Understanding Total Dissolved Solids (TDS) In Swimming Pools



Any time you enter a pool, you may be quietly praying to yourself, “Dear God, I hope this water is clean.”

Of course, we all know about the role of chlorine and how it works to keep a pool sanitized. 

But what we hear less talk of is the presence of total dissolved solids (TDS) that are floating around in the water.

What Are Total Dissolved Solids (TDS)?

Total dissolved solids, or TDS as it’s commonly referred to, is a measurement of all the things, good and not so good, that are dissolved in water. It’s measured in parts per million (ppm).

For reference:

  • Distilled water has a TDS of 0 ppm
  • Drinking water has a TDS of roughly 500 ppm
  • Sea water can have a TDS over 30,000 ppm

In a swimming pool, the TDS level is made up of everything from minerals (ie. calcium, sodium, chlorides, etc.), to algae residue, pollen, metals, and organic waste that swimmer’s introduce to the water.

The issue with TDS is that you can’t see what’s dissolved, and have little way of knowing if they’re an issue just by looking at a swimming pool — but those substances still exist, and they still play a role in your pool water chemistry.

It’s like when you dissolve sugar in water, the water is still sweetened even though you can no longer see the grains of sugar.

How Do You Measure TDS?

Total dissolved solids are measured using a digital testing meter.

It requires taking two samples of water – one from the mains, and one from the pool itself.

The tester operates on electronic frequencies to get an accurate reading of the level of TDS in water. 

However, it doesn’t introduce an electronic charge in the water, but rather measures the electrical conductivity of the water. This is determined by how much dissolved solids are present.

For example, the water from your mains may have a TDS reading of 500 ppm, and the pool water might measure at 1300 ppm. The difference, 800 ppm is the amount of TDS that is actually in the pool water.

Why Is TDS Always On The Rise?

TDS in a pool is always going to be rising because outside elements are always affecting the chemistry of the water.

When you’re balancing your pool water levels, minerals like calcium and sodium can be added or subtracted at will. Likewise, if you have a saltwater pool, your sodium level will most likely be higher than if you have a freshwater pool.

If you’ve ever had an algae problem and successfully eradicated it, there will still be leftover algae residue that contributes to TDS. 

Two more TDS culprits are atmospheric influences like pollen and dust mites, as well as organic matter that people bring into the water. This can be anything from body oils, to sweat, sunscreen, deodorant, hair gel, shampoo residue, and even urine.

As a preventative measure, you should always shower before entering a pool. This simple act sets off a beneficial chain reaction for any pool.

First, you’ll be limiting the amount of pollutants you add to the water by washing most of them off your body.

Second, this lack of pollutants means the chlorine won’t be working as hard to clean the pool. This translates into prolonging chlorine’s longevity, meaning you won’t be using (or buying) as much of it. 

Similarly, you won’t have to keep re-balancing the pool’s chemical levels, which can mean the addition of TDS minerals, and more money from your pocket as well.

What Is The Correct TDS For Pools?

You’re going to see varying information on the proper level of TDS for a pool to have. These levels also depend on if you have a freshwater, or saltwater pool.

Freshwater Pools

Generally, you want to give yourself about 1500 ppm of headroom. 

Some people believe a TDS level of 1500 ppm is too much, while others say closer to 4000 or 5000 ppm is when you should start to be concerned. This is because you have to take into account how the addition of various chemicals add to the water’s overall TDS level. 

That would make 1500 ppm a relatively low threshold, and at times most times, unrealistic.

Saltwater Pools

In keeping with this, saltwater pools require between 2800 and 3500 ppm of sodium in the pool at all times. This makes being overly concerned about a TDS level of 1500 ppm almost a moot point.

With an ideal sodium level of 3200 ppm, saltwater pools are already technically high in TDS levels. But for the most part, concern should be tempered until a test comes back around 4000 ppm.

Why Does TDS Matter For Pools?

When total dissolved solids levels get too high, it can result in issues that may be hard for you to fix through traditional water balancing methods, sometimes requiring the help of a professional pool service.

Suppressed Free Chlorine Levels

Free chlorine is the main sanitizing agent in the majority of pools. Without free reign (pun intended) to do its thing, the pool won’t stay clean.

One theory of why this occurs is because TDS particles bond to the chlorine and disrupt its efficiency (known as chlorine efficacy) , while another theory suggests they interfere with chlorine test results, which will show false low readings.

Cloudy Pool Water

Cloudy water isn’t necessarily a problem caused by total dissolved solids because once they actually dissolve, you can’t see them anymore. 

However, it can certainly be a factor as the water has an oversaturation of dissolved substances in it, which can lead to circumstances where the pool water becomes cloudy.

Contributes To Corrosion

Again, with a higher saturation of TDS in the water, it can turn the water against the pool itself, corroding through metal ladders, pipes and underwater light fixtures.

Algae Problems

Not only does TDS contribute to the blooming of algae in your pool, but the introduction of algaecides will also add by-products such as nitrogen and copper to the water. 

Salty Water & Scale Issues

With overly saturated sodium and calcium levels present in the water, issues with salty water and scale can result. These will either give the water a salty taste, an oily feel, or leave calcium deposits on your pool’s finish.

How Do You Remove Total Dissolved Solids?

There is basically only one way to remove TDS levels from your pool, and it’s the same way you would reduce high salt, calcium hardness or cyanuric acid levels — through dilution/drainage of the pool.

Draining a pool is a big job and usually only done every 5 years or so, so it’s important to check that whatever issues you may be experiencing with your pool aren’t caused by other factors.

  • Check to see if you’re running your pool filter long enough, with 8 to 9 hours being standard during pool season.
  • Check if the filter is clean, especially if it’s been awhile since you’ve cleaned your cartridge filters or backwashed sand filters or DE filters.
  • Check that the water flow in your system is working optimally. Ensure all valves are open, the pump and skimmer baskets are both clean, and your pump’s impeller isn’t clogged up.
  • Test your water to make sure the levels are all proper: pH, alkalinity, chlorine, and CYA. It’s also recommended to check your phosphate levels. If they’re high, your pool may become cloudy and algae can bloom.

If you’ve ruled out all of the above and you’re certain TDS is the problem, you’ll need to drain the pool and refill it with fresh water.

You should wait until the cooler months when doing a drain, to reduce the chances of your plaster or aggregate finish cracking, as the heat of the sun can dry them out.

Finally, some pools have a reverse osmosis filtration system installed to keep the water clean and eliminate the need for draining your pool.  An RO system filters the water through a semipermeable membrane that can separate pure water from larger dissolved molecules and bacteria.

Reverse osmosis technology can also benefit pools with high levels of cyanuric acid, calcium hardness, and waterborne diseases. This makes it a viable option when compared to draining, although it can be costly to install, run, and can also produce an increased amount of wastewater.

Now You Know!

The argument that total dissolved solids are a major pool issue is a debate that can rage for days. When it comes down to it, TDS is something that pool owners should be aware of, but for the most part, won’t affect most pools in a major way. 

However, now that you know about them and how to test the water, you also know how to mitigate them however you see fit.

Categories: Pool Care, Pool Chemistry