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What is the role of TDS in drinking water?

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Have you been drinking water which is colorless, odorless but also bland to taste? Then, you are probably consuming water with very low TDS. It is very important to know the composition of anything you put into your mouth and water is no exception. After all, it has short-term and long-term health consequences for you.

So what is TDS?

Water being a universal solvent, dissolves most substances it comes in contact with. These substances add up and constitute the Total Dissolved Solids (TDS) quotient of the water. So, TDS is nothing but the total organic and inorganic matter dissolved in a quantity of water. The principal constituents are usually calcium, magnesium, sodium, and potassium cations and carbonate, hydrogencarbonate, chloride, sulfate, and nitrate anions. The TDS content of any water is expressed in milligrams /litre (mg/l) or in parts per million (ppm).

Typically, when you test the TDS of a water sample, you will get the total anion and cation count present in the sample. Though a TDS meter will give you the quantity of these ions present, it doesn’t have the ability to differentiate between the beneficial and toxic ions. For instance,  if the water contains beneficial minerals, 150-250 ppm can be an excellent TDS range for optimal health. However, if the contaminant is residual pesticides from agricultural runoff, 100 ppm would be an extremely high reading. 

What is the source of TDS in water?

The dissolved organic matter can include hydrocarbons, herbicides, and pollutants. Water from lakes, waterfalls, and springs contains microorganisms and salts — some natural water supplies even contain phytoplankton, a microscopic plant. Organic compounds derived from soil, such as fulvic and humic acids, are also considered dissolved solids. 

Organic compounds can originate from urban or agricultural run-off, industrial wastewater, sewage, chemicals used in water treatment, carbonate or salt deposits, seawater intrusion, stormwater, or even the water pipes underneath towns and neighborhoods.

Synthetic chemicals in drinking water commonly come from pharmaceutical drugs, urban and industrial runoff, and chemicals used in manufacturing processes.

What is the desired level of TDS in water?

As per a publication by the WHO, the palatability of drinking water 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.

How is TDS measured?

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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. High TDS concentrations can also be measured gravimetrically, although volatile organic compounds are lost by this method.

Conductivity method

For a baseline measurement of the TDS in water, you can use a basic TDS meter. Dissolved ions allow water to conduct electrical currents. This means you can measure the level of ions in the water by testing its electrical conductivity. The higher the amount of dissolved ionized solids, the higher the water’s electrical conductivity. The practical quantitation limit for TDS in water by this method is 10 mg/litre. The other limitation is that some hazardous substances don’t actually produce ions when dissolved and hence can go undetected. But even on a relative scale, if you notice a sudden spurt in the TDS of water, it means there is serious contamination you must deal with. The ideal thing to do is sending a sample of your water to your nearest water lab.

Gravimetric method

This is the method employed in laboratories and hence way more accurate. Gravimetric analysis is a method of quantitative chemical analysis in which the constituent sought is converted into a substance (of known composition) that can be separated from the sample and weighed. The steps commonly followed in the gravimetric analysis are (1) preparation of a solution containing a known weight of the sample, (2) separation of the desired constituent, (3) weighing the isolated constituent, and (4) computation of the amount of the particular constituent in the sample from the observed weight of the isolated substance.

Potential effects of high TDS in water:

The presence of high levels of TDS in drinking 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).

Organic materials, high disinfectant content, and inadequate water treatment can all change the color of your water. Inorganic solids including metals can also affect color shifts — non-hazardous elements like iron and manganese react to oxygen and turn the water into a rusty orange or blackish-brown.

Tooth pitting or discoloration can also come from high TDS levels, particularly if the TDS includes high amounts of fluoride.

For a more in-depth understanding of TDS, check out our guide, TDS in drinking water.

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