Soft water is water with a near absence of calcium and magnesium salts. The Water Quality Association (WQA) defines soft water according to American National Standards NSF/ANSI 44 and NSF/ANSI 330: Soft water has less than one grain per gallon (gpg) of total hardness. According to the WQA, hardness is expressed in grains per gallon, or ppm, of calcium carbonate (CaCO3) equivalent.
The U.S. Geological Survey (USGS) defines soft water as having less than 60 ppm. The WQA definition has to do with scale formation and soap efficiency. The USGS definition has more to do with corrosion and is applied mainly to naturally soft water, which tends to be acidic.
Soft water might be best described in terms of what it isn’t. Consider these characteristics of hard water:
- Hard water forms a precipitate when heated.
- Hard water reacts with soap to form soap scum.
- Hard water precipitates with an increase in pH.
- Hard water forms rock-hard scale when it dries.
- Hard water can react with other chemical constituents to form precipitate in formulary.
Soft water does none of these. At least, that’s the theory.
Hardness reacts to form scale. Is something magical about the value of one grain per gallon that makes this characteristic disappear? The answer is no. Scale formation is a qualitative reaction. That means it occurs at any measurable level of hardness rather than at some set minimum level above which it does and below which it doesn’t.
If you have ventured into the industrial markets for soft water, you are likely aware that even medium-pressure boilers may demand soft water with less than 2 ppm of hardness. Above that level, the boiler develops scale and must be shut down for cleaning. Below that, it still forms scale, but the need to shut down for cleaning becomes less frequent. Higher-pressure boilers may require less than 0.3 ppm of hardness—a far cry from 17.0 ppm, which is considered by most to define soft water.
So, 17.0 ppm hardness water is not really soft. But it should be considered soft enough for residential application.1 It is fortunate that the demands for soft water performance for residential needs are so forgiving. Why? Because if a residential softener fails, it might not be noticed for several days or even weeks. If an industrial unit fails, it can be noticed almost immediately by losses in efficiency and costly damage.
Despite the forgiving nature of the residential softening application, the designing of the systems for residential use requires certain considerations to keep the customer happy. These considerations are not for proper sizing but for setting up the proper regeneration cycle and frequency. Although a softener of one cubic foot can handle 99 percent of household demands,2 it must be designed to handle the variables in the feed stream, such as the level of hardness, the ratio of hardness to sodium, total dissolved solids (TDS), water temperature, and level of hardness leakage that can be tolerated for the specific household.
The Equilibrium Relationship
Figure 1 represents softening by ion exchange, an equilibrium reaction.
Figure 1. The softening equilibrium equation.
As water passes down through a bed of resin, the driving force of the calcium (hardness) lessens, which slows the reaction. At the same time, the amount of sodium on the resin lessens, which also reduces the driving force for the reaction. Subsequently, the amount of hardness on the resin goes up and the amount of sodium in the water increases, as well. Both changes push the equilibrium in the opposite direction. Eventually, we hit an equilibrium where the reaction stops, and hardness starts to pass through the bed. This signals breakthrough, or exhaustion.
Prior to exhaustion and throughout the run cycle, a small amount of hardness leaches off the ion-exchange bed. This is called leakage. It occurs when the prior regeneration does not strip 100 percent of the hardness from the bottom of the resin bed.
Studies show that a freshly regenerated softening bed may have 15 percent or more of its sites still in the calcium form. With the very low hardness feed stream passing over it, some of the residual hardness leaches off. The feed stream is in equilibrium with the resin immediately above it, which has a lower residual hardness, assuming co-flow regeneration. As predicted from the equilibrium equation in Figure 1, this low-hardness stream regenerates the bottom of the bed. Leakage decreases as the run progresses because there is increasingly less hardness to strip off.3 Leakage and capacity are controlled by the regeneration level used.
Figure 2. Capacity and leakage vs. regeneration salt level.
Two relationships emerge: From the equilibrium equation, we see that the progress slows as the level of sodium in the water increases.4 Although we assume that the increased sodium is exchanging off the resin, the equation is valid for the sodium level in the original feed water, as well. In short, the higher the TDS of the feed water, the more difficult it is to soften the water.
The second discovery is that, as seen in Figure 2, the higher the regeneration level, the lower the leakage (and higher the capacity). Can increasing the regeneration level overcome the negative effects of an increasing TDS (and increasing sodium-to-calcium ratio) in the feed? What is that relationship?
Figure 3. Total TDS vs. salt dose for a given leakage.
Figure 3 illustrates that for a given TDS made up of a 50/50 ratio of sodium to hardness, the leakage can be decreased by increasing the regenerant dosage. The blue curve shows the salt level needed to produce one grain per gallon (gpg) at increasing TDS. In 2,000 ppm feed water, it will take about nine pounds (lbs) of salt per cubic foot (cu ft) to just break even on the softening leakage of 1 gpg. Obviously, for better performance, a half a grain (8.5 ppm) would increase the salt dose to around 15 pounds. At 3,000 ppm, we need to regenerate at 17 lbs/cu ft to hit 1 gpg leakage, and at 4,000 TDS, we need 26 lbs/cu ft for the same quality.
The Effects of the
Figure 3 is for a 50/50 ratio of sodium (Na) to calcium (Ca). As the sodium in the water increases and the calcium in the water decreases, the equilibrium equation predicts that the softening process will become more difficult. The leakage will go up, and the salt required to produce a given leakage will increase. It is probably not practical to attempt to soften water to half a grain with a single stand-alone softener with feeds above 3,000 ppm. The salt required is just too high for a residential user to handle.
The following table is a summary of the TDS limits for feed water having 100 percent calcium hardness, a 50/50 Na
/Ca ratio, a 75/25 Na/Ca ratio, and a 90/10 Na/Ca ratio. The values relate the TDS for a given Na/Ca ratio at 10 pounds and 15 pounds salt dose to produce 1 gpg leakage. Note that as the Na/Ca ratios increase, the TDS that can be handled for a given result decreases.
TDS Maximum for 1 gpg Hardness Leakage
|10 lbs/cu ft salt dose
|15 lbs/cu ft salt dose
Hardness value alone is not enough information to properly design a residential softener. There can be a 35 percent to 40 percent drop in the TDS tolerance between a high hardness/low sodium water and a low hardness/high sodium water. Producing acceptable leakage in a high-sodium feed requires a higher salt dosage. High TDS waters tend to be low in hardness. If the complete water analysis is unknown, assume a 50/50 sodium-to-hardness ratio. Your TDS limit for the typical regeneration level of eight pounds per cubic foot will be about 1,650 ppm. Ten pounds will lift your range to about 2,000 ppm.
Figure 4. Total TDS vs. salt level and leakage.
Figure 4 is a summary of hardness leakage for a given level of regeneration for a wide range of TDS values. This chart is for 100 percent calcium waters, so you must reduce the TDS values by about 20 percent. To use the chart as a starting point, use a TDS value 20 percent higher than your analysis. Example: You have 2,500 TDS water. Read the 3,000 TDS line vertically to the 10 ppm leakage value. Your starting salt dose is 15 lbs/cu ft.
- Michaud, C. F. 2012. “How Soft Is Soft Water? Soft Water Is a Green Technology.” Water Conditioning & Purification International Magazine, March 5, 2012. https://wcponline.com/2012/03/05/soft-soft-water/
- Michaud, C.F., “Differentiating: Residential, Commercial and Industrial Systems,” paper presented at the Water Quality Association Convention & Exposition, April 1-3, 2020, Orlando, Florida.
- Michaud, C.F., “What’s So Hard About Soft Water?” paper presented at the Pacific Water Quality Association 65th Annual Trade Show & Convention, October 11-12, 2022, Burbank, California.
- Michaud, C. F., “Ion Exchange Kinetics,” paper presented at the Pacific Water Quality Association 2021 Virtual Convention, October 12-14, 2021.