When it comes to some industry standards, there are a few recommendations that could stand to see a little reevaluation.

One of these is the recommendation about total dissolved solids (TDS) in residential swimming pools. When the TDS level goes beyond 1,500 ppm over the source water, the standard says that it’s time to drain and refill. This is recommended regardless of what the TDS may be composed of. It’s a perfectly reasonable limit for pools with high levels of disinfection byproducts, but it doesn’t make as much sense if the TDS is mostly salt, which is true for most residential pools.

With so many areas of the country so often experiencing drought, the recommendation to drain and refill when this limit has been reached seems arbitrary and wasteful.

It is almost universally recommended that recreational water be replaced on some schedule.

This is based on the idea that filtration and disinfection will not remove all pollutants and that dilution limits the buildup of pollutants from bathers.

The fact is, high TDS by itself is not a measure of the water quality. For example, if salt chlorine generators are used, TDS levels of 4,000 may be normal. TDS is a measure of all the dissolved species in the water, including mineral content, algicides, bather waste, disinfectants, disinfectant byproducts, and anything else that gets in the water.

For swimming pools and spas, it is the disinfectant byproducts that give rise to concerns about water quality because some of them are toxic or carcinogenic. Many of these are produced when chlorine combines with ammonia and organic compounds contained in bather waste (like sweat or urine) to form chloramines. Monochloramine and dichloramine contribute to the bad odor that some pools give off. Of the disinfection byproducts normally found in pool water, trichloramines is the most offensive and irritating. There is also some evidence that high levels of trichloramine are linked to asthma.

For commercial or public pools, high TDS can be indicative of improper sanitation, especially if the pool is indoor and poorly ventilated. In highly used pools, this built-up, partially oxidized bather waste increases chlorine demand.

However, because the concentration of disinfection byproducts is proportional to bather load, high TDS does not usually pose a health risk in the lesser-used residential pools.

For these pools, high TDS is composed primarily of simple salt, sodium chloride, a natural byproduct of chlorine’s oxidation reaction.

Where water conservation is a concern, for residential pools, it may be irresponsible to advise draining and refilling on the basis of TDS alone.

What is the problem with high TDS?

Even if high TDS is not by itself problematic for health reasons, it can contribute to structural damage. High TDS composed of salt raises the rate of corrosion, which can be damaging to soft stone surfaces and rust metal equipment. This is a problem that people who are familiar with salt chlorine generators are all aware of. And there are preventative steps people take to avoid such problems. It’s why people seal their decks and install corrosion-resistant heaters.

One problem associated with high TDS is that other chemical parameters may have climbed out of range.

For example, pools using trichlor or dichlor may have exceptionally high cyanuric acid levels. This can be problematic for a few reasons.

For one thing, high cyanuric acid slows the rate of chlorine’s oxidation reactions. With slowed oxidation, algae may be more difficult to control at recommended chlorine levels. Furthermore, bather wastes may be only partially oxidized, and disinfection byproducts may accumulate.

Another problem associated with high TDS is that calcium levels may have drifted out of range. This may occur if calcium hypochlorite is used as the sanitizer.

If water is oversaturated with calcium, calcium carbonate will precipitate out on pool walls and equipment as scale. The ideal range of calcium is between 200 and 400 ppm, although PHTA standards allow for up to 1,000 ppm calcium.

For these reasons and more, it is a good idea to try to limit TDS buildup in the first place.