This article originally appeared in WQP May 2020 issue as "The Perfect Choice"
It seems like there are a thousand choices for softening resins available. Some are made in China or India, while others are made in North America. There are solventless sulfonated types, residential grade, fine mesh, coarse mesh, highly crosslinked surface sulfonated, macroporous, uniform particle size, amber, black, food-grade, NSF or Gold Seal certified, even Kosher or Halal certified and more.
Navigating through the choices to find the perfect resin for your application can be a frustrating and bewildering experience. The following pages outline the most important aspects to consider when choosing a type of resin and a supplier.
Gel Versus Macroporous
Gel resins are essentially solid plastics. The pores are so tiny they cannot be seen even with an electron microscope. They are translucent in appearance. Gel resins are the most common type used for water softening.
Macroporous resins have additional pores added that are large enough to see with a high-powered microscope. These pores allow water and salts to directly access the interior of the resin beads. The pores take up space and reduce the capacity for any given level of crosslinking. They also give macroporous resins their opaque appearance. Although macroporosity is independent of crosslinking, almost all commercially available macroporous softening resins are also highly crosslinked.
What About Crosslinking?
Crosslinking is the three dimensional component or glue that holds the beads together. Higher crosslinking result in higher capacity but also makes the beads more brittle and difficult to regenerate. There are basically four levels of crosslinking commercially available, ranked from lowest to highest here.
- Residential grade;
- Industrial grade;
- High crosslinked grade (sometimes called premium grade); and
- Super crosslinked grade (usually sold as a macroporous type).
Residential grades of softening resin should have moisture content less than 50% and total capacity greater than 40 kilograins/cubic foot (>1.8 eq/L). Although this grade has somewhat lower capacity than the other available grades, they are also easier to regenerate. For waters that do not contain chlorine, a residential grade softening resin is perfectly adequate. Be forewarned though, this type of resin has significantly reduced life with even a modest concentration of chlorine (<0.5 mg/L) in the inlet water.
Industrial grade softening resins have somewhat higher capacity and lower moisture than residential grades. Capacity should be greater than 44 kilograins /cu ft (2.0 eq/L) and moisture less than 45%. Industrial grade softening resins are the workhorse of the softening industry and have the best all-round performance, high capacity, good efficiency and moderate resistance to chlorine attack.
High crosslinked grades are also known as premium grades, and they certainly cost more than standard grades partly because they have higher crosslinking and partly because they have lower moisture; thus you are paying for more plastic and less water. Capacity should be greater than 48 kilograins per cubic foot (> 2.2 eq/L) and moisture less than 42%. This type of resin has better chlorine resistance and a higher temperature rating than industrial or residential grades. However, the increase in capacity does not keep pace with the increase in cost, so for most softeners, it would be more cost-effective to just add in a little more resin to make up for the capacity difference.
Super high crosslinked cation resins are usually made from macroporous polymer, otherwise they would be too difficult to regenerate. Along with the macroporosity comes higher moisture (to fill the pores) and lower capacity. The resin manufacturer chooses the amount of “discrete porosity” to add; therefore capacity and moisture vary from manufacturer to manufacturer. This type of resin is generally used for condensate polishing where severe service conditions and high-operating temperatures are expected. They have generally lower capacity and worse salt efficiency than other grades and are more expensive. They are used in a handful of softening applications primarily for their best-in-class chlorine resistance.
Which Resin is Best for Iron Removal?
This subject is one of the most controversial and least understood, primarily because iron can be present in several different forms, and water analysis seldom tells us anything except the total concentration. Briefly, iron comes in four forms:
- Dissolved clear water iron (Ferrous or Fe+2);
- Dissolved but organically bound (sometimes called “heme iron”);
- Insoluble but colloidal iron (often from ferrous iron that has been exposed to air); and
- Insoluble suspended iron (the larger bits and pieces from corroding piping, ect.).
Clear water iron is well removed by softening resins, just like the hardness ions calcium and magnesium. However, the water in the brine tank contains enough oxygen that during regeneration, ferrous iron that comes out of the resin can precipitate as ferric iron, thus fouling the resin. There are lots of rules of thumb for limiting this fouling, mostly involved in “compensating” by decreasing iron loading by limiting throughput. Using a sulfite- or phosphate-based resin cleaner with every regeneration can also help prevent fouling from clear water iron. Any grade of softening resin is about equal with respect to clear water iron removal, though fine mesh and uniform particle size (UPS) types may have some advantages over the larger sized “standard mesh” products.
Dissolved but organically bound iron is not removed by softening resins. Some organic iron can be removed by organic trap resins, but this is site specific and beyond the scope of this article to discuss in detail. “Heme iron” actually refers to only a small subset of organic iron that which is bound to animal proteins.
Colloidal iron forms when ferrous iron precipitates. Removal by softening resins is unpredictable but is generally less than 75%. This is because resins typically filter to about 2 microns in size and colloidal iron is typically less than 0.5 microns in size. Softening resins have coalescing properties, so some agglomeration occurs. Again, fine mesh and UPS resins have advantages due to their smaller size and greater surface area. Resins that win the “ugly” contest due to irregular shape and significant broken pieces are also notably better than perfectly beautiful spherical resins. However, this can be a slippery slope because in resin beds that contain a lot of small fragments, these fines can block the flow spaces in between the resin beads and lead to high pressure loss, channeling and other problems with unequal flow. Since removal of colloidal iron occurs at the surface of the beads and not inside them, the grade or resin is irrelevant.
Particulate iron, those bits and pieces larger than about 1 or 2 microns in size, are fairly well removed by softening resins regardless of grade. However, as with any filter, the inability to completely purge the suspended solids during regeneration can cause problems. The typical backwash cycle that comes before regeneration is generally not effective, and resin cleaners are also not effective. Various techniques, such as turbulators or vortex plates, are effective, but it is still necessary to compensate to prevent excessive loading.
Certifications of Cleanliness
The most common certification is the NSF 61 certification, offered by NSF, the Water Quality Association (WQA) and others. This certification is provided through the resin manufacturers for any grade of softening resin and covers potential toxic leachables that could come out of new resins, including inorganic and both volatile and semi-volatile organic compounds. This is a must for any potable water use and may limit some possible choices of resin.