Technical Articles
Technical Bulletin: Total Dissolved Solids (TDS)
If you were to make a list of critical pool water chemistries to monitor, would you include total dissolved solids? Surely you would include chlorine, pH, alkalinity, and hardness, yet you may not pay much attention to TDS . TDS can be critical -- don’t ignore it!
What is TDS?
Total dissolved solids is the measure of the total amount of dissolved matter in the water, e.g. calcium, magnesium, chlorides, sulfates, rubber ducks, etc. Well, maybe not rubber ducks, but if they dissolved, they would count. Basically, TDS lives up to its name, as it is anything that is dissolved in the water. For a quick example, if the calcium hardness level is 500 ppm, then it is 500 ppm worth of total dissolved solids. The National Spa and Pool Institute standard suggests that an ideal level of TDS is 1000-2000, where 3000 ppm, or 1500 ppm at pool startup, is high.
How is TDS Introduced into the Water?
Even adding fresh water to your pool or spa can increase the total dissolved solids. Your tap water contains certain levels of salts like sodium chloride, minerals like calcium and other dissolved solids, which will increase the TDS. Pools and spas in areas with high evaporation rates can really see the effects of TDS. Water that evaporates from the pool or spa is pure so, it leaves behind the dissolved substances. When fresh tap water, with its own balance of TDS, is added to a pool that already contains a TDS level, then the dissolved solid level becomes elevated.
How Does TDS Affect the Water?
High TDS levels tend to fool you. The water may appear balanced or imbalanced when challenges like cloudy water, algae, corrosion or reduced efficiency of the chemicals sneak up on you. A problem like cloudy water is generally attributed to a high calcium hardness level or a
high pH level, but the true cause may be a high TDS level. Free chlorine may look great, but algae may mysteriously appear when the levels of TDS are extreme. Elevated TDS levels also cause corrosion of equipment and fixtures, as well as scale buildup. Chemicals, including chlorine and other sanitizers, find it more difficult to do their job when TDS is too high.
With a list of potential problems like those noted above, it is easy to see why you should measure and consider TDS when trouble-shooting a water chemistry problem in a pool or spa. If you add another chemical treatment without checking TDS first, you may be raising a TDS level that is already too high. Only one thing will change the TDS and significantly reduce the its level – fresh water.
What’s the Treatment?
Partially draining and adding fresh tap water will usually reduce high TDS levels in swimming pools. A spa has a smaller water volume so it should be completely drained and refilled. Of course, the water used to refill the pool or spa must be lower in total dissolved solids than the starting point. High TDS in the source water might indicate poor water quality from corrosive mineral salts, humus or organic matter. You may need to consult your local water authority if initial TDS is high. If you need to lower the TDS, you should do so before adding any other chemical treatments, otherwise you may be wasting your chemicals (and your money).
How are Total Dissolved Solids Tested?
It is always a good idea to test the TDS at least once a month in pools, even more frequently in spas, and most importantly, whenever there is a problem. For proper swimming pool and spa maintenance, NSPI suggests the following operational guideline levels for TDS: 300 ppm - minimum, 1000-2000 ppm - ideal, 3000 ppm – maximum, or 1500 ppm maximum at pool startup. Pools with salt generators can generally maintain a slightly higher level. The primary component of TDS in these pools is sodium chloride, which is very soluble.
There are two ways to measure TDS levels. A portable conductivity meter is one common method. TDS meters measure conductivity of certain species in a sample and then use factors to compute guideline TDS ppm values. These instruments require periodic calibration to maintain accuracy.
As an alternative to the conductivity meter, an inexpensive, dip-and-read test strip has been recently introduced that allows you to obtain TDS readings. You simply compare the reacted color of the strip to a color chart to determine the TDS level. It is simple enough that anyone can use it, and it does not require calibration.
Armed with this information now you’re ready to keep the TDS in line. Adding total dissolved solids to your list of critical water parameters to monitor will help you maintain healthy a pool or spa. If you overlook TDS you may waste time and dollars trying to troubleshoot water balance problems.
-Joe Sweazy, © 2001 Environmental Test Systems, Hach Company.
