TDS - Safe Level in Drinking Water
.
WHO/SDE/WSH/03.04/16
Total dissolved solids in Drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
GENERAL DESCRIPTION
Identity
Total Dissolved Solids (TDS) is the term used to describe the inorganic salts and small
amounts of organic matter present in solution in water. The principal constituents are usually
calcium, magnesium, sodium, and potassium cations and carbonate, hydrogencarbonate,
chloride, sulfate, and nitrate anions.
Organoleptic properties
The presence of dissolved solids in water may affect its taste (1). The palatability of drinkingwater
has been rated by panels of tasters in relation to its TDS level as follows:
Excellent: Less than 300 mg/litre;
Good: Between 300 and 600 mg/litre;
Fair: Between 600 and 900 mg/litre;
Poor: Between 900 and 1200 mg/litre; and
Unacceptable: Greater than 1200 mg/litre.
Water with Extremely Low Concentrations of TDS may also be Unacceptable because of its flat,
insipid taste.
ANALYTICAL METHODS
The method of determining TDS in water supplies most commonly used is the measurement
of specific conductivity with a conductivity probe that detects the presence of ions in water.
Conductivity measurements are converted into TDS values by means of a factor that varies
with the type of water (2,3). The practical quantitation limit for TDS in water by this method
is 10 mg/litre (M. Forbes, personal communication, 1988). High TDS concentrations can also
be measured gravimetrically, although volatile organic compounds are lost by this method
(4). The constituents of TDS can also be measured individually.
ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE
Water
TDS in water supplies originate from natural sources, sewage, urban and agricultural run-off,
and industrial wastewater. Salts used for road de-icing can also contribute to the TDS loading
of water supplies.
Concentrations of TDS from natural sources have been found to vary from less than 30
mg/litre to as much as 6000 mg/litre (5), depending on the solubilities of minerals in different
geological regions. Thus values, expressed as the sum of the constituents, were below 500
mg/litre in 36 of 41 rivers monitored in Canada (6), while, in a survey of the Great Lakes,
levels ranged from 65 to 227 mg/litre (7). The levels of TDS in all of the Great Lakes except
Lake Superior have increased in the last 70 years, by 50–60 mg/litre in Lakes Erie and
Ontario (7–10). Between 1960 and 1980, a threefold increase in TDS was observed in the
Kent River, Australia (5). Between 1955 and 1970, a tenfold increase in the salinity of the
groundwater at Burlington, MA, was noted, resulting from road de-icing. The use of de-icing
chemicals was prohibited thereafter (5).
EFFECTS ON HUMANS
No recent data on health effects associated with the ingestion of TDS in drinking-water
appear to exist; however, associations between various health effects and hardness, rather
than TDS content, have been investigated in many studies (see Hardness).
In early studies, inverse relationships were reported between TDS concentrations in drinkingwater
and the incidence of cancer (11), coronary heart disease (12), arteriosclerotic heart
2
disease (13), and cardiovascular disease (14,15). Total mortality rates were reported to be
inversely correlated with TDS levels in drinking-water (15,16).
It was reported in a summary of a study in Australia that mortality from all categories of
ischaemic heart disease and acute myocardial infarction was increased in a community with
high levels of soluble solids, calcium, magnesium, sulfate, chloride, fluoride, alkalinity, total
hardness, and pH when compared with one in which levels were lower (17). No attempts were
made to relate mortality from cardiovascular disease to other potential confounding factors.
The results of a limited epidemiological study in the former Soviet Union indicated that the
average number of "cases" of inflammation of the gallbladder and gallstones over a 5-year
period increased with the mean level of dry residue in the groundwater (18). It should be
noted, however, that the number of "cases" varied greatly from year to year in one district, as
did the concentration of dry residue in each district, and no attempt was made to take possible
confounding factors into account.
OTHER CONSIDERATIONS
Certain components of TDS, such as chlorides, sulfates, magnesium, calcium, and carbonates,
affect corrosion or encrustation in water-distribution systems (4). High TDS levels (>500
mg/litre) result in excessive scaling in water pipes, water heaters, boilers, and household
appliances such as kettles and steam irons (19). Such scaling can shorten the service life of
these appliances (20).
CONCLUSIONS
Reliable data on possible health effects associated with the ingestion of TDS in drinkingwater
are not available. The results of early epidemiological studies suggest that even low
concentrations of TDS in drinking-water may have beneficial effects, although adverse effects
have been reported in two limited investigations.
Water containing TDS concentrations below 1000 mg/litre is usually acceptable to
consumers, although acceptability may vary according to circumstances. However, the
presence of high levels of TDS in water may be objectionable to consumers owing to the
resulting taste and to excessive scaling in water pipes, heaters, boilers, and household
appliances (see also the section on Hardness). Water with extremely low concentrations of
TDS may also be unacceptable to consumers because of its flat, insipid taste; it is also often
corrosive to water-supply systems.
In areas where the TDS content of the water supply is very high, the individual constituents
should be identified and the local public health authorities consulted. No health-based
guideline value is proposed for TDS. However, drinking-water guidelines are available for
some of its constituents, including boron, fluoride, and nitrate.
REFERENCES
1. Bruvold WH, Ongerth HJ. Taste quality of mineralized water. Journal of the American Water Works Association, 1969, 61:170.
2. International Organization for Standardization. Water quality—determination of electrical conductivity. Geneva, 1985 (ISO 7888:1985).
3. Singh T, Kalra YP. Specific conductance method for in situ estimation of total dissolved solids. Journal of the American Water Works Association, 1975, 67(2):99.
4. Sawyer CN, McCarty PL. Chemistry for sanitary engineers, 2nd ed. New York, McGraw-Hill, 1967 (McGraw-Hill Series in Sanitary Science and Water Resources Engineering).
5. WHO/UNEP, GEMS. Global freshwater quality. Oxford, Alden Press, 1989.
.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
