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Inside Total Dissolved Solids

By Kim Skinner

KimSkinnerInsideTDSJust how much do swimming pool chemicals add to the total dissolved solids (TDS) content of the water? Do all chemicals of equal amounts (by weight) add identical amounts of TDS to swimming pool water?

These are important questions, because elevated TDS levels can - depending on the type of compounds that make it up - have significant implications for water quality as well as the service life of circulation equipment and interior surface material (more on that below).

For now, let's take a look at how chemicals added to water influence TDS numerically.

BY THE NUMBERS

When one pound of a chemical is added to 20,000 gallons of water, the usual amount of TDS being added to the water is six parts per million (ppm). While this is true with most chemicals, it is not true with all chemicals, some of which will escape from the water after being added and will not contribute to TDS. Another possibility is that portions of some chemicals will convert into water and thus will no longer be part of TDS.

Let's look first at a pure chemical that provides the full increase of its weight to the TDS content of the water: chlorine gas, also known as elemental chlorine. At levels of 1,000 parts per million or below, chlorine gas is nearly 100 percent soluble at a pH of 5.0 or above. Therefore, if one pound of pure chlorine is dissolved into 20,000 gallons of water, the result will be six ppm of chloride (as TDS).

One way of calculating this is as follows: 20,000 gallons of water weighs about 166,700 pounds - which is, conveniently, one-sixth of a million. So you multiply the one pound of chlorine by six to get the six ppm of TDS.

As with all acidic sanitizers, after chlorine gas is added, it is usually necessary to add sodium carbonate (soda ash) or sodium bicarbonate (bicarb) to adjust the pH and alkalinity upward. Interestingly, however, adding these alkaline chemicals does not ultimately add the full amount of TDS per pound added.

In fact, when soda ash and bicarb are added to pools, these chemicals will react with acid that has been added to form carbon dioxide, which will release into the atmosphere over a period of time. Only the sodium element from these two chemicals remains in the water and increases TDS.

The following equations illustrate these points:

Na2CO3+ 2HCl = 2NaCl + H2CO3

             H2CO3 = CO2 + H2O

In the first reaction, soda ash (Na2CO3) reacts with acid (HCl) from acidic sanitizers, muriatic acid or dry acid to form sodium chloride (NaCl) and carbonic acid (H2CO3). Sodium chloride remains in the water as salt, which does contribute to TDS, while carbonic acid will dissociate into carbon dioxide (CO2) and water (H2O).

The pure water that is formed does not contribute to TDS, and carbon dioxide will eventually escape into the atmosphere, thereby not contributing to the TDS content. Sodium bicarbonate also reacts with acid and forms the same compounds - salt and carbonic acid.

DIGGING DEEPER

To determine the actual amount of TDS that is being added by these chemicals, a look at their molecular weight is needed. The (rounded off) molecular weight of soda ash is 106; of sodium bicarbonate, 84. Sodium by itself has a molecular weight of 23, and there are two sodium molecules in soda ash, only one in sodium bicarbonate. The sodium element (which adds to the TDS content) constitutes about 43.4 percent of the soda ash and about 27.4 percent of the sodium bicarbonate.

Thus, when adding one pound of soda ash, only 43.4 percent of it would contribute to TDS, while with sodium bicarbonate, 27.4 percent would contribute to TDS.

The calculations are as follows: If one full pound of a material in 20,000 gallons of water would normally equate to six ppm of TDS, then one pound of soda ash, with only 43.4 percent of its total weight (sodium) remaining as TDS, contributes about 2.6 ppm of TDS. Similarly, one pound of sodium bicarbonate (with 27.4 percent as sodium) adds about 1.65 ppm.

(Note: Until these chemicals actually react with an acid, the entire contents of the various compounds would be present for a time as TDS).

Another group of chemicals that do not introduce the full TDS of their weights are compounds that add hydroxide or form hydroxides when added to water. Hydroxide (OH) will likely combine with a hydrogen ion to form pure water (H2O) and no longer be TDS. Sodium hypochlorite (often referred to as liquid chlorine or bleach) is an example of this type of compound.

Every gallon of sodium hypochlorite (in this example, 15 percent trade or 12.5 percent by weight) includes about 1.25 pounds of chlorine and about 1.6 pounds of sodium hydroxide for a total of 2.85 pounds of potential TDS. This would normally equate to about 17 ppm of TDS for every gallon added to a 20,000 gallon swimming pool.

But because hydroxide accounts for about 42.5 percent by weight of sodium hydroxide (and since hydroxide will eventually react with hydrogen and convert to water), this component is subtracted from the compound. Therefore, together with the chlorine (contained in bleach), about 2.2 pounds in a gallon of sodium hypochlorite are added to the TDS content of a solution. So one gallon of sodium hypochlorite adds about 13 ppm of TDS (rather than 17 ppm) to 20,000 gallons of water. (Bleach also adds a small amount of alkalinity to the water).

With 10 percent (by weight) bleach, about 10.5 ppm are added per gallon to 20,000 gallons of water. With 5.5 percent (by weight) household bleach, about five ppm of TDS per gallon are added to that 20,000 gallons of water. Also, chemicals that include oxygen or nitrogen will likely lose a portion of their weight, thus making a lower overall TDS contribution. Calcium hypochlorite (65 percent) is an example of a compound that adds oxygen. The percentage of the calcium hypochlorite contribution that ends up as oxygen is about 22 percent.

Calcium hypochlorite (cal hypo) also contains a small amount of water. Therefore, its contribution to TDS is only about 75 percent of its total weight. Lower-strength cal hypo will result in a slightly lower percentage of TDS contribution per pound. (Of course, part of the TDS contributed by calcium hypochlorite is calcium, which is added to the water and increases its hardness).

Trichlor, sodium dichlor, cyanuric acid and regular salt added to swimming pool water will result in the full increase of their weight to TDS.

Muriatic acid (hydrochloric acid) is product that, when added to pool water, will contribute significantly to TDS content. Muriatic acid at 31.45 percent strength will add about three pounds of TDS per gallon. Although the hydrogen from this acid will react with hydroxide to form water, it is such a small percentage of the total weight of the acid that it will not significantly affect the contribution of this acid to TDS. Therefore, one gallon of muriatic acid in 20,000 gallons of water will increase TDS by 18 ppm.

EVALUATING TDS

It's important to keep in mind that not all TDS has the same type of influence on water quality and water's ability to scale or corrode metals and cementitious materials such as plaster or tile grout. An increasing TDS value, for example, if derived entirely from an increase in calcium content, indicates a scaling possibility. This would be represented by a higher Saturation Index (SI) value.

But when the TDS value is high or rising as the result of elevated chlorides and/or sulfates, this can lead to the corrosion of pool plaster - the opposite of scale. This results in a lower SI value and may indicate a need for adjustment. High sulfate content (from dry acid) is known to be corrosive. Of course, the service industry doesn't normally test for sulfates, but technicians should take this factor into consideration. Interestingly, high sulfate and high calcium levels can combine and create severe scale.

There are also other unwanted dissolved solids that can build up but for which we don't normally test. These include various organics, bather sweat, nitrates, phosphates, foam and the like, all of which can cause various problems with water balancing. While these contaminants generally contribute only a small amount of TDS, they may cause significant problems just the same. And of course, high amounts of these contaminants become more of a concern in spas or other small bodies of water.

APSP-11 advises that when the TDS reaches 1,500 ppm above tap water content, then draining is recommended. For full-size pools, a reverse osmosis (RO) system to reduce TDS is also an option. Because we are testing for an actual amount of dissolved solids (and not getting hung up on what the TDS components are), and then using that result to determine draining intervals, this is an example of using TDS as an indicator or as a surrogate for assumed increases in salt, sulfates or unwanted organic contaminants.

It should also be noted that high TDS from regular salt (sodium chloride) as created in saltwater or chlorine-generation systems does not cause decreased sanitizer efficacy or cloudy water.

TDS is the measure of all solids dissolved in water, so its constituents will always vary depending not only on service routines, but also on bather activity and a range of environmental factors. As a result, it is difficult to generalize about the effects of given TDS levels on a given body of water.

Despite that, we can say with relative certainty that maintaining levels below the recommended threshold will in general have a beneficial impact on water quality and the longevity of pool surfaces, equipment and fittings. Furthermore, understanding how the chemicals added to the water influence TDS can inform the choices made not only by service technicians, but also by designers, engineers and builders who specify automated treatment systems during the project-development process.

Finally, it's always important to bear in mind that TDS is only one among many important factors in overall pool/spa water chemistry and should be considered only as part of an overall water-management program.

 

Kim Skinner has been part of the pool industry for 40 years as a service technician and plasterer and as president of Pool Chlor, a chemical service firm. He has conducted laboratory and field research on pool water chemistry and on the relationships between water chemistry and pool plaster surfaces; he also developed the bicarbonate start-up method for new plaster pools. Currently, he is with onBalance, a consulting group that also includes Que Hales and Doug Latta. For more information, go to www.poolhelp.com.

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