There are two commercially practiced ways of getting fresh water from salt water (brine): distillation and reverse osmosis. The process is known as desalination, or desalting.
Distillation Boiling is familiar to all of us. Distillation is the next step: condensing the steam to get liquid water. There are two ways to do the boiling step, either raise the temperature or lower the pressure. A problem with heat distillation is that dissolved limestone and other rocks in the water attach as a crust on heat exchange surfaces. Most salts, like sodium chloride (table salt), dissolve better in hot water, but calcium carbonate (limestone) is a reverse solubility salt, which forms scale where the water gets hot. Scale insulates the brine from the heat, making heating less efficient. Vacuum distillation avoids the scale problem by using mechanical energy instead of heat to produce steam. Reverse Osmosis Approximately half of the world’s installed desalination capacity is in reverse osmosis plants. Reverse osmosis (RO) desalination is done by means of a membrane separating brine from product water. High pressure on brine causes low salinity water to permeate through to the other side of the membrane. Chemical diffusion is the conventional explanation of how reverse osmosis works. The applied pressure in reverse osmosis is sufficient to overcome the osmotic pressure at the given concentration of salts in the brine. Higher salinity requires higher pressure. Brackish water reverse osmosis pressure is between 17 and 27 bar (one bar is one times the atmospheric pressure, 105 Pa or 105 N/m2, which in English units is 14.7 pounds per square inch). Seawater operating pressure is between 52 and 60 bar, typically about 1000 psi in English units. Desalting seawater is 3 to 5 times more expensive in energy consumption than desalting brackish water, which is twice as expensive as ordinary municipal drinking water treatment. For potable water, the target is total dissolved solids (TDS) of less than 500 parts per million (ppm). The disadvantages of reverse osmosis are: (1) the energy required for operating pressure, which in turn increases the carbon dioxide problem, (2) the necessity of extensive pretreatment upstream of the membrane, (3) costs and downtime due to membrane fouling, and (4) a voluminous stream of reject brine that pollutes the environment. RO reject brine is classified as industrial waste by the US Environmental Protection Agency. Dewatering the RO reject brine using reverse osmosis would require very high pressure because of the very high osmotic pressure that must be overcome. Although there is presently no economical solution to the reject brine problem (viz. Yuma Desalting Plant), dumping or hiding the reject brine should not be considered. And any dumping or hiding would entail significant costs for transporting the reject brine to the dumpsite.
The Yuma Desalting Plant Debacle An example of the unsolved problem posed by the voluminous stream of RO reject brine is the largest reverse osmosis plant in the United States, the Yuma Desalting Plant located in Yuma, Arizona. This very expensive modern facility has been idle since a 6-month test period ending in 1993 because dumping of its voluminous 9,400 ppm RO reject brine stream proved environmentally unacceptable. If operated at full capacity, with feed of saline agricultural drainage water (TDS 2,900 ppm) from the Wellton Mohawk Valley of approximately 390 million liters (102.7 million gallons) per day, the Yuma Desalting Plant could produce about 275 million liters (72.4 million gallons) of desalted water per day, to meet the growing demand of the arid American southwest. Las Vegas, Phoenix, and Los Angeles really need more water. But the YDP sits idle. The Yuma reject brine stream (TDS 9,400 ppm) is a daunting 117 million liters, or 117,000 m3 per day. Although the waste brine is relatively low in salinity, over time the salts accumulate where it is dumped, poisoning local fauna and creating a putrid trap for migrating waterfowl. From the cautionary tale of the YDP it should be clear that concentration of RO reject brine is an important unsolved problem in the art of desalination and a critical need for environmental protection as humanity struggles to increase water supply. Before making any agreement for a reverse osmosis plant, responsible policymakers should have an answer to this question, and not blunder blindly forward like the Yuma Desalting Plant.
Seawater Reverse Osmosis Reject Brine The pitch for reverse osmosis seldom includes a frank discussion of the reject brine problem. Like the carbon sequestration problem, we would all prefer to pretend it’s not there. Deal-busting prudence has never been popular or profitable. Seawater salinity is approximately 35,000 ppm. Most of the salt is sodium chloride, but calcium carbonate and sulfate salts are also present in high concentrations, and these other salts are what cause scale, an insulating crust on heat exchange surfaces, making distillation difficult. These scale-producing salts also block reverse osmosis membranes, so a necessary step is pretreating the brine upstream of the membrane. The pretreatment step adds to the operating cost. Getting potable water from seawater is much more challenging for reverse osmosis than the Yuma reject brine (TDS only 9,700 ppm). The energy requirement is very large, due to the high pressure required. RO desalination of seawater has a typical recovery rate of only 50% , so the reject brine would have a salinity of approximately 70,000 ppm, or 7%, and it would be as voluminous a stream as the stream of potable water produced. Dumping such a voluminous stream of highly concentrated industrial waste is not a long-term option in a world with rapidly increasing water needs. A high salinity plume dumped into the ocean is not a sustainable solution because no one wants a local Dead Sea. Various proposals for hiding the waste stream underground or in the ocean have transportation problems and are not really solutions at all. The only known dewatering method to reduce the volume of reject brine is evaporation ponds, which require valuable space and blight the environment near production facilities. Evaporation ponds are a toxic trap for migrating waterfowl. Industrial waste dumpsites, even temporary ones, are not satisfactory solutions to the unsolved problem of reject brine from reverse osmosis.
Physical Water Treatment Removing scale-forming ions from feed going to boiling or reverse osmosis can be done by non-chemical means, known collectively as physical water treatment. Various physical water treatment devices and processes use electromagnetic force in combination with flow velocity to cause Lorentz force on ions in the feedwater. Laminar flow is considered to be good, because turbulent flow would remix the ions. However, because Lorentz force is proportional to flow velocity, and laminar flow must be slow, prior art in this area has significant limitations.
Produced Brine from Oil and Gas Wells Another major cause of environmental blight is oil and natural gas wells, which produce 20 to 30 billion barrels of brine each year. This water ranges in salinity from a few thousand to 463,000 ppm. The volume is 70 times the total of all liquid hazardous wastes generated in the U.S. Approximately 95% of this brine is reinjected, at least by responsible operators, but that still leaves a huge stream being dumped into evaporation ponds or transported for reinjection. A need exists for a better way to concentrate produced brine from oil and gas wells. There are valuable organic compounds in the produced brine, and it would be a bonanza if they could be economically recovered. The transportation costs could be minimized by reducing the volume. However, separating a very large stream of mixed oil, water, mud, sand, and salt economically is an unsolved problem.
Counter-rotating disks within the separator create powerful radial flow effects that use centrifugal force to separate the brine components of different weights, while a RF pancake coil electrical inductor controls the flow of components according to their electrical properties. These combined effects create a continuous radial counterflow pattern, with purified fluids flowing inward toward the central axis and increasingly concentrated waste products flowing outward into a collection tank. No pretreatment, added heat, chemicals, dead-end filters, membranes, ion exchange, electrodialysis, or distillation are needed. It represents an inexpensive and easily scalable improvement in industrial and municipal wastewater processing, cleaning of brine waste from oil and gas production, and field water purification.