Desalination

Reverse Osmosis Filters at the Carlsbad Desalination Plant

Desalination holds the potential of providing a highly reliable water source for some communities; however, there are major cost and environmental concerns that must be addressed.

Toolbox Main Page IconAs drought and growing demand continue to put strain on water supplies for California communities, desalination is increasingly getting consideration as some see a seemingly drought-proof supply of water lapping at the state’s doorstep. Certainly, a seawater desalination plant would provide water regardless of the prevailing hydrological conditions. Many coastal California water agencies are looking at ocean desalination as a way to provide a more reliable supply of water during droughts and to reduce reliance on the Delta and Colorado River.

However, desalination proposals are not without controversy. Desalinated water remains one of the most expensive municipal supply options and many question the cost as compared to potentially cheaper and more efficient alternatives such as water conservation. Others express concern for the substantial undesirable impacts that ocean desalination could potentially have on marine ecosystems and near-shore habitats. And regardless of how well they are designed, all desalination facilities currently consume a great deal of energy and have the potential to increase greenhouse gas emissions.

For most communities in California, desalination is not practical due to their distance from the coast or lack of other suitable saline source water, nor is it economically feasible as more cost-effective supplies are usually available. However, recent advances in technology have reduced energy use and the associated costs, making desalinated water more competitive with other sources for some communities. Additionally, as of 2010, water suppliers are required to evaluate the potential for desalination in their urban water management plans.

THE DESALINATION PROCESS

Pretreatment tanks at Carlsbad Desalination Plant

Desalination is the process that removes salt and other dissolved minerals from saline water sources. There are five key components to a desalination system:

• Intakes—the structures used to bring the source water into the facility; these can be surface intakes or subsurface intakes, such as slant wells, or beach well galleries;
• Pretreatment— prepares the source water for further processing by removing suspended solids;
• Desalination— the process that removes salts and other dissolved solids from the source water, creating the product water;
• Post-treatment—the addition of chemicals to the product water to prevent corrosion of downstream infrastructure piping, and additional processing to meet drinking water standards;
• Concentrate management—the handling and disposal of the brine discharge from the desalination system.

The most common process for municipal desalination in use today is reverse osmosis, which works by pushing the saline water under intense pressure though semi-permeable membranes to remove dissolved salts and other impurities. The membrane allows only water molecules to pass through, leaving behind the salt, minerals and other impurities such as bacteria and viruses. Depending on the specific membrane used, the reverse osmosis process can remove 90-98% of all dissolved salts, organic molecules, microorganisms, colloids and suspended matter; the membranes are capable of rejecting particles with diameters as small as 0.0001 μm.

IMPACTS OF DESALINATION

There are three main environmental considerations when building an ocean desalination plant: the intakes that bring seawater in, the energy needed to separate the drinkable water out, and what is done with the brine discharge afterwards.

  • Intakes and ecosystems: One of the primary concerns of coastal desalination plants is how the source water is drawn into the facility. Surface water intakes of seawater result in impingement and entrainment of marine organisms. The mortality of marine organisms can be lessened and greatly reduced by proper design of open water intakes or by using subterranean intakes where feasible.
  • Energy costs/greenhouse gases: To remove the salts, water is pumped through filters at a very high pressure. Doing this with thousands of gallons of water per minute requires a lot of energy, making energy costs a significant portion of the cost of producing the water. Depending on the type of power used to produce the water, desalination facilities could have significant greenhouse gas emissions associated with their operation, as well as other environmental impacts from energy generation.
  • Brine management: The desalination process produces a stream of concentrated salts and other constituents that must be managed and carefully disposed of. Management options include discharge to the ocean or other surface water, discharge into a dedicated brine line, or injection into a deep well aquifer. The discharge of brine into water bodies can be problematic as the higher saline brine is denser than ocean water and if not properly discharged, will tend to sink to the bottom, adversely affecting damage bottom-dwelling communities, including fish, plants, and algae.

POTENTIAL BENEFITS

Desalination can provide a reliable water supply that is more dependable and drought-proof than freshwater sources that rely on annual or multi-year precipitation, runoff, or recharge rates. Both seawater and brackish desalination processes are capable of producing very high quality water.

For those communities with limited water supplies and a viable saline water source, desalination can provide a local water source, thereby reducing dependence on imported supplies and vulnerability to water supply reductions that can result from drought, climate change, or disruption of imported water supplies.

Some communities have established desalination as an emergency supply option and have standby facilities that only operate when other sources aren’t available. While this allows water suppliers to maintain a base supply in times of extreme shortage, maintaining these facilities incurs higher capital and maintenance costs than other sources of water.

POTENTIAL COSTS

Despite technological advances in recent years, desalination still remains one of the most expensive municipal water supply options; a large part of the costs is attributed to the energy needed to power the plant, which can be anywhere from 28% to 50% of the total annual costs. According to the California Water Plan, seawater desalination is estimated to cost between $1600 to $3000 per acre-foot. Water from the Carlsbad plant is estimated to cost about $2000 per acre-foot, which is about twice as much as the cost to purchase water from Metropolitan Water District.

Since energy requirements are a direct function of the salinity of the source water, desalination of brackish water is less energy intensive than for seawater desalination with cost estimates ranging from $600 to $3000 per acre-foot.

NEW TECHNOLOGIES

Advancements are continually being made in technologies that hold promise for reducing the costs and impacts of desalination. New processes under development include forward osmosis, concentrated solar stills, and closed circuit desalination. Membranes made from new materials such as nanoparticles, carbon nanotubes or graphene show promise in terms of water permeability and salt rejection. However, these technologies have yet to be scaled up to commercial operation and their research and development is ongoing.

Energy recovery devices installed at the Carlsbad Desalination Facility.

With energy being a significant factor in the costs of desalination, energy recovery devices have been developed to help to reduce those costs by capturing some of the energy created by the high pressure brine stream produced during the reverse osmosis process and transferring that to the incoming seawater. Renewable energy such as solar power or wind power is being considered as possible energy sources to reduce costs and greenhouse gas emissions; the world’s first utility scale, solar powered desalination plant is under construction in Saudi Arabia.

In 2013, Israel completed construction of the world’s largest modern seawater desalination plant, capable of providing more than 185,000 acre-feet per year. The $500 million reverse-osmosis plant incorporates advances in engineering and materials such as highly efficient pumps and energy recovery devices, making the plant’s energy consumption among the lowest in the world for large-scale desalination plants.

DESALINATION IN CALIFORNIA

So far, seawater desalination has not comprised a significant amount of the California’s water supplies. There are only seven seawater desalination plants in operation along California’s coast as of May 2016, the largest of those is the 56,000 acre-feet Carlsbad facility which began operations in December of 2015. While the Carlsbad facility is currently the largest in the nation, the remaining seawater desalination plants are considerably smaller with three of them producing water only for industrial purposes.

Several more seawater desalination facilities have been proposed, with nine along the California coast and two in Baja California, Mexico that would provide water to Southern California communities. If built as proposed, those eleven facilities could potentially produce over 400,000 acre-feet per year.

Most of the desalination plants operating in California are brackish groundwater desalination plants. Brackish desalination refers to desalination of water with a lower salt content than sea water; it can be naturally occurring, such as in an estuary or from the intrusion of seawater into coastal groundwater basins, or from human activities, such as farming and other land uses.

As of May 2016, there were 23 operating groundwater desalting plants with the capacity to produce nearly 140,000 acre-feet per year; 15 additional plants are proposed, nearly all of them located in Southern California.

PERMITTING AND REGULATORY FRAMEWORK

There are over 30 federal, state, and local agencies that have some regulatory or permitting authority over desalination plants, depending on the site and the technology used. This includes the federal fish agencies, the US EPA, the Coast Guard, the State Water Resources Control Board, the California State Lands Commission, the Air Pollution Control District, county agencies, and local governments. Maneuvering through the maze of regulatory process to acquire the necessary permitting can take years. A case in point: the Carlsbad desalination plant took fourteen years from the beginning to the end of construction.

In May of 2015, the State Water Resources Control Board adopted a Desalination Amendment to its Water Quality Control Plan for the Ocean Waters with regulations that apply to new or expanded seawater desalination projects along California’s coast. The Desalination Amendment requires new or expanded seawater desalination plants to use the best available, site, design, technology, and mitigation measures feasible to minimize intake and mortality of all forms of marine life. The amendment specifies preferred technologies that are based on the best available science; however, alternative intake and disposal methods can be used if demonstrated to be as protective of marine life as the preferred technologies. The amendment also specifies mitigation measures in order to address harmful impacts on marine life.

IN CONCLUSION …

Desalination can provide a highly reliable water source for some communities, one that is immune from hydrologic variability, climate change, and disruptions to imported water supplies. However, there are major concerns with desalination that must be addressed, including cost, environmental impacts, and greenhouse gas emissions. The best way to address these impacts is to engage in a methodical planning process with extensive community involvement that weighs the impacts against other water supply options and the supply reliability of desalination.

Click here to download this resource management strategy from the California Water Plan.

 

Photos of the Carlsbad Desalination Plant courtesy of the San Diego County Water Authority.

FOR MORE INFORMATION

Helpful documents and websites …

Agencies and organizations working in this area …

More on desalination from Maven’s Notebook …

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