Professor POU/POE – August 2013

Aug. 1, 2013

Q We design many commercial softeners and filters. Occasionally we experience very short capacity on both the softeners and filters. We use a lot of …


We design many commercial softeners and filters. Occasionally we experience very short capacity on both the softeners and filters. We use a lot of fiberglass tanks and the lower distribution systems that come with them. I’m thinking that the same distribution system should not be used on the softeners and the filters. Can you help with this?

                                                                                                   – Wisconsin


The design of outlet, or lower, distributors is often overlooked when designing commercial softeners and filters. Each distributor has to be matched to the design flow rate of the particular tank involved.

Water softeners and other ion exchange equipment

The principles are quite simple. An outlet distribution (collection) system, designed by you or the tank manufacturer, requires that certain conditions be met. The treated outlet flow must be equally distributed over the entire area of the tank. This requires some pressure (psi) loss. With no psi loss there will not be equal distribution through the resin bed. What controls these is a lower collection system, which must incur only a small amount of pressure loss. Without this loss the treated water will flow toward the center of the distribution system, thereby causing a “funnel” shape, which will not utilize the resin near the exterior of the bed.

If the distributor system incurs too much pressure loss it will be undesirable for the whole system. An optimum pressure loss in the outlet system is about 1 psi. This assures that the overall psi loss during treatment will be minimized and that the water being treated will be equally distributed over the entire area of the ion exchange bed.

The design requires some inquiry into the manufacturer’s specifications of the lower distributor that comes with the specific tank in question. These should be expressed as Cv (the flow rate which yields a 1 psi loss) values or as a graph which displays a set of curves showing the pressure loss of all the available lower distributors at various flow rates. If this examination is not done properly, either the treated flow rate will incur too high a loss or the water will not be distributed uniformly over the bed area. Either of these issues will cause the ion exchanger to fail to perform as expected.

Since backwash and fast rinse flow rates are higher than service flow rates, they are not a particular concern. If the above rules for service flow are followed the pressure loss will not be a concern for the backwash and fast rinse because they are going to the drain and thus do not require the same pressure as does the building.

Water filters

The same parameters which apply to softeners also apply to filters. However, if a filter and a softener with the same diameter are used it is unlikely that the same lower distributer will be correct for both. Most filter media require very low service flow rates compared to softeners.

Because filtration rates are typically much lower than softeners, it is commonly the case that the diameter of a filter used with a softener is greater. This obviously indicates that a softener and an associated filter of a different diameter require different outlet distributors. Take this into consideration when designing a filter/softener combination.

If the lower distributors that come with fiberglass tanks cannot meet the above requirements, you will need to design your own. You can do this by adapting the tank’s bottom outlet fitting to a network of PVC pipe with individual plastic nozzles (much like the ones used on residential equipment) properly spaced and with a correct quantity to yield the approximate 1 psi referred to above.


I’m dealing with a well water supply that has a TDS of 1,000 ppm and a hardness of 250 ppm, all as CaCO3. Please advise whether a water softener is required. Does the TDS affect the treated water hardness or the resin’s capacity?

– India


The hardness of 250 ppm converts to 15 grains per gallon (gpg) and the TDS of 1,000 ppm converts to 59 gpg. Fifteen gpg would normally require softening. It is important to know what influence the TDS has on softening.

Assuming a resin bed depth of 30 inches, a single-pass downflow and a feedwater of less than 85 percent sodium, the TDS will cause the continuous hardness leakage to be between 2 ppm and 24 ppm. The more specific leakage is determined by the salt dose.

At a salt dose of 5 pounds per cubic foot of resin the product water hardness (leakage) will be about 24 ppm (1.4 gpg). At a dose of 10 pounds per cubic foot the leakage will be about 5.6 ppm (0.3 gpg). Either of these dosages would be acceptable for home use. If, on the other hand, the water is for low pressure-low temperature boiler feedwater, the hardness should be less than 2 ppm. A salt dose of 17 pounds per cubic foot will yield a hardness of 2 ppm.

At a higher TDS level the hardness leakage is greater. At a lower TDS level the leakage is lower. This is caused by the fact that increased TDS means more sodium (Na) and potassium (K) are present, both of which are used as regenerants for softeners. During softening, increased Na and K act upon the resin as they do during regeneration. They displace hardness ions but to a lesser degree. Hence, higher TDS leads to greater hardness leakage. At higher TDS levels you could approach the point where it would not be economically practical to soften.

TDS also affects the capacity of softening resin. Given your numbers and assuming a five-pound per cubic foot dose at a temperature of 600° F, a flow rate of 6 gpm per cubic foot, a 30-inch bed depth, and a 1 gpg endpoint (1 gpg over the continuous leakage) would yield a capacity of about 15,000 grains per cubic foot. Under the same conditions, but with a 10-pound dose, the capacity would be about 23,000 grains per cubic foot. A 15-pound dose would about yield about 28,000 grains per cubic foot.       

David M. Bauman, CWS-VI, CI, CCO, is technical editor of Water Technology® and a water treatment consultant in Manitowoc, Wis. He received his B.A. from the University of Illinois in Industrial Design. He can be reached by email at: [email protected].

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