Colorful California Landscape: A Water-Friendly Garden
Colorful California Landscape:  A Water-Friendly Garden

Desalination

According to the US Geological Survey (USGS) Water Science School, saline water can be made into freshwater, which everyone needs everyday.  The process is called desalination, and it is being used more and more around the world to provide people with needed freshwater.  Most of the United States has, or can gain access to, ample supplies of fresh water for drinking purposes.  But, fresh water can be in short supply in some parts of the country (and world).  As the population continues to grow, shortages of fresh water will occur more often, if only in certain locations.  In some areas, salt water (from the ocean, for instance) is being turned into freshwater for drinking.

What is meant by "saline water?"  Water that is saline contains significant amounts (referred to as "concentrations") of dissolved salts. In this case, the concentration is the amount (by weight) of salt in water, as expressed in "parts per million" (ppm).  If water has a concetnration of 10,000 ppm of dissolved salts, then one percent (10,000 divided by 1,000,000) of the weight of the water comes from dissolved salts.

Here are the USGS parameters for saline water:

  • Fresh water - Less than 1,000 ppm
  • Slightly saline water - From 1,000 ppm to 3,000 ppm
  • Moderately saline water - From 3,000 ppm to 10,000 ppm
  • Hightly saline water - From 10,000 ppm to 35,000 ppm

By the way, ocean water contains about 35,000 ppm of salt.

Source: Saline-water resources of North Dakota, USGS Water Supply Paper 1428, 1958.

The worldwide need for freshwater

The scarcity of fresh water resources and the need for additional water supplies is already critical in many arid regions of the world and will be increasingly important in the future.  It is very likely that the water issue will be considered, like fossil energy resources, to be one of the determining factors of world stability.  Many arid areas simply do not have fresh water resources in the form of surface water such as rivers, lakes, etc. and have only limited underground water resources that are becoming more brackish as abstraction of water from the aquifers continues.  The world-wide availability of renewable energies and the availability of mature technologies in this field make it possible to consider the coupling of desalination plants with renewable energy production processes in order to ensure the production of water in a sustainable and environmentally friendly scheme for the regions concerned.  Solar desalination is used by nature to produce rain which is the main source of fresh water on earth.  All available man-made distillation systems are a duplication on a small scale of this natural process.  Recently, considerable attention has been given to the use of renewable energy as sources for desalination, especially in remote areas and islands, because of the high costs of fossil fuels, difficulties in obtaining it, attempts to conserve fossil fuels, interest in reducing air pollution, and the lack of electrical power in remote areas.

The science of desalination

Desalination has evolved into a viable water supply alternative allowing to tap the largest water reservoir in the world – the ocean.  Seawater desalination technology, available for decades, made great strides in many arid areas of the world such as the Middle East, the Mediterranean, and the Caribbean.  The figure below shows the location of the existing desalination plants worldwide.

Desalination plants operate in more than 120 countries in the world, including Saudi Arabia, Oman, United Arab Emirates, Spain, Cyprus, Malta, Gibraltar, Cape Verde, Portugal, Greece, Italy, India, China, Japan, and Australia.  Since early spring of this year the first US large seawater reverse osmosis (RO) desalination plant began operation in Tampa, Florida.  Worldwide, desalination plants produce over 3.5 billion gallons of potable water a day.  The installed RO desalination plant capacity has increased in an exponential scale over the last 30 years

Reverse Osmosis (RO) Desalination

Historically, the key concern related to the use of seawater desalination in a larger scale has been the high cost of water production.  Cost-saving innovations in seawater desalination technology are transforming the once expensive option of last resort into a fiscally viable alternative.  The "engine" of every desalination plant that turns seawater into fresh potable water is the RO membrane.

Typically, a large seawater desalination plant has thousands of membrane elements connected into a highly automated and efficient water treatment system which typically produces 1 gallon of fresh water from approximately 2 gallons of seawater.  The productivity, energy use, salt separation efficiency, cost of production and durability of the membrane elements by enlarge determine the cost of the desalinated water.  Technological and production improvements in all of these areas in the last two decades are rendering water supply from the ocean affordable.  Membrane productivity – i.e. the amount of water that can be produced by one membrane element, has increased two times in the last twenty years.

In seawater desalination plants salts are separated from the fresh water applying pressure to the seawater, which is 60 to 70 times higher than the atmospheric pressure.  After the salt/water separation is complete, a great portion of this energy stays with the more concentrated seawater and can be recovered, and reused to minimize the overall energy cost for seawater desalination.  Dramatic improvements of the membrane element materials and energy recovery equipment for the last 20 years coupled with enhancements in the efficiency of RO feed pumps, and reduction of the pressure losses through the membrane elements have allowed to reduce the use of power to desalinate seawater from 114 kWh/1,000 gallons in 1979 to 14 kWh/1,000 gallons of produced fresh water today. Taking under consideration that the cost of power is typically 20 to 30 percent of the total cost of desalinated water, these technological innovations contributed greatly to the reduction of the overall cost of seawater desalination.  Novel energy recovery systems working of the pressure exchange principle are currently available in the market and use of these technologies is expected to further reduce the desalination power costs with 10 to 15%.

The pressure exhangers transfer the high pressure of the concentrated seawater directly into the RO feed water, with an efficiency of 95% or more. Future low-energy RO elements could operate at even lower pressures to continue to improve the RO technology cost effectiveness.

The recent trend of building large capacity seawater desalination plants is driven by the cost benefits offered by the advantage of size and centralization.  The economy of scale related to building fewer large capacity RO plants rather than a large amount of small facilities has also contributed to the overall reduction of the cost of desalinated water.  Typically, the economy of scale of facilities larger than 20 MGD yields additional cost of water reduction in a range of 5 to 10 %.  Use of existing intake and discharge facilities of power plants has also contributed to the desalinated water cost of reduction.  Co-location of desalination of power plant in a large scale was first introduced by Poseidon Resources for the 25 MGD Tampa seawater desalination plant, and since than has been considered for numerous plants in the US and worldwide.  Today, the construction of large desalination plants is possible mainly due to the availability of large-size off-the-shelf high pressure pumps and large energy recovery systems.

Orange County, California Needs It's Own Water Source

According to Jeff Thomas, President, Municipal Water District of Orange County (MWDOC) Board of Directors, there is a misconception that the tap water we use when we brush our teeth, shower, wash our clothes or water our lawns is overpriced.  The water delivered directly to our homes and businesses is clean, filtered and is monitored more closely than bottled water, which is about 300 times more expensive than tap water.  Tap water costs less than one penny for three gallons.  Bottled water costs more than a dollar per gallon.  Still, our water bill seems to go only one direction – up. USA Today released a new report that stated water bills in Southern California have increased anywhere from 67 to 141 percent over the past decade.  Much of this increase is related to outside rate hikes from imported water sources.  Today, Orange County is about 50 percent self-reliant on local water supplies and local water projects while the other half of our water is imported from Northern California or the Colorado River.  To protect ourselves from unpredictable rate increases as well as future droughts and natural disasters that could cut off our imported water supply, it is imperative that we develop new local water projects that will make Orange County more self-reliant and less reliant on outside forces that may or may not be able to deliver in a crisis.

One new water project that is in the offing is a proposed seawater desalination facility in Huntington Beach.  Built through a public-private partnership, no tax dollars would be at risk in its development and public water districts would only be responsible for purchasing the water if and when it is delivered at the quantity and quality agreed to beforehand.

So far, this project has been more than 10 years in development as it winds its way through the cumbersome permitting process.

Another project is the South Orange Coastal Ocean Desalination Project, based in Dana Point, which is undergoing a feasibility analysis.  As another new source of drinking water, this project would provide an additional 15 million gallons of drinking water for south Orange County.

Desalinating seawater will cost more than importing water today, but we have a responsibility to look beyond today.  We must consider the future price of water and the cost of remaining so heavily reliant on imported water.  What is the value of tapping into the world’s largest reservoir – the Pacific Ocean?  Keep in mind that a Northern California earthquake that rates 6.7 or higher on the Richter scale would likely destroy the levies in the Delta, which would cut off our drinking water supply from Northern California for up to three years.  The value of desalination plants providing 65 million gallons of fresh drinking water every day to Orange County residents and businesses would be incalculable.

Consider that when the Orange County Water District’s Groundwater Replenishment System (GWRS) – Orange County’s internationally acclaimed wastewater recycling program – was originally planned, the cost estimate of purified drinking water from this facility was more than the cost of imported water at the time.  Due to technological advancements and efficiencies combined with the increasing cost of imported water, the GWRS water has become cost-competitive with imported water supplies.  It is not unreasonable to think that desalinating seawater will follow that same formula.  Over the past decade, the cost of desalinated seawater has plummeted as membrane technology improves and energy recovery systems are put in place.  Meanwhile the cost of imported water inexorably keeps climbing.

By investing in and supporting local water projects, we can be sure Orange County will become an island of self-reliance in Southern California and that our well will never run dry.

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Stewardship requires the recognition that we are all caretakers of the environment and economy for the benefit of present and future generations. We must balance the impacts of today's decisions with the needs of future generations. - The Minnesota Round Table on Sustainable Development (1998)