The combination of an ever-increasing population and a diminishing usable water supply threatens the sustainability of humanity’s existence on a global scale, especially in California, where agriculture is so integral to the state’s economy. Due to widespread recognition of this problem, there has been a growing trend in the development of alternative water resources, one of which involves the desalination of salt or brackish waters. Several different desalination technologies exist, including microfiltration, multi-stage flash, and multi-effect distillation.
This senior project investigates the economic viability of implementing reverse osmosis desalination for treatment of agricultural wastewater or other unusable water sources. These water sources might include brackish groundwater or municipal wastewater. The capital and operational costs of implementing such technology were compared to the delivery prices for existing water sources in the San Joaquin and Coachella Valleys and along the Central California Coast. Along with the costs associated with each option, the environmental, social, and political concerns were considered, as well.
Field visits and personal interviews of current desalination plant operators, in conjunction with desalination pricing, were compared to the current cost of irrigation water delivery in the San Joaquin and Coachella Valleys, as well as the Central California Coast. The results showed that the cost of desalination, while significantly less expensive in the past few years, was still too great to offset the relatively low prices of irrigation water delivery. The concern of brine disposal in the Central and Southern California Valleys also poses environmental problems. Although desalination is not currently economically viable, it seems only a matter of time before either the cost of water becomes too great or the cost of desalination becomes affordable.
California’s current population of 35 million is expected to increase by approximately 12 million by the year 2030, which will impact the state’s water demands significantly (Karajeh et al 2005). Agricultural irrigation represents a considerable portion of fresh water demand, with an estimated 65% global water demand and nearly 90% of the water demand in California (Abu-Zeid 1998). This creates a challenge for water supply reliability and availability as technologies shift from the construction of new dams, reservoirs and conveyance canals, and move toward water conservation and reclamation. As a result, saltwater and brackish water reverse osmosis desalination is becoming of greater interest, largely because technological advances have caused the cost of membranes to decrease dramatically (Karajeh et al 2005). In the same time that desalination costs have been declining, the costs of surface and groundwater have been increasing, making desalination a more competitive source of water for both municipal and agricultural purposes (Beltrán and Koo-Oshima 2004).
A study by Sorour et al (1992) investigated various desalination technologies for agricultural drainage water applications. The study ranked reverse osmosis (RO) desalination as the highest performing desalting technology, when compared to ion exchange, electrodialysis and vapor compression. This high ranking was due to lower desalting costs, higher tolerance to changes in salinity, and ability to remove dissolved organics. Per the results of the study, reverse osmosis desalination became the recommended technology for agricultural drainage desalination (Sorour et al. 1992).
The process of reverse osmosis is, simply stated, the removal of contaminants by pushing water through a membrane with the use of hydraulic pressure (Solt and Shirley 1991). RO membranes are generally nonporous and will pass water, while retaining most solutes, including ions. The separation of salts and other minerals from the water is achieved by reversing the natural osmotic flow with the application of pressure to the side of the concentrated solution as illustrated in Figure 1 (Vigneswaran et al 2004).