Total dissolved solids
(endorsed 2011)
Guideline
No specific health guideline value is provided for total dissolved solids (TDS), as there are no health effects directly attributable to TDS. However for good palatability total dissolved solids in drinking water should not exceed 600 mg/L.
General description
Total dissolved solids (TDS) consist of inorganic salts and small amounts of organic matter that are dissolved in water. Clay particles, colloidal iron and manganese oxides and silica, fine enough to pass through a 0.45 micron filter membrane can also contribute to total dissolved solids.
Total dissolved solids comprise: sodium, potassium, calcium, magnesium, chloride, sulfate, bicarbonate, carbonate, silica, organic matter, fluoride, iron, manganese, nitrate, nitrite and phosphates.
The palatability of drinking water can be rated according to TDS concentrations and a breakdown is provided below, based on World Health Organization guidelines (WHO 2004):
0 ā 600
good
600 ā 900
fair
900 ā 1200
poor
> 1200
unacceptable (unpalatable)
Precisely what level of TDS an individual water supply system decides to accept is a function of community acceptance, available water resources, and the cost and practicality of effecting any change to natural TDS levels.
High TDS values may be associated with excessive scaling in pipes, fittings and household appliances. Water with very high or very low TDS may also be corrosive.
Typical values in Australian drinking water
In major Australian cities, TDS values can range from below 100 mg/L to more than 750 mg/L; regional supplies can have TDS values up to 1000 mg/L and some rural and remote communities may have TDS in excess of 1000 mg/L, owing mainly to groundwater characteristics.
Measurement
Three methods can be used to determine to determine TDS. The most accurate entails a complete analysis of the sample and summation of the concentration of all the anions and cations. The most common and least expensive method is to convert electrical conductivity measurements to TDS values by multiplication by a factor that varies with the type of water (APHA Method 2510A 1992). Gravimetric measurement (i.e. by evaporation and weighing) can also be used (APHA Method 2540C 1992). As a rough guide, electrical conductivity, measured in micro Siemens per cm (, also known as EC, or electroconductivity units), is multiplied by 0.64 to estimate TDS. The relationship, however, is dependant on both chemistry and temperature, and factors of between 0.50 and 0.64 are used across Australia. The factor is also likely to vary between raw and treated water. Inferring a TDS value therefore has to be based on local circumstances.
Treatment of drinking water
It is difficult to remove dissolved solids from drinking water. Suitable technologies include reverse osmosis, ion exchange, and distillation, but all of these require considerable energy input and can be expensive to operate. Lime softening may also be effective where high TDS is mainly due to hardness.
Health considerations
No health effects have been associated specifically with high TDS concentrations. The health effects of individual components of TDS are discussed separately in the discussions on inorganic chemicals (Section 6.3.1 and relevant Fact Sheets). Indirectly, high TDS water, being less palatable than that with a low TDS, might discourage consumers from drinking tap water, leading to use of potentially less healthy water (from alternative sources, natural or manufactured) and/or other less healthy drinks.
Guidelines in other countries
The 2004 World Health Organization guidelines do not specify a value for TDS but do describe values according to palatability.
The European Communities (1998) Directive 98/83/EC nominates a conductivity value of 2500 at 20ĀŗC and, in a footnote, requires that the water not be aggressive. An online converter function (Lenntech 2010) converts that value to 1600 mg/L of TDS (the standard for EC is intended to be at 25ĀŗC).
The United States Environmental Protection Agency nominates a TDS level of 500 mg/L in its list of National Secondary Drinking Water Regulations (USEPA 2009) and explains the category as follows:
National Secondary Drinking Water Regulations (NSDWRs or secondary standards) are non-enforceable guidelines regulating contaminants that may cause cosmetic effects (such as skin or tooth discoloration) or aesthetic effects (such as taste, odor, or color) in drinking water. EPA recommends secondary standards to water systems but does not require systems to comply. However, states may choose to adopt them as enforceable standards.
The Federal-Provincial-Territorial Committee on Drinking Water of the Federal-Provincial-Territorial Committee on Health and the Environment (2008) provides Guidelines for Canadian Drinking Water Quality, and a TDS level of 500 mg/L is nominated as an āaesthetic objectiveā.
References
APHA Method 2510A (1992). Conductivity: Laboratory method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
APHA Method 2540C (1992). Total Dissolved Solids Dried at 180ĀŗC. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.
Drinking Water Federal-Provincial-Territorial Committee on Drinking Water of the Federal-Provincial-Territorial Committee on Health and the Environment (2008). Guidelines for Canadian Drinking Water Quality. Government of Canada.
European Communities (1998). Council Directive 98/83/EC of 3 November 1998 on the quality of water intended for human consumption. Available at https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A31998L0083 (accessed 14 January 2021).
Lenntech (2010). Online converter-calculator. Available at https://www.lenntech.com/calculators/tds/tds-ec_engels.htm (accessed 14 January 2021).
USEPA (United States Environmental Protection Agency) (2009). National Secondary Drinking Water Regulations. United States Government, Washington D.C.
WHO (World Health Organization) (2004) Guidelines for Drinking Water Quality.
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