Supply from the Sea—Exploring Ocean Desalination
As the world’s population continues to expand into areas of limited water supply, planners and politicians will have few options but to turn to the ocean as a supplemental source of freshwater.
Although there are still many barriers to large scale implementation of ocean desalination (desal), advances in technology, water policy, economic allocation and public awareness will continue to drive the development of ocean desal projects well into the foreseeable future.
This article provides a brief overview of ocean desal including global and domestic trends, a comparison of desal technologies, a summary of the economics involved and a summary of implementation considerations.
Ocean desal trends
Ocean desal as a source of drinking water is not a new concept. Egyptian, Persian, Hebrew and Greek civilizations studied various desal processes. Aristotle and Hippocrates both advocated the use of distillation in the fourth century B.C. By 2001, there were more than 15,000 ocean desal plants worldwide with a total production capacity of nearly 6.2 bgd. Most ocean desal facilities are located along the coasts in the energy-rich Middle East. However, ocean desal is a growing market in Spain, England, the U.S. and Mexico. Trends impacting advancement of ocean desal in the U.S. and abroad include improvements in technology, increased governmental subsidies, increased consideration of co-location with power plants and more public/private approaches to project development.
Nothing has contributed to ocean desal’s increasing viability and growth as much as the continuing improvements in technology.
In 1959, the first reverse osmosis (RO) membrane was developed by Loeb & Sourirajan at the University of California, Los Angeles. These original cellulose acetate membranes allowed researchers to apply high pressure to specific ionic species from water molecules, producing freshwater from a salty solution.
Since the early days of RO, there have been significant advances in membrane technology to improve salt rejection and reduce transmembrane pressure and membrane fouling. RO membrane manufacturers continue to refine the manufacturing, packaging and fabrication processes. Additionally, researchers and manufacturers continue to improve the efficiency of high-pressure pumping and energy recovery systems and the effectiveness of RO pretreatment. These technological advances have contributed to a substantial decrease in the capital, operations and maintenance costs of RO for ocean desal.
Although technological advances continue to reduce the cost of ocean desal, its economic viability is usually limited when compared to alternative sources of additional freshwater supply.
However, the inevitability of ocean desal as a viable freshwater source is causing some regional water agencies to subsidize the development of large-scale desal projects.
Examples of agencies promoting large-scale ocean desal projects include Tampa Bay Water in Florida and the Metropolitan Water District of Southern California.
Additionally, state and federal agencies such as the Texas Water Development Board and the U.S. Department of Energy continue to offer grants and financial incentives in support of desal. These subsidies not only create a market that drives private investment in research and development, but allow some water suppliers to begin to economically integrate ocean desal into their existing supply portfolio.
Co-location with power plants
Throughout North America and Europe, ocean desal plants are almost exclusively co-located with power generation facilities. This trend can be attributed to the ready availability of a reliable and inexpensive source of electricity, availability of existing intake and outfall structures and potential for utilizing a higher temperature feed water source. Because energy usage is one of the most significant cost factors when considering ocean desal on a life-cycle basis, reliable and inexpensive energy can dramatically decrease its overall cost.
Utilizing existing intake and outfall structures not only limits the environmental impact of a new ocean desal facility, but it can also substantially reduce construction cost.
Environmental impact is limited during construction because new sub-marine facilities are usually not required. Impact is limited during operation because the return flows from power plant cooling systems offer a significant pre-dilution for the concentrated brine waste discharge.
Additional benefits may be gained by drawing the RO feed water from the power plant’s spent cooling water. With warm water feed, RO membranes can operate at a higher rate, which can reduce pressure and/or footprint requirements.
However, with warmer feed water, salt rejection across the RO membranes is typically diminished, and multiple “passes” of RO membranes may be needed to achieve similar product water quality.
Public/private project development
Considering the limited large-scale application of ocean desal in the U.S., project development typically involves significant risk related to maintaining environmental compliance, implementing advanced technology, ensuring treatment performance, maintaining system reliability and controlling operations and maintenance costs.
Interested water utilities and public agencies must therefore develop approaches to mitigate these risks. Developers have attempted to fill this need by promoting their ability to deliver new ocean desal plants while applying approaches to manage overall project risk. These may include options contracting and bundled procurement approaches including design-build and design-build-operate. Although third-party private developers have helped promote and implement ocean desal throughout the U.S., their role will likely diminish as the market matures and public water agencies become more comfortable managing these risks directly.
Is desal cost prohibitive?
When considered on a cost-per-volume treated basis, the cost of ocean desal has increased dramatically over the last 20 years.
For example, the Tampa Bay Seawater Desalination Plant is expected to deliver water at a total cost of around $2 per 1,000 gal. When this trend is superimposed onto a curve representing the cost of other new supplies in areas with limited supply options (such as expanded import systems), “crossover” is either imminent or has already occurred. There is no question that ocean desal is an expensive solution for obtaining additional freshwater supply. However, in some areas of the country where population growth will far outstrip the availability of conventional freshwater sources, ocean desal may in fact be the “least cost” alternative for new suppliers.
Although a straight comparison of cost per volume of new freshwater supply is an important step in considering the economics of new sources, this approach fails to take into account an equally important factor: reliability.
Associated with each new supply is an inherent reliability. Contributing factors may include regional weather patterns, environmental regulations, infrastructure conditions, natural disasters and variability in water quality. Water agencies must consider their ability to provide sufficient freshwater to their customers under a wide range of reliability conditions.
Financial analysis offers a useful analogy when considering supply economics. As every investor knows, the first rule of financial management is diversification.
Each investment has an inherent risk and return. By combining investments of varying risk and return, an optimally efficient portfolio can be achieved, which maximizes the return for a given level of aggregate risk. The same approach can be applied to water supply planning. However, instead of considering risk versus return, water planners consider reliability versus cost. Although ocean desal represents a relatively high-cost supply alternative, it can be highly reliable. Therefore, by combining ocean desal with lower cost, lower reliability supplies, water managers can achieve an optimally efficient supply portfolio that maximizes reliability for a given aggregate cost.
Getting the salt out
There are two primary mechanisms for ocean desal: thermal processes and membrane processes.
Thermal processes rely upon induced evaporation to separate water vapor from a salt solution, followed by a condensation step that returns the water vapor to liquid form.
Membrane processes rely upon selectively permeable membranes that reject dissolved ions while allowing water molecules to pass under high pressure.
Thermal processes include a multi-effect distillation, multi-stage flash distillation and vapor compression. Membrane processes include RO and sequential nanofiltration.
From vision to reality
As with any large investment in capital facilities for water supply, implementation of ocean desal requires the application of thorough and comprehensive management and engineering approaches.
Significant considerations may include: supply planning and economics; water quality analysis; site evaluation; technology evaluation; energy consideration; residuals management options; environmental compliance and facility permitting; public outreach; supply integration; project aesthetics; and project procurement options.