Water desalination processes separate dissolved salts and other minerals from water. Different processes are used for this matter, such as vacuum distillation and membranes to desalinate. Membrane separation requires driving forces including pressure (applied and vapor), electric potential, and concentration to overcome natural osmotic pressures and effectively force water through membrane processes.
Seawater desalination has the potential to reliably produce enough potable water to support large populations located near the coast. Reverse osmosis (RO) and Nano filtration (NF) are the leading pressure driven membrane processes. Operating pressures for RO and NF are in the range of 50 to 1000 psig (3.4 to 68 bar, 345 to 6896 kPa). Conventional macro-filtration of suspended solids is accomplished by passing a feed solution through the filter media in a perpendicular direction.
Examples of such filtration devices include:
- Cartridge filters
- Bag filters
- Sand filters
- And multimedia filters.
Macro-filtration separation capabilities are generally limited to undissolved particles greater than 1 micron.
For the removal of small particles and dissolved salts, crossflow membrane filtration is used.
Electrodialysis (ED) and Electrodialysis Reversal (EDR) processes are driven by direct current (DC) in which ions (as opposed to water in pressure driven processes) flow through ion selective membranes to electrodes of opposite charge. In EDR systems, the polarity of the electrodes is reversed periodically. Ion-transfer (perm-selective) anion and cation membranes separate the ions in the feed water. These systems are used primarily in waters with low total dissolved solids (TDS).
Membrane Distillation (MD) is a water desalination membrane process. MD is a hybrid process of RO and distillation in which a hydrophobic synthetic membrane is used to permit the flow of water vapor through the membrane pores, but not the solution it. The driving force for MD is the difference in vapor pressure of the liquid across the membrane.
Desalination (also called “desalinization” and “desalting”) is the process of removing dissolved salts from water, thus producing fresh water from seawater or brackish water. Desalting technologies can be used for many applications. The most prevalent use is to produce potable water from saline water for domestic or municipal purposes, but use of desalination and desalination technologies for industrial applications is growing, especially in the oil & gas industry.
KASRAVAND Believes developments in desalination technologies are specifically aimed at reducing energy consumption and cost, as well as minimizing environmental impacts. Advancements include such new and emerging technologies as forward osmosis, low temperature distillation, membrane distillation, pressure retarded osmosis, biomimetic and graphene membranes.
There are two types of membrane process used for desalination: reverse osmosis (RO) and electro dialysis (ED). The latter is not generally used in Latin America and the Caribbean. In the RO process, water from a pressurized saline solution is separated from the dissolved salts by flowing through a water-permeable membrane. Permeate (liquid flowing through the membrane) is encouraged to flow through the membrane by the pressure differential created between the pressurized feed water and the product water, which is at near-atmospheric pressure. The remaining feed water continues through the pressurized side of the reactor as brine. No heating or phase change takes place. The major energy requirement is for the initial pressurization of the feed water. For brackish water desalination the operating pressures range from 250 to 400 psi, and for seawater desalination from 800 to 1000 psi.
In practice, the feed water is pumped into a closed container, against the membrane, to pressurize it. As the product water passes through the membrane, the remaining feed water and brine solution becomes more and more concentrated. To reduce the concentration of dissolved salts remaining, a portion of this concentrated feed water-brine solution is withdrawn from the container. Without this discharge, the concentration of dissolved salts in the feed water would continue to increase, requiring ever-increasing energy inputs to overcome the naturally increased osmotic pressure.
A reverse osmosis system consists of four major components/processes:
3- Membrane Separation
4- Post-Treatment Stabilization.
Pretreatment: The incoming feed water is pretreated to be compatible with the membranes by removing suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused by constituents such as calcium sulphate.
Pressurization: The pump raises the pressure of the pretreated feed water to an operating pressure appropriate for the membrane and the salinity of the feed water.
Separation: The permeable membranes inhibit the passage of dissolved salts while permitting the desalinated product water to pass through. Applying feed water to the membrane assembly results in a freshwater product stream and a concentrated brine reject stream. Because no membrane is perfect in its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains in the product water. Reverse osmosis membranes come in a variety of configurations. Two of the most popular are spiral wound and hollow fine fiber membranes. They are generally made of cellulose acetate, aromatic polyamides, or, nowadays, thin film polymer composites. Both types are used for brackish water and seawater desalination, although the specific membrane and the construction of the pressure vessel vary according to the different operating pressures used for the two types of feed water.
Stabilization: The product water from the membrane assembly usually requires pH adjustment and degasification before being transferred to the distribution system for use as drinking water. The product passes through an aeration column in which the pH is elevated from a value of approximately 5 to a value close to 7. In many cases, this water is discharged to a storage cistern for later use.
As a general rule, membranes with high water permeability (low feed pressure) also have a higher salt permeability compared to membranes with lower water permeability.
The permeability of solutes decreases (the rejection increases) with an increase in:
- Degree of dissociation (Weak acids such as lactic acid are rejected much better at higher pH when the dissociation is high)
- Ionic charge (Divalent ions are better rejected than monovalent ions)
- Molecular weight (Higher molecular weight species are better rejected)
- Non polarity (Less polar substances are rejected better)
- Degree of hydration (Highly hydrated species e.g. chloride, are better rejected than less hydrated ones e.g. nitrate)
- Degree of molecular branching (Iso-propanol is better rejected than n-propanol)