Cost of Desalination

Options for pretreatment of seawater include beach wells, media filtration, and ultrafiltration. 

Beach wells use the natural filtration abUity of beach sand to remove particulate matter. While beach wells provide excellent feed water quality, their use is normally limited to small plants where sufficient beach area is available. 

Ultraflltration membranes have been considered as an alternative to multimedia filtration for seawater RO pretreatment in recent years. There have been pilot studies at numerous sites. The benefits of UF over media filtration pretreatment, lower fouling rate, has been demonstrated in long-term tests (2 years) by Jamaluddin et al. (1998). UF has been recently selected as pretreatment for a large-scale desahnation plants in Saudi Arabia, Japan, and China (Wolf et al., 2005). 

A comparison of typical design conditions for the two alternatives, including reverse osmosis is presented in Table 7.4. It was assumed the pretreatment system would have to treat (at least occasionally) degraded water, and that two stages of media filtration with coagulant addition were required. 

Capital and total fife-cycle costs for 75,000-m /day plants with each of the two pretreatment options are presented in Table 7.5. The comparison shows a premium for ultrafiltration of 6.5 and 3.6%, respectively, for capital costs and total fife-cycle costs. 

---

In some places and under certain conditions, freshwater can be obtained more cheaply by desalination of seawater than by transporting water. This is tme when all the costs of extremely large monetary investments in dams, reservoirs, conduits, and pumps to move the water are considered. Before the rapid escalation of fuel costs between 1973 and 1980, the cost of desalination of seawater to adequately supply southern California would have been less than that of transport to the Peripheral Canal. This would have been the case even if there were an unlimited supply of water in the mountains of northern California, a condition that does not appear to exist. It has been shown that before 1973 a seacoast town could have been suppHed with 7-12 x lO" /d of freshwater more cheaply by desalination than by damming and piping water a distance of >160 km km (7). Indeed, the 1987—1992 drought in California has compelled the city of Santa Barbara to constmct a water desalination plant, and a 76,000-m /d plant is plaimed for the western coast of Florida (8). 

Despite the cost of desalination technology, it has made water available in places where it was not before. Not only has water become available in these places, but the quantities available have also opened prospects for industrial development. This has led to important improvements in the standard of living with prospects for even further improvements in countries fortunate enough to be able to meet the cost of the technology. 

Some of the largest plants for seawater desalination, wastewater treatment and gas separation are already based on membrane engineering. For example, the Ashkelon Desalination Plant for seawater reverse osmosis (SWRO), in Israel, has been fully operational since December 2005 and produces more than 100 million m3 of desalinated water per year. One of the largest submerged membrane bioreactor unit in the world was recently built in Porto Marghera (Italy) to treat tertiary water. The growth in membrane installations for water treatment in the past decade has resulted in a decreased cost of desalination facilities, with the consequence that the cost of the reclaimed water for membrane plants has also been reduced. 

The costs of desalination by ED and RO are very close. ED is generally the less expensive process26 where the salinity is low, because the current is proportional to the amount of salinity reduction. RO is considered to be less expensive for seawater, but low-resistance membranes and thinner solution compartments could make ED competitive for seawater desalting. 

Quite interesting furthermore is a comparison of the cost of desalination by various processes as a function of the feed water salinity, as shown in Figure 15. 

R. Raucher, J. Clements, True costs of desalination projects. World Water Water Reuse Desal, Spring (2011) 26-28. 

Figure 4.15 Historical cost of desalination as compared to the goal of the ADC demo (1.6 kWh/m ). (From Stover, 2006, p. 75.)...

Some idea of typical operation costs of desalination processes is given in Figure 4.12, plotted against the salt concentration of the water to be desalted. Although the cost data are by now a little old, the relative costs as between the various processes are still roughly right. It can be seen that, for all but the lowest concentrations, reverse osmosis is the least expensive option. 

---