Water Desalination & RO

Abstract: While the importance of desalination has been increasing steadily over the past decades, views on the merits of desalination among water professionals differ. Some portray desalination as a panacea for much of the world's water woes. Others perceive desalination very negatively because of its alleged high costs, energy intensity and environmental impacts. While none of these extreme positions appears to be justified, it remains even more necessary to gain an objective understanding of the real stakes in desalination in the context of integrated water resources management. The World Bank has undertaken a regional study that aims at improving the understanding of desalination within the Bank and among some of its clients in the Middle East, North Africa and Central Asia. It also tries to clarify the conditions under which desalination can help in reaching the United Nation's Millennium Development Goals (MDGs) for water supply and sanitation. The study, which has been funded by the Bank-Netherlands Water Partnership, includes case studies of Algeria, Cyprus, Jordan, Malta, Tunisia and Uzbekistan. A key conclusion of the study is that desalination alone cannot deliver the promise of improved water supply. The ability to make the best use of desalination is subject to a series of wider water sector related conditions.

INTRODUCTION: In the United States, economic growth increasingly requires that greater volumes of freshwater be made available for new users, yet supplies of freshwater are already allocated to existing users. Currently, water for new users is made available through re-allocation of existing water supplies—for example, by cities purchasing agricultural water rights. Water may also be made available through conservation efforts and, in some locales, through the development of “new” water from non-traditional sources such as the oceans, deep aquifer brackish groundwater, and water reuse. Developing “new” water from non-traditional sources relies primarily on membrane filtration (e.g., reverse osmosis) and to a lesser extent distillation. Both are technology-intensive processes whose future depends on the development of, respectively, better membranes and more efficient heat transfer approaches. Membrane filtration in particular carries with it weighty pre- and post-treatment concerns: How to prevent the formation of filter-clogging inorganic and biologic scales? How to dispose of residual concentrated brines? How to chemically stabilize the water that is produced? Water development in the 20th century largely involved the physical collection and transfer of freshwater by dams and aqueducts. Because of tightening supplies and public resistance to further dam building, water development in the 21st century will increasingly rely on creating or reclaiming water from non traditional sources by membrane filtration and other desalination processes. Since the latter are tied so strongly to technology, the absolute volume of new water produced in the United States in the 21st century will likely reflect the extent of new research and development investment in desalination. This has happened before. In the 1950s and 1960s, the U.S. government invested a total of roughly $900 million (in 1985 dollars) in desalination research conducted through the Office of Saline Water in the Department of the Interior. Reverse osmosis (RO) desalination is one of the notable inventions that resulted from this federal investment: RO is presently a $2 billion per year global market, and the technology is used to produce six billion gallons of water per day. Increasing the availability of new water will likely require a renewed national commitment to desalination research and development.

Abstract: Concentrate disposal is a major cost for desalting operations, particularly for inland sites. For many water recycling applications, only partial desalting is needed, with sodium chloride the most common target for removal. A process to preferentially remove sodium chloride could result in significant savings for concentrate treatment and disposal. A pilot study was conducted to demonstrate the viability of one such approach. This pilot study examined the salt passage characteristics of several nanofiltration (NF) membranes to segregate sodium chloride from other dissolved ions. The pilot study testing unit used a two-pass system, combining an NF pass with a second reverse osmosis (RO) pass. In this approach, unlike typical two-pass systems, the concentrate from the first NF pass is blended with the permeate from the second RO pass. Since the NF effectively retains total organic carbon (TOC), chemicals of emerging concern (CECs), and any pathogens, these contaminants are blended into the product water. Thus, further treatment may be necessary for potable reuse applications. The pilot study consisted of a scalable two-pass NF-RO membrane system operated in parallel with full scale RO units. Water quality, power consumption, and chemical consumption were tracked for 6 months at varying system conditions. Findings indicate that sodium chloride can be preferentially removed from the reclaimed water, chemical and power consumption can be reduced when operated at system recoveries comparable to typical RO systems, and much higher recoveries are achievable with modest increases in power and chemical usage.