As demand for fresh water increases, communities look to the sea and to previously unusable aquifers
With the world’s population consuming water in record amounts and water scarcity causing a host of geopolitical and humanitarian problems, technology can be harnessed to help meet the demand.
One increasingly important solution to freshwater scarcity is treating saline water through a process known as desalination, or desalinization.
Many areas already are turning to desalination to stave off shortages of fresh water. The process can be used to treat saltwater from the ocean, and also to treat groundwater that has concentrations of salt that make it unfit for human consumption.
The U.S. Geological Survey defines saline water by the following concentrations of salt, expressed in parts per million:
- 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
- Highly saline water: From 10,000 ppm to 35,000 ppm
- Ocean water: About 35,000 ppm
Desalination Around the World
As of 2002, approximately 12,500 desalination plants in 120 countries were producing 14 million m3/d of fresh water, according to the U.S. Geological Survey. This is less than 1% of the total world consumption of water. Nations that heavily rely on desalinated water include Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, Bahrain, Libya, and Algeria. By contrast, the United States is one of the largest users of desalinated water among industrialized countries. Facilities are predominantly found in California and parts of Florida.
A 2010 market survey by Pike Research forecasted that the Middle East and North Africa “will continue to be the global hub of desalination plant construction,” with Saudi Arabia, the United Arab Emirates, the U.S., China, and Israel leading the world market for desalination technologies. The study also predicted that worldwide desalination capacity will go from 76 million to 126 million m3/d between 2010 and 2016.
People have been purifying salty water for ages. Most systems duplicate nature’s process for creating rain. One of the earliest forms of desalination used human-controlled evaporation, or steam distillation, to remove salt from water. The distillation process also reduces other drinking water contaminants.
Other types of distillation include multiple-stage flash evaporation, which in the 1960s was considered the most feasible large-scale desalination technology; multiple-effect distillation; and vapor-compression. Non-distillation methods include ion exchange, and processes such as reverse osmosis, which uses membranes to filter salt.
One concern associated with reverse osmosis technology is biofouling, the accumulation of microorganisms, plants, or algae on wet surfaces. Graeme K. Pearce, principal at Membrane Consultancy Associates, an independent consulting firm, told WaterWorldthat biofouling “increases both energy use and chemical cleaning frequency, giving a higher carbon footprint and creating a waste sludge for disposal.” It’s also prompting more research into new materials.
New technologies are making seawater desalination more affordable and readily available. For example, Fluence custom designs seawater desalination systems with energy recovery systems using pressure exchangers or turbine pumps, reducing energy requirements by as much as 40 percent.
Conventional desalination methods typically rely heavily on power-hungry, fixed facilities. But, some larger operations may be cogeneration plants, harnessing leftover energy from power generation desalinate water.
A research team led by the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) is studying graphene membranes as an energy-efficient desalination technology. This proof-of-concept work shows seawater can be desalinated using a freestanding membrane made of graphene — a form of carbon that is one atom thick.
While large desalination plants require a significant investment of money and space, researchers, including scientists at MIT, have been working on portable, low-cost solutions that would provide drinking water after natural disasters or in locations where it’s not feasible to site and build huge plants. These containerized desalination solutions also are practical for small, self-contained communities like military bases, construction sites, and resorts.
A solution gaining traction based on its small footprint and affordability is Fluence’s NIROBOX™. This small, decentralized treatment system can be commissioned rapidly, allowing users to start producing potable water in less than two months after the need is identified. And, because it is containerized, installation is easy, requiring minimal construction. Nirobox’s modular nature means it can be easily expanded as needed.
Nirobox uses existing, proven desalination technologies in a three-stage process – disc filtration, ultrafiltration, and reverse osmosis with an energy recovery device – reducing power consumption by roughly 30 percent. The system components are installed in a new, 40-foot-long high-cube container. Other equipment can be added for post-treatment processes to make the water suitable for drinking water or use in industrial processes.
A single Nirobox can produce as much as 264,000 GPD (1,000 m3/d) of fresh water from seawater or brackish water, depending on the model chosen. By using five units in parallel, a user can produce as much as 1.3 million GPD of fresh water.
Brackish Water Desalination
The need for desalination can be seen worldwide, including in the U.S. More state officials, including those in Texas, are advocating its use to meet increase water demands. Water demand in Texas should exceed 18.4 million acre-feet by 2020. By 2070, demand is forecast to be near 21.6 million acre-feet.
There are about 100 desalination facilities currently in operation in Texas, producing about 138 million GPD of desalinated water, according to the Texas Desalination Association. But none of the plants is sourcing water from the Gulf of Mexico. Instead, they are using a portion of the approximately 2.7 billion acre-feet of brackish groundwater in aquifers under the state.
Brackish water reverse osmosis poses particular challenges. The feed water comes from a wide variety of sources, including surface water and wells, many of which contain a wide range of contaminants including arsenic and nitrates. Technologies such as ultrafiltration and media filtration prepare this water for reverse osmosis.
But, Texas now has two pilot seawater desalination plants. One is operated by the Brownsville Public Utility Board, and the other is operated by the Laguna Madre Water District, which serves the coastal areas of South Padre Island and Port Isabel.