Everything Old Is New Again
Both ion exchange resins and membranes have been used for decades in the field of quality control of drinking water. Ion exchange resins were first used to provide potable drinking water to homes in mid-1900s. At the same time, the U.S. military used resins to provide drinking water as a replacement to the Lister Bags that were used in World War II. The military then began to conduct a series of tests of these resins at various temperatures. However, the resins were soon overtaken by membranes, which then were subjected by the military to the same tests that were being conducted on the resins.
However, both these technologies now have a variety of new applications. In this article, we will examine the current uses of resins and membranes for the provision of potable water.
Resins generally are the result of sulfonation of divinylbenzene copolymers (of various crosslinkages). They were first known as Dowex 50 resins for the home market. Calgon used them to exchange sodium for the "hardness" ions -- calcium and magnesium especially. Generally, these materials were removed before they were completely saturated by the hardness cations. In the case of industries, such as television tube manufacturers, where complete removal of ions was required, the cation exchanger was followed by an anion exchanger, or else, the two exchangers were co-mixed. The anion associated with the quaternary was usually the chloride ion. The anion exchanger was regenerated with sodium hydroxide.
Membranes soon replaced resins. The membranes were made by taking sheets of the polystyrene divinylbenzene material and either sulfonating or quarternarizing them as required. The resulting process then was standardized.
However, there was a major problem with membranes. This problem became evident during a large project carried out by the U.S. government. The United States had committed to providing to the country of Mexico water from the Colorado River that had a certain level of salinity. This required processing millions of gallons of water from the Colorado River on a daily basis. By 1968, reverse osmosis membranes had only been used successfully for units of up to 100,000 gallons per day. The result was that the membranes became contaminated with precipitation of calcium salts. Consequently, a pilot plant was set up at Yuma, Arizona to try to undo this precipitation of salts, or to understand it better. That plant was created by the Office of Saline Water, which since that time has gone out of business and the pilot plant is now operated by the Office of the Bureau of Reclamation.
How These Technologies Work
A column of a typical cation exchange resin is placed in the sodium form. A water containing hardness (calcium and magnesium ions) is passed through it. The hardness ions replace the sodium ions. This does not happen evenly. Eventually, the hardness ions will start leaking through the column. Long before that happens, the column of used cation exchanger is replaced with a fresh one. The activity then continues. The replacement is effected by passing a concentrated salt (NaCl) through it. The column is thus regenerated and ready for use again.
EPA is actively encouraging U.S. drinking water treatment facilities to consider using ion exchange as one possible method to remove arsenic from raw water supplies.
In the case of total deionization, the cation exchanger, in the hydrogen (H+) form releases that hydrogen ion. The effluent solution then goes to an anion exchanger in the hydroxyl (OH-) form. The hydrogen and the hydroxyl ions combine to form water. The cation exchanger is regenerated with acid (HCl). The anion exchanger is regenerated with a base (NaOH). Sometimes, the cation exchanger and the anion exchanger will be mixed together. In those cases, regeneration must be done very carefully.
Membranes work through a method called reverse osmosis, where the cations go out of one electrical end and the anions out of the other, leaving alternate effluents of pure water and highly concentrated materials. As mentioned in above, the membranes tend to be contaminated with calcium or magnesium salts,
Some Current Applications
Ion Exchange Resins Used in Desalination
Electrodialysis using ion exchange resins is an established process for desalting brackish water. Electodialysis is based on the development of membranes that are selective for the passage of ions of a given charge.1 Two different membranes are used; one is more selective to anions, and the other is more selective to cations. Electric current aids the diffusion of these ions, and the electric energy required is proportional to the concentration of salts in the saline water. Consequently, the process is more attractive for desalting of brackish (low salt concentrations) waters.
Ion-exchange resins, which comprise 60 percent to 70 percent of the membrane, are solidly hydrated, strong electrolytes and might be regarded as solid sulfuric acid or as caustic solid. The resin most commonly used is polystyrene cross-linked with divinylbenzene.
Electrodialysis units with capacities from 10,000 to 650,000 gallons per day have been installed. The process has major advantages for brackish water but is considered too costly in electric power requirements for desalting seawater. Additionally, keeping the membrane surface clean is a major problem. Prefiltering and chemical treatment of feed has kept plants operating. Membrane replacement costs are a major part of producing fresh water by this method.
Removal of Arsenic from Drinking Water by Ion Exchange
In January 2001, the U.S. Environmental Protection Agency (EPA) adopted a new standard for arsenic in drinking water at 10 parts per billion (ppb), replacing the old standard of 50 ppb. Public water systems must comply with the 10 ppb treatment standard by January 23, 2006.
EPA is actively encouraging U.S. drinking water treatment facilities to consider using ion exchange as one possible method to remove arsenic from raw water supplies.2 The anion exchange method is very reliable, simple and cost-effective for removing arsenic in the As(V) form from drinking water. The treatment process removes arsenic using a strong base anion exchange resin in either the chloride or hydroxide form, with the chloride the preferred form because salt can be used as the regenerant. The ion exchange process is a proven method of removing As(v) from water supplies with low sulfate levels.
The Wide Variety of Membrane Applications
Today's water suppliers are examining membranes for achieving the removal of microbes (bacteria, giardia and cryptosporidium cysts) and trihalomethane (THM) precursors and pesticides.3 Furthermore, they are also starting to use membranes to extract potable water from brackish, alkaline sources or seawater or color from surface water. Potentially, membrane filtration can provide for one-step removal of turbidity and THM precursors, as well for disinfection.
Electrodialysis using ion exchange resins is an established process for desalting brackish water and converting it into drinking water.
The four commonly accepted categories of membranes are defined by the size of the material being removed from the carrier fluid. From the smallest to largest pore size they are: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF).
RO eliminates dissolved solids, bacteria, viruses and other organisms from water. RO is mainly used for producing potable water from salt-bearing (primarily brackish) water.
NF concentrates on removing salts, bacteria, particles, and other constituents that have a molecular weight greater than 1,000. It works in situations requiring high organic and moderate inorganic removals.
The successful use of the UF for the concentration of the rejected solutes depends primarily on particle size and, to some extent, on particle charge. Typically water-plant rejected species include bio-molecules, polymers and colloidal particles.
By far the most widely used membrane process, MF essentially provides sterile filtration, restraining pass-through of microorganisms and material of colloidal size and larger. Muff's pores are 0.1 to 10.0 microns -- two to five orders of magnitude larger than that for other membrane classes. As a point of reference, the smallest bacterium, pseudomonas diminuta, has a size of 0.3 micron.
The list of organic and inorganic substances to be removed from raw water supplies will inevitably get longer. As EPA constantly seeks to improve U.S. drinking water supplies, more advanced technology will be needed to accomplish this goal. No doubt ion exchange resins and membranes will continue to play important roles in providing safe drinking water to the citizens of our country.
Dr. Bregman wishes to acknowledge the agony suffered by his wife Mona as she learned to use a laptop computer when typing the first draft of this article. Also, he acknowledges the work of Joan Jackson, his executive assistant, and Robert Edell, president and CEO of Bregman and Company. He also wishes to point out that Harry Gregor was known as the "father" of the theory of how ion exchangers work and K. Channabassappa and Fred McGowan were responsible for the use of membranes in practical applications.
1. Liu D.H.F., Liptak B.G., and Bouis A.B. Environmental Engineers' Handbook. 1997. Lewis Publishers. Pages 994-995.
2 U.S. Environmental Protection Agency. Removal of Arsenic from Drinking Water by Ion Exchange Design Manual. 2003
3. Hersch P. "Picking Their Membranes ? Part 1", Environmental Protection, September 2001, Vol. 12, No. 9, Available at no charge at Environmental Protection?s Web site (www.eponline.com) under "Archives." Pages 33-36.
This article originally appeared in the June 2005 issue Environmental Protection, Vol. 16, No. 5.
This article originally appeared in the 06/01/2005 issue of Environmental Protection.