EXPLAINER: The role of desalination in California’s water supply

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

As drought and growing demand continue to strain California communities’ water supplies, desalination is increasingly being considered 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 compared to potentially cheaper and more efficient alternatives such as water conservation. Others express concern for the substantial undesirable impacts that ocean desalination could have on marine ecosystems and near-shore habitats. 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 California communities, desalination is impractical 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

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;
• Brine management—the handling and disposal of the brine discharge from the desalination system.

The most common process for municipal desalination in use today in the United States is reverse osmosis, which works by pushing the saline water under intense pressure through semi-permeable membranes to remove dissolved salts and other impurities. The membrane allows only water molecules to pass through, leaving behind salt, minerals, and other contaminants 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 primary 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 afterward.

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 significantly reduced by proper design of open water intakes or by using subterranean intakes where feasible.

Energy costs/greenhouse gases: Water is pumped through filters at a very high pressure to remove the salts. 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. The higher energy use makes operations vulnerable to cost fluctuations and can create uncertainty in planning long-term operational costs and comparing them to other water supply options. Depending on the type of power used to produce the water, desalination facilities could have significant greenhouse gas emissions and 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. The amount can be substantial: every 1,000 gallons of seawater contains almost 300 pounds of salt. Management options include discharge to the ocean or other surface water, 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 bottom-dwelling communities, including fish, plants, and algae.

POTENTIAL BENEFITS AND COSTS

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.

However, despite technological advances, desalination remains one of the most expensive municipal water supply options. A significant portion of the costs is attributed to the energy needed to power the plant, which can account for anywhere from 28% to 50% of the total annual costs.

The cost of desalinated seawater in California varies depending on the plant and location. For example, the Claude “Bud” Lewis Carlsbad Desalination Plant is set at about $3,400 per acre-foot for fiscal year 2024. Meanwhile, water from the Doheny plant is expected to cost $2,058 per acre-foot in its first year.

Since energy requirements are a direct function of the salinity of the source water, desalination of brackish water is less energy intensive than seawater desalination and ranges from approximately $840 to $1,700 per acre-foot.

However, improvements in the efficiencies of desalination technologies and increases in costs of traditional surface water supplies can, in some cases, make desalination comparable with other water supplies.

DESALINATION IN CALIFORNIA

So far, seawater desalination has not comprised a significant amount of California’s water supplies. Only twelve seawater desalination plants are in operation along California’s coast as of 2019; the largest is the 56,000 acre-feet Carlsbad facility, which began operations in December 2015. The remaining seawater desalination plants are considerably smaller, with three producing water only for industrial purposes.

Most of the desalination plants operating in California are brackish groundwater desalination plants. Brackish desalination refers to the desalination of water with a lower salt content than seawater; 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.

The Department of Water Resources determined that in 2020, about 106,000 acre-feet of brackish water was desalinated for drinking water, representing two-thirds of the desalinated water produced and used in California. Most brackish groundwater facilities are in the inland groundwater basins of Southern California, serving large urban areas and rural communities.

PERMITTING AND REGULATORY FRAMEWORK

The State Water Resources Control Board’s Ocean Plan outlines essential standards aimed at safeguarding the water quality and beneficial uses of the Pacific Ocean along California’s coastline. It specifies the requirements and preferred methods for managing ocean discharges and intakes. According to the California Water Code and the Ocean Plan’s desalination provisions, seawater desalination facilities are mandated to employ the best available site, design, technology, and feasible mitigation measures to minimize the intake mortality of all marine life forms. Compliance with the Ocean Plan is required for establishing or expanding seawater desalination facilities.

The Ocean Plan highlights that subsurface intakes are preferred for coastal desalination facilities unless deemed infeasible by the Regional Water Quality Control Board. Moreover, it recommends commingling brine with wastewater discharge as the best technology for brine disposal to reduce the impact on marine life. The Ocean Plan is regularly updated to address evolving concerns, with the next revision scheduled for 2024. This update will consider recommendations from the Seawater Desalination Siting and Streamlining Report, along with other issues that have emerged since the last update in 2018.

Besides the State Water Resources Control Board, over 30 federal, state, and local agencies 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 California Coastal Commission, the California State Lands Commission, the Air Pollution Control District, county agencies, and local governments. Maneuvering through the maze of regulatory processes 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 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 significant 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.