In response to requests from Plumbing Manufacturers Intl. (PMI) and its members, as well as from other supporters of the U.S....
The pros and cons of adding various filter media to softening resin
Using a water softener is a popular and effective method for residential water treatment. A water softener improves water quality by removing hardness and iron, and in most cases, the benefits of the system far outweigh the equipment and maintenance costs.
Basic water softeners require little regular attention. Having a high quality cation softening resin on top of a gravel support bed resolves many residential water conditioning concerns; however, the gravel support is optional, and in many applications not used. The media is regenerated, and under normal conditions, its capacity does not become significantly reduced for many years. In residential applications, it is not unusual for a softener resin to continue operating well for more than 10 years.
This is not usually true of the many filter media types often used in conjunction with softeners. It is very popular among residential water conditioning dealers to “supercharge” their softeners by adding various types of filter media to the systems. These additions can be a great advantage in many cases, providing an effective and economical course to treat multiple contaminants that might otherwise require redundant equipment. When sized, applied and maintained properly, these multifunctional water treatment units make excellent point-of-entry water conditioning systems.
In an effort to provide a unique product, water treatment dealers sometimes combine various incompatible filtering media with softening resin in a single vessel. It seems that each dealer is trying to exceed the other by offering a system with more “stages” of filtration than the competitor’s system. This often leads to poor performance, low softening capacity and poor filtration capability. Consequently, the end user may be left with a system that provides minimal benefits.
For example, in the same tank, combine manganese dioxide, which requires about 30 gpm/sq ft for backwash, with a softening resin that requires about 5 gpm/sq ft, and then try to find a workable backwash rate. In one extreme case, a dealer was offering an 18-stage softening system that had a minuscule amount of nearly every type of media available. The product contained so little of each that attaining any benefit at all would be by accident rather than by design.
This can be overcome, however, by incorporating divisions within the mineral tank to make otherwise incompatible materials operate properly together. Dealers can offer carefully engineered multi-tasking systems that provide great benefits, rather than a sales gimmick.
Granular activated carbon is frequently combined in the same tank with water softening resin. Carbon can remove or significantly reduce a wide range of foul-smelling water contaminants, and it offers significant benefits for a multitude of other water treatment concerns. Carbon and softening resin make a decent combination in relation to their backwash requirements and overall compatibility. In most cases, however, the resin and carbon tend to mix together inside the mineral tank, rather than create the desired separate layers that would offer better performance. The practice of merely adding a few pounds of carbon to the softening resin offers minimal short-term benefit and almost no long-term capacity.
Using 1/2 cu ft of 12x40 granular activated carbon will offer chlorine removal for about 300,000 gal or approximately three to five years in average residential conditions. The intermixing of the carbon and resin will result in only partial oxidation protection for the resin, but this is not typically the primary reason carbon is added to softeners. It is of utmost importance to perform regular maintenance on these systems with carbon and resin mixtures. Operated beyond its reasonable capacity, the carbon will not only stop removing contaminants, but it will start to unload previously removed undesirable constituents back into the effluent supply.
There are some specialty carbons that tend to remain in a separate layer and may be replaced without removing a significant amount of the resin. With most granular activated carbon products, however, the carbon and resin should be replaced at the same time because it is generally not worth the time and effort required to separate the two. The primary cost when performing this service is the time required, not the media cost. A hydraulically operated media extractor can significantly reduce the labor time required to remove the media from the tank. These are available already set up, or they can be constructed using an inlet outlet head and a few standard plumbing parts.
Some anion resins can be successfully combined with cation softening resin to accomplish specific contaminant removal tasks. This is especially popular for the reduction of nitrates and tannins in conjunction with softening. Many of these anion resins are regenerated with salt, so the combination appears to be a perfect match.
There are issues, though, such as the susceptibility of many anion resins to high influent hardness levels. The lighter, less dense anion resins will settle at the top of the tank so that on standard cocurrent flow systems, the influent hard water encounters the anion resin first. There is no magic cutoff point, but as the hardness level increases above about 7 grains per gal, precipitation of hardness salts will foul the anion resin, robbing from the capacity and shortening the usable lifespan. In these cases, it is best to use separate systems with the softener installed ahead of the anion unit.
Calcium carbonate is another notable media often combined with water softeners. This seems like a strange combination, as they are in some ways promoting opposite functions. Typically used to increase acidic pH levels, calcium carbonate also adds hardness and total dissolved solids back into the water. This highly versatile media is excellent for many water treatment purposes, but it works best when used with a dedicated backwash valve.
In mixed systems, the influent water is softened as it passes through the cation resin, and then some hardness is added back when the water encounters the calcium carbonate near the bottom of the tank. This may or may not produce desirable results based on individual applications, but in any case, the amount of hardness added will be difficult to estimate and control. The calcium carbonate product will slowly dissolve as water passes through it, leading to a continuously lower quantity of calcium carbonate in the tank. That is, until or unless the depleted media is replenished.
While capable of providing beneficial results, calcium carbonate combined in the same tank with softening resin is usually used as a sales tool to promote the system as a neutralizer. If not maintained on a regular basis, the calcium carbonate will be depleted and cease to serve this neutralization function.
Many other types of filtering media are combined with softeners at some time or another. No matter what media is used, it is important that multi-bed systems utilize a well-planned configuration, engineered to operate properly together. There must be a sufficient quantity of any given media to actually provide the benefit intended, and keep in mind that each has particular service and backwash flow rate parameters.
Squeezing more media into a mineral tank often results in inadequate freeboard, leading to particulate accumulation, channeling and poor long-term performance. A basic softener is usually all that is required, but if your customers want a supercharged system, be reasonable with the design and prepare for the increased maintenance.