Generally defined as an effective and environmentally friendly process of purification, separation, and decontamination of aqueous solutions through the exchange of ions.
The following information is summarized from a patent disclosure containing much the same information as found in several other resources:
At this time, known techniques will not permit one to employ economically the ion exchange process for the treatment water having a salt content greater than 1,000 mg. per liter of solution. Beyond this limit the operating costs become higher than those resulting from the use of other techniques. (Seawater has a concentration of 35 ppt or about 36,050 mg/L)
This (patented) method permits one to obtain demineralized water at low cost and overcomes the difficulties experienced when traditional ion exchange processes are used to process water having a high concentration of salt:
Summary of Invention:
...fluids having high mineral content can be demineralized economically by contacting the fluid first with strong base anion exchange resin... followed by contact with a weak acid cation exchange resin. The fluid is then subjected to a polishing operation consisting of contacting the fluid with strong acid cation exchange resin followed by contacting said fluid with a weak base anion exchange resin. Regeneration is carried out by passing a bicarbonate solution first through the strong base anion exchange resin and then through the weak base anion exchange resin. The cation resins are regenerated by first passing acid regenerant through the strong acid cation exchange resin and then through the weak acid cation exchange resin.
The process is more complex than what the team is looking for; resins, reservoirs, and a lot of upkeep. Also, as stated in the patent and other articles found online, it is not terribly economical, sustainable, energy efficient, etc. Ion exchange is not the best method comparatively, but can be used effectively as a final treatment. Several sites recommended electrodialysis as an alternative to ion exchange.
EDR (electrodialysis reversal desalination) is a process in which an electric current migrates dissolved salt ions (including fluorides, nitrates and sulfates) through an electrodialysis stack consisting of alternating layers of cationic and anionic ion exchange membranes. Periodically, the direction of ion flow is reversed by reversing the polarity applied electric current.
Electrodialysis depends on the following general principles:
- Most salts dissolved in water are ionic, being positively (cationic) or negatively (anionic) charged.
- These ions are attracted to electrodes with an opposite electric charge.
- Membranes can be constructed to permit selective passage of either anions or cations.
An electrodialysis unit is made up of the following basic components:
- Pretreatment train
- Membrane stack (several hundred cell pairs bound together)
- Low-pressure circulating pump
- Power supply for direct current (a rectifier)
Electrodialysis has the following characteristics that lend it to various applications:
- Capability for high recovery (more product and less brine)
- Energy usage that is proportional to the salts removed
- Ability to treat water with a higher level of suspended solids than RO
- Lack of effect by non-ionic substances such as silica
- Low chemical usage for pretreatment
Electrodialysis units are normally used to desalinate brackish water. The major energy requirement is the direct current used to separate the ionic substances in the membrane stack.
This process is more like a reverse fuel cell, and would be just as complicated. Not effective to recreate for our small scale (and small budget) application.