Total dissolved solids: the facts
By APSP Recreational Water Quality Committee
Monday, 25 March, 2019
Total dissolved solids play a significant role in water chemistry. While pH, total alkalinity and calcium hardness levels get all the attention, TDS should not be overlooked.
With the increased popularity of electrolytic chlorine generators (ECGs), TDS levels have been steadily climbing, making it more important than ever to understand what total dissolved solids are and how they work.
What is TDS?
Total dissolved solids (TDS) is the total of all dissolved solid matter such as minerals, metals, salts and contaminants in water. TDS levels are often determined with a conductivity meter, which gauges water’s ability to conduct an electric current. The greater the concentration of charged particles (including both positive and negative ions), the more freely electric current can flow through water. TDS levels can also be measured via test kits and test strips.
It is important to note that while testing methods measure charged particles in the water, true TDS also includes non-charged contaminants such as oils, lotions, cosmetics and other swimmer waste. Other tests will provide better information about organic contaminants in pools and spas, but are much more costly and time-consuming to perform in comparison to the simple conductivity test.
In residential pools and spas, TDS testing should be performed monthly. In commercial pools, spas and water features, testing should be performed several times a month as necessary.
TDS and the Langelier Saturation Index
Rising TDS levels have traditionally been used as an indicator of the accumulation of contaminants in the pool or spa water. Contamination may take the form of unoxidised or partially oxidised pollutants and include nitrogenous products from swimmer waste. TDS concentration increases over time as dissolved materials are added to water from source or fill water, pool treatment chemicals, swimmer waste and environmental contaminants. Evaporation also increases TDS, as water evaporates leaving behind more concentrated dissolved solids.
Of the different water balance indices currently used in the pool industry to help predict the scale forming or corrosive tendencies of pool and spa water, the major index utilised today is the Langlier Saturation Index (LSI).
The LSI is a mathematical/chemical formula that considers five factors or properties of pool/spa water, including pH, bicarbonate/carbonate alkalinity, calcium hardness, temperature and TDS. In traditional LSI calculations, carbonate alkalinity, calcium hardness and temperature levels were assigned numerical factors while the TDS content was assigned a constant of 12.1, which reflected TDS levels from 0 to 1000 ppm.
Historically, the LSI was developed for use with closed systems, like pipelines, wherein the water balance remained relatively fixed for a given body of water. The system was later applied to swimming pools.
Initially, TDS was easily controlled by draining pools when levels were too high. However, this solution is not always practical for open pool systems, as TDS levels can climb because of several factors like added pool chemicals, evaporation, new replacement water and reactions with the atmosphere, to name a few. The concept of trying to provide a better arithmetic formula to protect tile grout, equipment and plaster surfaces resulted in adding the LSI TDS constant of 12.2 for the 1000–20,00 ppm range.
For years, a level of 3000 ppm was considered the maximum allowable TDS for pools. That began to change in the early 1990s when ECGs became more prevalent; the sodium chloride (salt) introduced into these pools causes TDS levels to exceed the 3000 ppm range. By the early 2000s, the NSPI (now APSP) adopted the additional TDS constant of 12.3 for levels from 2000–3000 ppm. These TDS constants or factors were updated by APSP again in 2009 to cover TDS ranges from 0 up to 5000 ppm.
Application and use of TDS data
The recommended maximum allowable TDS for pools and spas is 1500 ppm greater than TDS at initial pool or spa start-up. Start-up TDS incorporates both balanced water TDS and salt (sodium chloride) added at start-up.This maximum allowable level can be used to prevent common issues associated with ageing pool/spa water, including:
- Reduced efficiency of disinfection/sanitising chemicals due to elevated organic contaminants.
- Corrosion of fixtures as TDS increases due to greater conductivity of the water.
- Surface staining and/or etching that can result from elevated TDS.
Exceeding the maximum recommended TDS level may indicate the need to partially or completely drain water. For spas, TDS can be used to calculate the water replacement interval (WRI) or to determine when the spa needs to be completely drained. This is likely to be required if either of the following conditions are met:
- TDS in the spa exceeds the source water TDS by 1500 ppm or more (see example 1); or,
- The WRI is less than or equal to the number of days since the last time the water was drained (see example 2).
WRI is calculated as:
WRI = (0.33) (Spa volume in gallons ÷ Maximum # of bathers per day since last change)
Example 1 — TDS exceeds source water TDS by 1500 ppm or more
The TDS of the original source water was measured and recorded to be 800 ppm. The TDS of the spa water now reads 2500 ppm. The difference is greater than 1500 ppm (2500 ppm - 800 ppm = 1700 ppm). Therefore, the spa should be drained immediately.
Example 2 — WRI less than or equal to # of days since last drain
Consider a 600-gallon (2.3 m3) spa last drained and refilled on Sunday evening, with a usage pattern as follows:
The WRI is computed in the table below. When compared with the interval since the last change, if the difference WRI (days since last change) is less than or equal to zero, the spa should be drained.
The TDS numerical value is used to obtain the correct LSI factor to accurately calculate the LSI.
Although there is not a minimum level in ANSI/APSP standards, source water TDS should always be checked before start-up of new pools, spas and water features. Low TDS may be indicative of low calcium hardness and/or low total alkalinity levels, which may produce corrosive conditions that could affect tile grout, surfaces and equipment. High or elevated TDS levels (which include elevated sodium chloride levels) can:
- Indicate high organic contamination, which can influence the consumption of the sanitiser and its ability to properly disinfect and oxidise the pool or spa water. That in turn can lead to swimmer or bather safety concerns.
- Increase the conductivity of the water, which may lead to possible corrosion of fixtures, lights and equipment, especially if any stray voltage is present in the water.
- Damage some ECGs, which should be monitored to prevent problems. Consult the ECG manufacturer to determine at what point high TDS can cause problems.
- Increase the solubility of calcium carbonate and other calcium compounds. As has been known for years in industrial water treatment, high mineral content in water increases the conductivity of water, and can thus lead to possible corrosion if other factors that favour corrosion are present. With elevated TDS and salt (NaCl) levels, there is an increase in ionic strength as salt and TDS concentrations increase. For cementitious interior finishes, this increase in the solubility of calcium components of the plaster and other cement-based surfaces can be offset/managed by adjustment of the other LSI water chemistry parameters (pH, total alkalinity, calcium hardness and temperature) and use of the LSI to prevent corrosive conditions. This must include the correct factor for elevated TDS levels.
- Cause problems with masonry materials used in pool/spa decks. Both natural and man-made products subjected to frequent wet/dry cycles with water containing high TDS can experience deterioration of these materials. Always consult with the installer/contractor to confirm the compatibility of these materials to be placed on and around pools/spas with elevated TDS and salt levels.
The increased popularity of ECGs means many pools, spas and water features are running TDS levels in excess of 3000 ppm. Understanding the potential problems associated with elevated levels needs to be on every pool professional’s radar. Concerns including hazy ‘tired’ water, improper LSI water balances, surface staining, decreased sanitiser efficiency and deterioration of equipment should all be considered when performing a regimen of care to any pool, spa or water feature. The simplest remedy to these problems is often to perform a partial or complete drain of a vessel and replace with fresh water that has a lower TDS content.
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