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TDS is Total Dissolved Solids. Water dissolves the minerals present in the strata of soil it filers through in the case of ground water and, in the case of surface water, the minerals present in the soil over which it flows (rivers/streams) or over which it stands (lakes, ponds, reservoirs).The dissolved minerals in water are commonly referred to as Total Dissolved Solids (TDS). The TDS content of any water is expressed in milligrams /litre (mg/l) or in parts per million (ppm). These units are equivalent.
The minerals are basically compounds (salts) of Calcium(Ca), Magnesium(Mg) and Sodium(Na) What is commonly called as ‘hardness in water’ is due to the compounds/salts of Ca and Mg such as Calcium or Magnesium Chloride, Calcium or Magnesium Sulphate ( CaSo4, MgCl, etc).Some types of dissolved solids are specifically dangerous even in low quantities. This includes arsenic, fluorides and nitrates. There are particular standards for the acceptable amounts of these elements in water and in some cases like fluoride, there is some disagreement as to what constitutes safe levels.
Leaving aside the specific harmful chemicals fluoride and arsenic, drinking water for human beings should contain some level of minerals (TDS), but these levels should not be excessive.
India Standards: The standard that applies to India is the BIS 10500-1991 standard
This standard used the WHO standard as the basis and has been amended subsequently to take into account the fact that over exploitation of ground water which has the largest share of water supplied for human use has deteriorated to such an extent that the crucial parameters such as TDS, hardness, Chlorides, etc usually exceed the desirable levels substantially. Consequently, a higher permissible limit has been specified. Water used for drinking becomes unpalatable when the TDS level is above 500 mg/l, but lack of any better source enables people consuming such water to get used to its taste. The BIS standard applies to the purity level acceptable for human beings to drink. For practically all industrial and some commercial uses, the purity levels required are very much higher and in most cases demand water with virtually no residual dissolved solids at all.
BIS Standard says that the maximum desirable TDS is 500 mg/L and the maximum permissible level in the absence of a better source of water is 2000 mg/L. A related standard is the 'hardness measured as CaCO3" where the maximum desirable is 300 mg/L and maximum permissible is 600 mg/L.
"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 "
US EPA Standard:
The U.S. Environmental Protection Agency (EPA) recognises broadly two categories of drinking water standards, known as maximum-contaminant-level goal (MCLG) and secondary maximum contaminant level (SMCL). The MCLG is a health goal set at a concentration at which no adverse health effects are expected to occur and the margins of safety are judged “adequate,” while the SMCL is a non-enforceable guideline that presents no risk to human health. While fixing no limit for MCLG, the EPA has fixed an upper limit of 500 mg/L for SMCL. This limit has been fixed to avoid undesirable aesthetic effects of odour, taste and colour that could be felt by consumers and technical effects of corrosion, incrustation, staining, scaling and sedimentation of pipelines and other fixtures that convey water. Despite not fixing a limit to MCLG of TDS, high TDS water can have certain other constituents at harmful levels of SMCL to cause adverse health effects. Thus MCLG can be a few times more than the SMCL.
Very low TDS: Due to insipid or bitter taste and lack of useful minerals, too-low TDS also causes problems. There does not seem to be a generally accepted lower limit, but 80 mg/L may be used.
TDS can be measured very fast using a low-cost portable conductivity meter (TDS meter) calibrated to give TDS directly by anybody with extreme ease. It costs hardly Rs. 2000/- and the only recurring expenditure is occasional replacement of batteries. It is worthwhile for users of well water, piped water and packaged water and practitioners of rainwater harvesting and groundwater recharging to test water TDS as a matter of routine. It may be noted that TDS of rainwater is only a few tens of mg/L. Any sudden increase in TDS of water is a signal that water is getting contaminated with some high-TDS water.
UV, UF and other conventional filtration methods will not affect TDS. The only one which works is Reverse Osmosis
RO is the only commonly used domestic filtration system that removes even the dissolved impurities. RO is required if the Total Dissolved Solids (TDS) exceeds a certain value. (what is the upper limit ? Look for discussion on that elsewhere in IWP). RO is also suggested if you have reasons to believe that your water may be contaminated with sewage/ pesticides/ heavy metals/ industrial effluents.
A problem with RO is, it needs a lot of water. It divides the input water in two parts, and forces the dissolved solids out from one part in to other. Thus, the output comprises two streams of water – a “clean” stream with low TDS and cleaned of other impurities too. And a “reject” stream that is even more dirty than the input water. Typically, an input of 3 liters will give 1 liter of clean water and 2 liters of “reject”. Theoretically, the “reject” water can be used for mopping the floor etc. but few have the discipline to do that.
Reduction of TDS changes the taste and pH of water, and it is not good to reduce the TDS too low. Some manufacturers make a hybrid machine that combines RO with either UV or UF. Bulk of the water is processed by RO, to remove dissolved solids; and some is processed by either UF or UV, to kill micro-organisms, but retaining the dissolved solids. The two are combined to restore the dissolved solids to some lower limit. The ratio of mixing the two can be controlled by user.
The cost of RO systems is in the region of Rs. 10,000/- to 15,000/- The RO works under some pressure, which is developed by an internal pump, and therefore it needs electricity to operate.
With very high TDS levels in the 1000s, conventional domestic RO units may not be able to work effectively.
Rainwater harvesting is a useful permanent solution where other sources of water have unacceptably high levels of TDS or hardness. TDS of rainwater is a few tens of mg/L
Water softening does not reduce TDS. In water softening sodium replaces calcium and magnesium, in the dissolved solids which causes a minor reduction only in TDS.
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The FAQ is a compilation of answers recieved to TDS related questions on India Water Portal. Contributors include: