What is TDS?
Total dissolved solids (TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular (colloidal sol) suspended form. Generally, the operational definition is that the solids must be small enough to survive filtration through a filter with two-micrometer (nominal size, or smaller) pores. Total dissolved solids are normally discussed only for freshwater systems, as salinity includes some of the ions constituting the definition of TDS. The principal application of TDS is in the study of water quality for streams, rivers and lakes, although TDS is not generally considered a primary pollutant (e.g. it is not deemed to be associated with health effects) it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants. Source: Wikipedia
Water can be classified by the level of TDS in the water:
Freshwater: less than 500 mg/L TDS=0.5ppt
Brackish water: 500 to 30,000 mg/L TDS=0.5-30ppt
Saline water: 30,000 to 40,000 mg/L TDS=30-40ppt
Hypersaline: greater than 40,000 mg/L TDS>=40ppt
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 the 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.
Permissible TDS level
As per World Health Organization(WHO):
Reliable data on possible health effects associated with the ingestion of TDS in drinking water 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. Source:
The Bureau of Indian Standards (BIS) fixes the upper limit of TDS in drinking water at 500 ppm. Crucially the standard also mentions that in case no alternative source of drinking water is available, then this upper limit can be relaxed to 2,000 ppm.
This TDS limit is for normal fresh water and in special cases where toxic contaminants like arsenic or fluoride are present in the water source, other special BIS standards will be applicable.
Some of the dissolved minerals in water are useful for human health and therefore very low or zero TDS is also not desirable. When the TDS is reduced severely it also affects the Ph of the water, which BIS specifies should be in the range 6.5-8.5. Water with very low TDS will taste insipid and while there is no official lower limit, TDS of at least 80 ppm is the accepted minimum level.
Excessive TDS, particularly with salts of calcium and magnesium, leads to hardness of water which then causes scaling in household devices that use water. Source:
How to purify water with high TDS:
Reverse osmosis( RO) is the most effective way of purifying water with high TDS content. RO removes almost any kind of dissolved or suspended impurities from water. UV, UF and other conventional filtration methods will not be effective for TDS.
Osmosis a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one.
Reverse-osmosis a process by which a solvent passes through a porous membrane in the direction opposite to that for natural osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure.
- It removes dissolved impurities from water. Eg: Lead, Mercury, Arsenic, Fluoride, Chlorine etc.
- Suitable for hard water.
- All dissolved and suspended particles in water are removed.
- Low-pressure RO requires no electricity (less efficient)
- It removes essential minerals too along with impurities rendering the water tasteless.Eg: Iron, Magnesium, Calcium, Sodium etc.
- A large quantity of water is wasted to produce potable water
- The membrane is vulnerable to sediments and bacteria which can grow on its surface
- Requires electricity for high inlet pressure. (more efficient)
- Chlorine needs to be removed using activated carbon filter to enhance the life of RO membranes
- Not effective in removing Synthetic organic chemicals as well as Volatile organic chemicals.
- Activated carbon filtration is preferred for organic contaminants