What is TDS?
TDS is the sum of all cations and anions in a body of water.
If we add the pure salt sodium chlorine (NaCl) to pure water (which has next to zero conductivity), it will dissolve (dissociate) completely into its ions. The water would contain sodium cations (Na+) and chloride anions (Cl-). The total of the Na+ ions plus Cl- ions then becomes the TDS of this water sample. TDS is expressed in parts per milligram (ppm).
As other molecules are added to the water they may or may not dissociate into their ions. If the molecules dissociate, they may do so completely or partially. Any resultant cations and anions produced will also add to the TDS of the water.
TDS in a Swimming Pool
In a swimming pool we intentionally contribute to the TDS by adding calcium chloride, sodium bicarbonate, hydraulic acid and other chemicals without a second thought.
The big one though, is that we are constantly increasing the TDS by sanitising the pool. You can build salt levels up 4,000 and 5,000 ppm plus over time by sanitising using liquid chlorine. So much so that if you convert the pool over to saltwater chlorination you don’t need to add salt. We experienced this when we converted several commercial pools from liquid chlorine pools to saltwater pools.
Why do we worry about TDS in Swimming Pools?
We don’t have a problem with the above TDS contributors. The problem lies with the TDS contributors that we don’t know a lot about when it comes to swimming pools. It's the TDS contributions from sunscreens, cosmetics, medications and bodily fluids that can build up in a pool that are of concern. We also need to mention the TDS contributions from algae and other biologicals and chemicals that find their way into a pool via wind and rain. Are there further chemical reactions with other chemicals in the pool creating lord knows what?
While there might be health concerns that we don’t know about when looking at TDS, our concern, in this blog, is on its effects on the quality of the water and our ability to maintain that quality.
Any service tech will tell you what a nightmare it is to look after a well-used spa in a complex that they visit only once a week. A spa is a compressed pool, if you like. It’s heated with people sitting very close to each other. On top of that, well, let your imagination run wild.
The use of TDS levels is one way that service personal determine that it is time to replace the water in a spa. That system works. The costs are minimal. Of course, when you do this for the first time you find out that the spa cannot be easily drained and that there is no water tap in the immediate area. The process can be unnecessarily labour intensive. Good planning, Frank.
While it might not be intentional, swimmers do swallow pool water, kids especially.
Several years ago, we dropped a new robotic cleaner into a pool in an apartment building that sat on the corner of an extremely busy traffic area. On its first run it left a clean strip down the middle of the very fine black sediment that had settled on the pool bottom. Makes you think, doesn’t it?
We can test for suspected TDS contributions such as phosphates and urea which we can treat and metal contributions that we can also test for and remove but we really don’t have any idea about what else is in the water. I’ll be blowed if I know of any way of testing for unknowns easily.
It wasn’t until students at a school or a college in the USA conducted chemistry experiments looking for antibiotics in natural water ways that we learnt about the problem.
How do you manage the unwanted TDS in Swimming Pools?
We only manage this by the seat of our pants (if at all), don’t we. It’s a guess, isn’t it?
I have read that we should replace water at the rates varying from 15 litres per swimmer per day to 60 litres per swimmer per day.
We mostly rely on backwashing to achieve this. That’s a start, but what happens when we use cartridge, diatomaceous earth and media filters on our pools? These filters are generally promoted as water saving.
To rely on backwashing, we should at least do a calculation to estimate how much water is replaced by a backwash and how many swimmers that volume covers.
The next step would be to work out the various sources of TDS contamination. A pool specifically used for hydrotherapy will get significant contributions of unknowns thanks to the age group using the pool. On the hand, with a ‘Learn to Swim’ pool you will be dealing with kids. We have a pretty good idea of their contributions and those are more easily managed, mostly by super chlorinating on a regular basis and treating for phosphates.
Knowing the various sources of your unwanted TDS will allow you to formulate a management plan for that pool.
Those of us who have spent a good amount of time in the pool service side of the industry will know that a new pool has far less problems than an old pool. The water becomes stale in the old pool. We can see it in the test results and in the look of the pool water. Of course, there will be other factors contributing to this. Usually, this means that the pool has reached a stage (regardless of the cause/s) that requires a water change and most likely some additional maintenance during that process.
The aim should be to avoid reaching that point, especially with a commercial pool. The cost of replacing the water in a commercial pool would be substantial, so having a good TDS management plan is essential.
Those costs would include water replacement, chemical replacement, reheating, labour costs, a bucket full of hidden costs and lost income.
How do we measure TDS in a Swimming Pool?
I’m not sure that we can accurately measure TDS that concern us, the unknown ones. Using TDS when managing a spa makes sense as we can have a pretty good idea that a high TDS would indicate that the spa is full of nasties. The source of the TDS is obvious while we might not know its makeup.
We usually use a TDS meter which is conductivity meter with a conversion factor (CF) built in. The following is an extract from the ‘Fondriest – Environment Learning Center” website
“When calculating total dissolved solids from a conductivity measurement, a TDS factor is used. This TDS constant is dependent on the type of solids dissolved in water and can be changed depending on the water source. Most conductivity meters and other measurement options will use a common, approximated constant around 0.65 32. However, when measuring mixed water or saline water (with a conductivity value greater than 5000 uS/cm), the TDS constant should be higher: around 0.735 and 0.8 respectively 20. Likewise, fresh or nearly pure water should have a lower TDS constant closer to 0.47-0.50”
Salt meters are conductivity meters with an inbuilt CF that have been calibrated to a known salt solution. My salt meter preference is for the Ultrapen PT1 which will measure NaCl. That tells us heaps. Where do we go to from here?
With an Aquarius Ultima Controller we can choose a reasonable CF, and the unit will give you a TDS read. But what does this tell you? Bugger all, I think, but it does give you a start point. How good would it be to have an accurate figure for TDS after you first fill a pool, before you add any chemicals. You could get the water laboratory tested. Knowing that figure and adding the contributions for the chemicals you add will give you a more accurate TDS start figure and therefore a more accurate CF factor.
In the “Standard Methods– For the Examination of Water and Wastewater” (22nd edition) there is section on page 2-65 that outlines the process for measuring TDS by drying a water sample. Getting a chemical laboratory test might be an option if you cannot pinpoint problems occurring with the pool chemistry in a pool that you manage.
Regardless of which start TDS figure you use; you should be able to use that figure minus your salt read to give you a reciprocal TDS. As time goes on, you will see changes in the reciprocal TDS which should indicate if you are managing TDS levels with any degree of success. I cannot see any other way of doing it. Frankly, I'm not sure that this would be of any help if it did work.
I tested this process on a commercial pool that I service. After the install of a new probe the conductivity was 6055 microsiemens per centimeter. The calibrated salt meter read was 4515ppm, calcium hardness was 360ppm, alkalinity 75ppm. Using only these figures, the CF calculates to 0.81 which sits with Fondriest range above. If I wanted to use the TDS figure to tell me remotely what the salt level is, then a CF of 0.74 would be accurate enough for this pool. That's pretty useful when remotely monitoring a pool that's using a saltwater chlorinator. Nothing in this tells me the level of unwanted TDS.
Using an Aquarius Ultima Controller, it can display either conductivity or TDS. You choose the CF you want to use. It's a tool you can use as you see fit.
I don’t think you can beat the process of assessing what is contributing to the TDS in a pool and implementing a water replacement program that will avoid any future expensive pool shutdowns.
What do you think, Frank?
