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.

TDS buildup is inevitable.  As we've explained, every time you add chemicals to water, you're increasing the TDS.  When the water evaporates, it leaves behind all of the solids that had been dissolved in it.

Just how much are you increasing the TDS when you chemically treat pool or spa water?  Well, for every pound of dry chemical that you add to a 15,000 gallon pool, you are increasing the TDS by about 8 ppm.

"Parts Per Million" is what scientists refer to as a "weight to weight" measurement.  That is, if you know how much the water weighs, and you know how much the stuff you are adding to the water weighs, you can calculate how many parts per million you are adding to the water.

As it happens, one gallon of water weighs 8.34 pounds.  So if you know how many gallons are in a pool or spa, and you multiply that number by 8.34, you will know how many pounds of water are in the pool or spa.

So, how many pounds of water are in a 450 gallon spa?  The answer is 3,753 pounds (450 x 8.34 = 3,753).  How about a 15,000 gallon pool?  Well, 15,000 x 8.34 = 125,100.  So, there are 125,100 pounds of water in a 15,000 gallon pool.

OK, so now you know how much the water weighs.  Now, take 1 million and divide it by the total weight of that water, and you will find out how many parts per million are contained in each pound of water.  For example, 1,000,000/125,100 (pounds of water in our 15,000 gallon pool) = 7.99.  We'll call it 8.  That is, every pound of material added to a 15,000 gallon pool will contribute about 8 parts per million.

How about our 450 gallon spa?  Take 1,000,000, divide it by 3,753 (the weight of the water in the spa), and you get 266.45.   We'll settle on 266.  So for every pound of stuff that you add to a 450 gallon spa, you will be increasing the TDS by 266 parts per million.                                                                                             

The above charts should give you a pretty good idea of why TDS builds up so rapidly in spa water and also why the effect of chemical treatment is so dramatic when you are dealing with a spa.  The accompanying charts - one for pools and the other for spas - will also give you an idea of how much of an impact 1 pound of chemical added to the water will have on TDS in vessels of different sizes.

Incidentally, if you wanted to design a pool that would contain almost exactly 1 million pounds of water, it would be a 120,000 gallon vessel.  In that pool, every 1 pound of solids dissolved in the water would increase the TDS by 1 ppm.

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

TDS is fairly easy to calculate for dry chemicals.  It's a bit more complicated for liquid solutions.  If a research lab were going to test the solids content of a liquid, they would take a precise volume of the liquid and then slowly heat it until the liquid itself had evaporated.  Then they would dry the remaining solids and weight them.

The two most common liquid solutions used in our industry are muriatic acid and liquid chlorine (sodium hypochlorite).  For your information, 1 gallon of muriatic acid will contribute 1.87 pounds of dissolved solids to the water.  1 gallon of liquid chlorine will contribute 2.2 pounds of dissolved solids.

You should also understand that every type of chlorine & sanitizer - including gas chlorine - eventually ends up contributing to TDS in the form of chloride.  So every time you add a pound of dry chlorine compound, or infuse a pound of gas chlorine into a 15,000 gallon pool, you will wind up increasing the TDS by about 8 ppm.

Short of a detailed laboratory analysis, the most dependable method of TDS testing is through the use of a portable TDS meter.  These meters actually measure the conductivity of the water  - its ability to conduct an electrical charge, which happens to increase as the TDS increases.

Hand-held TDS meters - usually in the $50.00 - $150.00 range, gggeenerally operate by either placing some water in a sample cell or dipping the meter directly in the pool water and pushing a button, which causes a small electric current to pass between two electrodes immersed in the water and separated by a specific distance.

The meter measures the current passing between the electrodes and uses that to deter mine the water's conductivity, the meter dial (or LCD readout) is calibrated to indicate TDS in parts per million.

The easieste way to reduce TDS is to drain the pool and refill it with fresh water.  This can also be done in stages, taking the water level down 1 or 2 feet at a time and refilling over a period of days or weeks.

Under normal circumstances, pool water can be expected to last anywhere from 3 to 5 years before it has to be completely changed.  Your choice of chemical treatment can help to determine just how long it lasts.