Ion exchange water softening. How do I find out if the installation functions properly?

If you have a water softener at home or you have to work with Na-softener, and it is not possible to make a chemical analysis for hardness, then there is an easy way to find out whether the softener installation functions properly.

To do this, it is necessary to measure the electrical conductivity of the source and softened water during the operation of the softening unit. In the vast majority of cases for fresh water, the value of the electrical conductivity of softened water will always be greater than the value of the source water.

A bit of theory.

During the process of ion exchange water softening, the multivalent ions contained in the source water are replaced by the monovalent sodium ion contained on the ion exchange resin. Multivalent ions of the source water are the hardness ions (calcium and magnesium).

Thus, the number of ions in water does not change, but their type changes. In this case, one sodium ion can transfer more electric charge than one calcium or magnesium ion in the presence of sulfates. Accordingly, softened water almost always has a higher electrical conductivity than the original non-softened water.  Only if there are no sulfate ions in the water (which is practically impossible for natural water), the softened water will have a lower electrical conductivity than the source water.

The approximate sequence of actions for measuring the electrical conductivity of water should be as follows:

  1. You must ensure that the softener is on
  2. Take two clean empty glasses. Take a sample of the source water in one glass and the softened water in the other
  3. Measure the electrical conductivity values of the source water then the softened water. During the measurement process, it is necessary to achieve stabilization of the electrical conductivity readings. The more the water temperature differs from 25 0C, the longer it will take to stabilize the electrical conductivity reading. Hold the conductivity sensor for at least 5 minutes in each sample.
  4. Record the electrical conductivity values of the source and softened water.

As a result of measurements, we get two values of electrical conductivity. If the electrical conductivity of the softened water is greater than the source water then the water is softened.

But the question arises, how complete is the softening of the source water?

To answer this question, you need to understand how the softener works. Figure 1 shows the dependence of the hardness of softened water on the amount of water passed through the softening filter. As it can be seen from the graph, most of the time the unit produces softened water with a hardness of no more than 0.5 mg-EQ/l. In this case, the electrical conductivity of the softened water will change slightly and only at the end of the filter cycle, the electrical conductivity of the source water will increase sharply and reach the electrical conductivity of the source water.  The dependence of the softened water hardness on the electrical conductivity is shown in figure 2.

Figure 1

Figure 2

Thus, it is necessary to measure the electrical conductivity of softened water several times at certain intervals in order to make sure that its value is approximately at the same level. As soon as the value of the electrical conductivity of the softened water begins to grow and becomes equal to the value of the electrical conductivity of the source water, this indicates that the water-softening unit must be regenerated with a solution of table salt.

It should be noted that this method of control is possible only with a relatively stable ionic composition of the source water. In other words, the source of water supply must be the same. If the source of water supply is a tank where, for example, river water and artesian water constantly mix in different proportions, then it will be difficult to use this method of control. Although in this case, it is not clear at all which filter cycle to set up on the softening installation.

A few words about the device for measuring the electrical conductivity of water – conductometer.

The simplest pocket conductometer is sufficient for our purpose.

I recommend buying the conductivity meters, which show the measured value in μS/cm. If the conductometer displays the measured value in mg/l (ppm), this conductometer is called TDS-meter. If the readings in μS/cm are relatively accurate readings that determine the electrical conductivity of water of a given ionic composition, then the readings in mg/l (ppm) are inaccurate. The reading in mg/l (ppm) is obtained by multiplying the reading in μS/cm by the conversion factor. Water with different ionic composition has a different conversion factor. At the same time, almost all conductometers have a conversion factor of 0.5, which assumes that only NaCl is present in the water.

To determine the conversion factor for a specific ionic composition of water, you can use this article – https://tiwater.info/en/the-influence-of-ion-composition-of-water-on-its-electrical-conductivity/

My practice shows that the conductometer should cost more than 30$. You may certainly get lucky with a cheaper one, but the probability of this is low.

The measurement range of the conductometer should be from 0.0 to 1999 μS/cm. Some conductivity meters can be dual-band. However, the range from 0.0 to 1999 μS/cm must be present in the conductometer. The resolution should be no more than 1.0 μS/cm.

If your water is brackish and has an electrical conductivity value of more than 2000 μS/cm, you should generally consider the feasibility of softening such water. Such water is not potable and requires the use of water treatment technologies that reduce the amount of salts in the water. However, in this case it is possible to use a conductivity meter with a higher measuring range.

If you measure the electrical conductivity of the source and softened water with two different conductometers, taking into account the errors of the devices, you may get an incorrect result.

To avoid measurement errors only one conductometer should be used for measuring both the source and softened water.

A more detailed description of the control technology of the ion exchange softening water plant is presented in the article –  https://tiwater.info/en/the-method-of-controlling-the-process-of-ion-exchange-water-softening/

Currently, this automated system for monitoring the water softening process has been implemented in several steam boilers.

I hope this information will be useful.

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