TDS – Total Dissolved Solids – And Your Swimming Pool

In Blog, Chemistry by Timothy Petsch

What is TDS? TDS – Total Dissolved Solids – is defined in the NSPF CPO® manual as “the total weight of all soluble matter in the water“.

Basically, anything that you throw into a pool – including chlorine, specialty or balance chemicals, even your fill water – will add to the dissolved solids level. Historically, the industry has dealt with total dissolved solids by partially draining the pool water any time the measured TDS level reached 1600 or more.

What is the rationale behind reducing the TDS level? Is there anything wrong with this approach?

The logic, according to the experts, was that a high TDS level would limit the effectiveness of chlorine and lead to unsafe sanitary conditions. That sounded good, but unfortunately it was not correct. I can only imagine how many millions of gallons of water, not to mention the BTU’s of heat, balancing chemicals, and chlorine that were wasted in the process

What was wrong with doing that and why was TDS thought of as the bad guy?

Let’s start with the bad guy. The rationality is that as the TDS level increased, the levels of chlorine inhibiting material, such as nitrates, phosphates and ammonia s also increased. This is only partially correct. Let me tell you a personal story of why TDS is near and dear to this author’s heart.

Back in the early ’90s I introduced residential chlorine generation to the greater Seattle market. When a home owner added the required salt (to bring salinity levels to 3,500 PPM) and took a sample of their water to a local pool store to have it tested, they were told in no uncertain terms that their TDS was way too high and that 1/2 of the pool water had to be drained. As you can imagine, that caused a credibility problem that had to be dealt with, and I thought to myself “How could the product that I add to the pool water to MAKE chlorine be a problem?”.

As it turns out, the TDS actually was not a problem, but that conclusion took over a decade to come to light. So instead of simply accepting high TDS as a problem, I asked “Why is TDS a problem and is there a difference between “good” TDS and “bad” TDS?

Let’s examine what “bad” TDS is. The Total Dissolved Solids that will inhibit the efficacy of chlorine are nitrates, phosphates and ammonia, as mentioned above. The rest –mainly salt and calcium –are actually “good” TDS, and by good I mean that salt is what chlorine is made from, and calcium is what is required to balance the water. So where did the concept that Total Dissolved Solids –all Total Dissolved Solids – were always unwanted over a certain level? The simple answer boils down to testing. In order to test for “bad” TDS a test called a Gravimetric test had to be performed. This was a test that was not only expensive but complex. Since this was not the kind of test done poolside, or in a Pool Store, the industry created a benchmark that went something like this: If the overall TDS is  x, then the “bad” TDS contained in that number must be y. When x gets to a certain level then y must be at a level which would inhibit the efficacy of chlorine. Therefore, the pool water should be partially (or even completely) drained, because the only suitable test that was available was to test for Conductivity, which tested mainly for…you guessed it…salt!!

As far as this author is concerned, our industry is responsible for millions of gallons of pool water being wasted. That’s my opinion, what do you think?