Understanding TDS for Certified Pool Operators: Total Dissolved Solids in Swimming Pools
One of the concepts discussed in a Certified Pool Operator (CPO®) certification class is Total Dissolved Solids (TDS). While many pool operators hear the term frequently, the chemistry behind it is often misunderstood. In simple terms, TDS represents the total mass of dissolved substances present in the water, including salts, minerals, metals, and organic materials.
From a practical standpoint, anything added to a pool that dissolves contributes to Total Dissolved Solids. Chlorine products, balancing chemicals, acids, contaminants from swimmers, and even the fill water itself all contribute to the dissolved mineral content of the pool.
Understanding how TDS accumulates, how it is measured, and how it affects water chemistry is an essential component of professional pool operator training and CPO certification.
The Basic Math Behind TDS
Water treatment math makes it possible to estimate how much a chemical addition increases TDS.
In a 10,000-gallon pool, one pound of any dissolved material will increase Total Dissolved Solids by approximately 12 ppm (parts per million).
This relationship comes from basic concentration calculations used in water treatment engineering:
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10,000 gallons = 37,854 liters
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1 pound = 453,592 milligrams
When distributed across that volume:
453,592 mg ÷ 37,854 L ≈ 11.98 mg/L
Since 1 mg/L equals 1 ppm in dilute aqueous systems, the result is approximately 12 ppm.
This mass-balance principle is widely used in water treatment engineering and environmental chemistry calculations.
Research in water chemistry confirms that TDS is fundamentally a measurement of the total mass of dissolved substances in water rather than the concentration of any single compound (Hem, 1985; WHO Water Quality Guidelines).
Why Every Pool Chemical Increases TDS
Because TDS represents the total amount of dissolved material, every chemical used in pool water treatment contributes to it.
For example, common sanitizers contribute dissolved compounds as they react in the water.
Sodium Hypochlorite (Liquid Chlorine)
Liquid chlorine is sodium hypochlorite (NaOCl), commonly sold in concentrations between 10–12.5%.
When added to water, sodium hypochlorite produces hypochlorous acid (HOCl), the primary sanitizing form of chlorine. As chlorine reacts with contaminants and is consumed, the reaction products ultimately become chloride ions, which remain dissolved in the water.
Research into pool chlorination chemistry shows that chlorine consumption ultimately produces chloride as a final product of oxidation reactions (Wojtowicz, 2004; Falk, 2013).
This means that although chlorine is consumed during sanitation, the dissolved salts remain behind and contribute to TDS.
Trichloroisocyanuric Acid (Trichlor)
Trichlor tablets are another widely used chlorine source in pools.
When trichlor dissolves in water, it produces:
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Hypochlorous acid (chlorine sanitizer)
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Cyanuric acid (stabilizer)
Stoichiometric analysis of the compound shows that 10 ppm of free chlorine added using trichlor increases cyanuric acid by approximately 6 ppm (Falk, 2013).
Because both cyanuric acid and the byproducts of chlorine oxidation remain dissolved in the water, trichlor contributes significantly to long-term increases in TDS.
Calcium Hypochlorite
Calcium hypochlorite introduces both chlorine and calcium ions.
When used to deliver 10 ppm of free chlorine, calcium hypochlorite increases calcium hardness by roughly 7–8 ppm as calcium carbonate equivalent, depending on product purity and available chlorine concentration.
This relationship is well documented in water chemistry literature discussing hypochlorite reactions and water balance calculations (Wojtowicz, 2004).
Again, the added calcium remains in solution and contributes to the overall dissolved solids content.
Acids and Balancing Chemicals
Even balancing chemicals contribute dissolved materials.
For example, muriatic acid (hydrochloric acid) adds chloride ions after neutralization reactions occur in the water.
Although the quantities are relatively small compared with sanitizer additions, over time these contributions accumulate and become part of the pool’s TDS load.
The “Age of the Water”
Many experienced pool professionals refer to TDS as the age of the water.
The concept is simple: the longer water remains in a pool without being replaced, the more dissolved materials accumulate from chemical additions, environmental contaminants, and swimmer load.
Fill water already contains dissolved minerals such as calcium, sodium, bicarbonate, and sulfate. Once the pool is filled, every chemical added increases the total dissolved content.
Research in water chemistry confirms that dissolved solids accumulate in closed aquatic systems unless water is physically removed and replaced (Hem, 1985).
Why TDS Usually Increases Over Time
In most pools, TDS gradually rises due to several processes:
Chemical Additions
Sanitizers, algaecides, acids, and balancing chemicals all add dissolved ions.
Bather Load
Sweat, urine, cosmetics, and organic debris introduce additional dissolved substances.
Environmental Contamination
Dust, soil, fertilizers, and atmospheric pollutants can dissolve in the water.
Evaporation
Evaporation removes only water molecules, leaving dissolved salts behind. As evaporation occurs, the concentration of dissolved solids increases.
This principle is fundamental in hydrology and water chemistry: evaporation concentrates dissolved minerals because the solutes do not evaporate with the water (USGS Hydrologic Cycle studies).
When evaporation losses are replaced with new fill water that also contains dissolved minerals, the total TDS increases even further.
When Should TDS Be Reduced?
Because dissolved solids accumulate over time, many aquatic health guidelines recommend monitoring TDS relative to the starting value of the fill water.
Several operator training programs and aquatic facility guidelines suggest that water replacement should be considered when TDS increases approximately 1500 ppm above the source water level.
This guideline is commonly referenced in aquatic operator training materials and technical discussions surrounding swimming pool water quality management.
The primary method for reducing TDS is dilution through partial water replacement.
As operators often say:
“Dilution is the solution to pollution.”
Measuring Total Dissolved Solids
TDS can be measured directly through laboratory evaporation methods, but in field applications pool operators usually rely on electrical conductivity meters.
These instruments estimate TDS by measuring how well the water conducts electricity.
Ions such as sodium, calcium, chloride, and sulfate increase electrical conductivity because they carry electrical charge in solution.
However, conductivity meters measure ionic dissolved solids, not all dissolved substances.
Non-ionic dissolved materials, such as sugars or certain organic compounds, may contribute to TDS without significantly affecting conductivity.
Environmental chemistry research explains this limitation clearly: electrical conductivity reflects ionic content rather than the full spectrum of dissolved solids (EPA water quality monitoring guidance).
Because of this, conductivity-based TDS readings are considered estimates rather than exact measurements.
TDS and the Saturation Index
Total Dissolved Solids also plays a role in water balance calculations, including the Langelier Saturation Index (LSI).
The LSI is used by pool operators to determine whether water is:
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corrosive
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scale forming
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chemically balanced
In many simplified pool industry LSI tables used in operator training, the TDS factor is typically approximated as:
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12.1 for TDS below 1000 ppm
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12.2 for TDS above 1000 ppm
Because this factor changes very little across common pool TDS ranges, the difference usually has minimal impact on the final index calculation in standard swimming pools.
However, in environments with very high TDS—such as saltwater pools or spas—the effect can become more significant.
TDS in Spas and Hot Tubs
Spas and hot tubs represent a different situation.
Because they contain much smaller volumes of water and experience higher bather loads and temperatures, TDS can increase rapidly.
In heavily used spas, TDS levels can increase by several hundred ppm within a matter of days or weeks.
For this reason, many professionals who maintain spas prefer to own a portable TDS meter and monitor dissolved solids more frequently.
Thoughts for Pool Operators
Total Dissolved Solids is not a contaminant by itself. Instead, it is a measurement of everything dissolved in the water.
Understanding how TDS accumulates helps pool professionals recognize why water eventually needs to be replaced and how chemical additions affect overall water chemistry.
For those studying for CPO certification, TDS is best understood through three key principles:
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Anything dissolved in the pool increases TDS.
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Evaporation concentrates dissolved solids.
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The only practical way to reduce TDS is through dilution with fresh water.
Mastering these fundamentals is part of becoming a knowledgeable pool operator and maintaining safe, balanced swimming pool water.
References
Hem, J. D. (1985). Study and Interpretation of the Chemical Characteristics of Natural Water. U.S. Geological Survey Water Supply Paper.
Falk, R. (2013). Chlorine Stabilization and Cyanuric Acid Relationships in Pool Water Chemistry. ResearchGate.
Wojtowicz, J. A. (2004). Swimming Pool Water Chemistry. Journal of the Swimming Pool and Spa Industry.
World Health Organization. Guidelines for Safe Recreational Water Environments.
United States Environmental Protection Agency. Water Quality Monitoring and Conductivity Measurement Guidance.
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Anything that is in the water that is not Dihydrogen Monoxide (aka: H2O).
DHMO is not aka: H2O
Hey Ian, Thanks for reading! Dihydrogen Monoxide although not the common name used in science for H2O, in recent years has become the name more recognizable to the public. 2 (di) parts Hydrogen and 1 (mono) part oxygen. I considered using hydroxic acid, hydrogen hydroxide, or oxidane which are all accepted terms by IUPAC standards, but they are far less familiar to those not in the scientific field. Of course, I could have just called it water. Thank you for your feedback. I definitely value your thoughts. Hoping you will enjoy some of the other pieces I have written as well.