Manufacturers recently have made great strides in developing ion exchange resins that effectively adsorb and desorb organics when regenerated in the brine cycle. These resins are usually referred to as organic scavengers (OS) or organic traps.
When do you know a customer has organics such as tannins in their water supply?
The water almost always has a faint yellow to tea-like color, which is sometimes accompanied by a sulfur-like smell. Tannins are what give tea its color.
Most water affected by tannins is in marshy, low-lying and coastal areas. Although they pose no real health threat, tannins affect water's aesthetic qualities and can ruin clothing washed in tannin-laden water.
Before the development of OS resins, tannins and other organics were removed by chlorination, chemical precipitation or polymerization. These methods use chemical feed pumps that need to be maintained and large retention tanks that take up basement space in customers' homes.
How does an OS resin function? Unlike conventional ion exchange resins, OS resins do not exchange organics. Instead, they adsorb them on the surface of the resin. Also, since OS resins are anionic, they tend to dealkalize water. Two types of OS resins are most frequently used by dealers selling to the residential market: Gel type 1 acrylic anions and type 1 strong base macroporous anions.
The difference between the two is that the gel type 1 anion has less surface area and is slightly prone to fouling. Most acrylic gel-type OS resins are preferred where there is less than 1 part per million (ppm) organics and the water is not turbid. A macroporous anion has large pore structures that can produce better results in more difficult cases.
Rules to Scavenge By
How do dealers choose the best OS for their applications? Source water testing is most important. Dealers should test water for sulfates, alkalinity, iron, total dissolved solids (TDS) and hardness.
Limit the use of any OS system to water that contains no more than 5 ppm tannins, 500 ppm alkalinity and 500 ppm TDS. Highly alkaline and TDS water may indicate salt intrusion, especially if the source water is from coastal areas.
Many dealers who apply OS resins also suggest eliminating iron and hardness because iron can coat the resin bead, limiting adsorption. Hardness can combine with sulfate, causing a calcium sulfate precipitate that will also inhibit the adsorptive properties of OS resin. If your tannins are greater than 5 ppm, then you may want to pre-treat with chlorine followed by a backwashable carbon filter, then polish the water with the OS.
One rule of thumb to remember: The darker the water, the more difficult it is to treat.
Unfortunately, since OS resins do not work under principals of ion exchange, there are no concrete formulas available to calculate system capacity. The only tried and true method for estimating bed life is to run a column test on the water to be treated
Treatment System Design
After selecting the best OS resin, the next step is to design the proper treatment system. Since OS resins work best in the chloride form, OS treatment systems resemble a water softener. Choose a reliable control valve, that can program every cycle.
Most manufacturers recommend service flow rates of 1 to 4 gallons per minute per cubic foot (gpm/cf). This may be a bit conservative, but use common sense when sizing and estimating the service requirements of your application.
For example, flow rates in many homes are intermittent; the average home may only have a peak flow rate of 6 gpm at different times of day. Therefore, OS systems may require no more than 1 cf of bed.
This may explain why, in some situations, dealers have good results using a dual bed system by placing as little as one-third cubic foot of OS on top of a cation bed. A dual bed system is appropriate because OS resins are much lighter than cation resins.
In any system design, upper distributor screens are recommended to prevent backwashing the resin to drain, especially if you are using OS on top of a cation bed. OS resins range from 16-50 mesh. Choose a distributor screen with a .010-.013 slot size. A gravel under-bedding is also recommended to help distribution and flow rates. Use gravel that is either 1/4" by 1/8" or 1/16"x1/8" in size. for proper backwash rates.
The most important aspect of an OS conditioner design is the brine cycle. Most organics like tannins take much longer to elute off the resin bead. The minimum duration that brine should dwell in the bed is 30 minutes. No more than 10 pounds of salt is required to regenerate 1 cubic foot of OS resin.
If possible, program the brine cycle to "stall" 20 minutes into the cycle. This will give the brine a chance to elute the organics off the resin. In some systems, brine stalling is impossible or unnecessary. But it is recommended if it can be incorporated into your system.
Regeneration Helpers
OS resins are extremely prone to fouling. Salt additives such as phosphoric acid, citric acid and soda ash aid the regeneration process. Resin manufacturers regard soda ash is a safe alternative to sodium hydroxide (caustic). Since some OS resins can dealkalize water, soda ash will replenish lost carbonates that might have depressed the pH.
To better understand the role soda ash plays in the regeneration process, imagine the OS resin bead as a tiny sponge. Brining contracts the resin bead; soda ash swells the bead. This microscopic expansion and contraction of the resin bead aids in the elution of organics. Soda ash also decreases the solubility of an organic like tannin, providing a more thorough rinse.
Oxidizers such as hydrogen peroxide, chlorine and ozone should be removed before treating with OS resin. Oxidizers can permanently damage OS resins because in order to obtain large pore structures, the best performing OS resins are manufactured with limited cross-linking as a framework.
Resin derives its cross-linking from a percentage of divinylbenzene used in the resin. As a general rule, the higher percent of cross-linking, the stronger the resin bead. No resin, however, is entirely oxidizer-proof.
Most improperly applied resins foul within 48 hours. However, even the most carefully applied OS resins may eventually foul. An indicator of a fouled bed is loss of capacity and random slugs of colored water during peak flow rates.
The most effective method to clean a fouled bed is a warm brine squeezing, for which you will need: a portable brine tank, a clean 5-gallon bucket, 10 pounds of solar salt and one pound of citric acid.
SYMBOL 183 \f "Symbol" \s 13.5 Place citric acid and solar salt in the brine tank
SYMBOL 183 \f "Symbol" \s 13.5 Add 5 gallons of warm water ( no hotter than 100 degrees F)
SYMBOL 183 \f "Symbol" \s 13.5 Mix thoroughly
SYMBOL 183 \f "Symbol" \s 13.5 Connect brine line to control valve
SYMBOL 183 \f "Symbol" \s 13.5 Cycle control valve to brine draw position
SYMBOL 183 \f "Symbol" \s 13.5 Hold the drain line until it feels warm
SYMBOL 183 \f "Symbol" \s 13.5 Stop and bypass the control.
Let the solution sit for a minimum of two hours, then let the control valve continue through its normal cycle. The drain water should run dark brown if the treatment was effective.
If the resin is still fouled after two warm brine squeezes, rebed and consider using a more suitable type of OS resin or treatment method.
As with any water conditioning system, success is determined by proper application, maintenance and research.
Michael Urbans is vice president of residential sales and manufacturing at Res-Kem Corp., Media, PA. Information for the article was provided by Ted Begg, Rico DiMattia, George Crits and Bud Kiefer.