal city development. The last water source is based on plans to construct new villages and cities along the Red Sea in order to accommodate a growing population (10.7 million by 2025). This scenario incorporates medium to large scale desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> plants into these establishments during initial construction. In comparison, seawater for tourism would utilize small scale desalination systems. For the three aquifers, though they are all brackish water, they vary in salinity<!–[if supportFields]> XE “salinity” <![endif]–><!–[if supportFields]><![endif]–>. The Nile-fed aquifers have the lowest salinity, whereas the coastal aquifers have high salinity bordering on seawater. The desert aquifers are medium salinity oases. However, Afify notes that as aquifer water is a relatively non-renewable resource, and are generally in areas of low development in Egypt, use of aquifer water for desalination should be limited.
Afify assigned values for each alternative (permutations of water source, desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> technology, and plant size) across five categories—investment costs, operation costs, quality of produced water, environmental impacts of brine outflows, and political preference. Costs were measured in Euros, water quality in ppm, and political and environmental factors were rated out of ten. To rank the categories, Afify used percentages of these values to create two weighting scenarios. In both scenarios, the best technology for each situation was the same: large-scale MED for coastal cities, small-scale MED for coastal resorts, small-scale ED for oasis aquifers, medium-scale ED for Nile<!–[if supportFields]>XE “Nile” <![endif]–><!–[if supportFields]><![endif]–>-fed aquifers, and medium-scale ED for coastal aquifers. Afify uses these results to recommend a plan of action for coastal development—one large MSF desalination plant for the city of Suize and five MED plants for all other cities along the Red Sea.

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