Reclamation Home

Reclamation Offices

Newsroom

Library

Projects & Facilities

Research Office

Thermal Processes

Thermal processes separate fresh water from salt water by boiling or freezing. Reclamation research has taken thermal desalination to a new level by funding research to quantify theoretical energy required for desalination and where energy is lost in the process, developing membrane distillation, investigating salinity gradient solar ponds as heat source, and examining clathrate formation to aid in harvesting fresh water ice crystals from deep below the sea.  Research topics in thermal processes include:

• Membrane Distillation (MD) is another process commercialized under the DWPR program by New Jersey Institute of Technology.  In MD, hot saline water and cool fresh water are separated by a hydrophobic micro- or ultra- filtration membrane.  The hot side vaporized into the cool side through the membrane pores.  Since the membrane is hydrophobic, the liquid water cannot get through as long as the membrane remains uncontaminated by oils or detergents.  Many forms of the process have arisen, differentiated by the direction of flow, whether the cool side is at a negative pressure, or whether there is an air gap between the hot and cold channels. 

• An improved and economical vaporization/condensation process, called Dew-Vap, was developed at Arizona State University in Tucson, AZ.  Corrugated plastic material was used to form towers of sandwiched channels for vaporizing saline water with steam heat.  Saline water is tricked down one channel from the top of the tower while steam flows up the channel.  As the steam rises to the top of the tower, it becomes saturated with additional vapor from the saline water.  At the top of the tower the vapor is cooled by contacting incoming saline water.  The vapor then condenses and falls down a separate compartment.  Dew-Vap technology has been commercialized by a company called Altela, as AltelaRain.

DewVaporation Demonstration Test at the 23rd Avenue Wastewater Treatment Plant, Phoenix, AZ.

• Distillation to condense pure water from seawater has been available to seafarers for millennia.  The process has been made more efficient by reducing pressure across multiple stages to get the most from the process. 
  
  
• As water freezes, salt ions are excluded from the ice structure.  In report #71, the University of North Dakota investigated the feasibility of desalting saline groundwater during the winter by spraying into the air and capturing the ice crystals for melting in the spring. However, the crystals need to be washed and, in most freeze desalination tests, the amount water needed for washing is almost that of the amount of the amount of water produced. Clathrates are hydrated gas molecules.  Water becomes structured around gas molecules, such as methane or carbon dioxide, at high pressure.  Reclamation funded a series of projects to test the feasibility and economics of harvesting gas hydrates from deep ocean water.  These projects did not overcome the hurtles of keeping the crystals from freezing solid in the pipe, washing them with a minimum of water, and recycling the gas without letting it escape into the environment. 

Desalting and Water Purification Research Program Reports