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ThermoSpas.com › hot tub accessories › TDS - Total Dissolved Solids explained

Total Dissolved Solids

"Bubbling Over" Newsletter Article - November 2005

Total Dissolved Solids, or TDS, may be the most misunderstood factor in the whole field of pool & spa water chemistry. It's misunderstood because no one knows exactly what effect it is going to have on any particular body of water. Let’s try to clear up some of the mystery.

As its name states, TDS is the sum total of all of the dissolved things in a given body of water. Every time you add anything to the water, you are increasing its TDS. This includes not only sanitizing and pH adjusting chemicals, but also conditioner, algaecides, tile and surface cleaners, airborne pollutants, bather waste and dissolved minerals in the fill water.

TDS buildup is inevitable because every chemical used in the pool and spa industry will eventually contribute to the TDS. Some will contribute more than others. Because sanitizing compounds often require the additional use of pH adjusting chemicals, the chemical maintenance regimen you choose can have a dramatic effect on the buildup of TDS.

At low levels, TDS does not present a problem. In fact, a certain amount of TDS is necessary for water balance. Hardness and Total Alkalinity are both part of TDS. But at high levels - above 3,000 parts per million - you are welcoming problems. The Association of Pool and Spa Professionals (APSP), in its standards for both swimming pools and spas, recommends an ideal TDS of between 1,000 and 2,000 ppm, with a maximum of 3,000 ppm.

TIP: Using your TDS Test Strips will help you determine whether your levels are safe of not. Test monthly as part of your regular spa maintenance routine.

No one knows exactly what mechanism is at work when you're dealing with high TDS water. One commonly held theory is that when you get a lot of dissolved substances in water, they interfere with the normal workings of sanitizers. They may do this by forming a chemical "shield" around bacteria, algae, and other substances normally attacked by chlorine or bromine. Or they may simply present "roadblocks" in the path of sanitizer molecules, preventing them from freely circulating. Other problems that may occur from high TDS levels include:

  • Corrosion of metal equipment and accessories, even though the water is balanced.
  • Eye and skin irritation, even though the pH is right and there are no chloramines in the water.
  • An algae bloom, even with a 2-3 ppm chlorine residual.
But, it is also possibly that none of these symptoms will occur. It is the uncertain nature of problems caused by TDS that makes it such a headache to deal with!

TIP: When everything else seems to be all right, but the water still acts funny, check the TDS.

Happy Hot Tubbing!

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