Total Dissolved Solids TDS

TDS or Total Dissolved Solids is one value which is used to evaluate the water quality. The TDS describes the total amount of all inorganic and organic substances dissolved in a solvent. In our case, here on this page, the solvent is water. The water can be drinking water, cooling water or wastewater. Total dissolved are measured in micrograms per liter or in PPM, parts per million. Whereas 1 ppm equals 1 microgramm per liter.
Calcium, magnesium, nitrates and chlorides are the most abundant minerals in drinking water. All minerals in the water influence the TDS. All these elements can also be found in sea water. And in addition a lot of salt, sodium chloride.

Just a few examples for what TDS rates different water can have. A pure water, such as destilled water from a steam boiler or filtered water like Reverse Osmosis water, has close to zero TDS. If there is no pollution in the air, rain water is close to zero also.
In Northern America a tap water, with TDS levels below 500 ppm, is considered to be a good drinking water. A lot of mineral water that is sold, has a TDS value of 500 and more. Hard water starts at 1000 PPM. Seawater can reach due to the high sodium content, TDS values of 40,000 and more. Whereas wastewater can hold very high amounts of total dissolved solids, far above 100,000 ppm.

TDS Meter used to check the waterUsing a TDS meter to meassure Total Dissolved Solids

TDS meters measure the electrical conductivity in Microsiemens of the liquid. The TDS meter is put in a water sample. The meter apply a small voltage between two anodes. The resistance in the water sample can be measured. And the conductivity is calculated. Since conductivity and TDS depend on each other, you can draw conclusions about the Total Dissolved Solids in the liquid.
However, the total amount of dissolved solids says nothing about the actual water quality. This total dissolved is only a parameter for calculating the reaction of water in the technical environment, for example when drinking water is heated.

Calcium and magnesium have the tendency to precipitate and form scale in pipes and machines. An absolute figure, from which amount of TDS the scaling starts, can not be given. The pH of the liquid is influencing this a lot as well. At a pH <7, which is seen as acidic, there is a higher tendency of dissolving carbonates, at a pH >7 liquid is seen as basic, the percipitation is increasing. In easier words scaling is happening more likely at a pH of 7 or higher.

Practical use of knowing the Total Dissolved Solids

Practically Total Dissolved Solids is used for instance to check the quality of drinking water or cooling water. There are limits set by the user of a cooling water system. If the TDS reaches a certain value, blow down has to be done.
Therefore users try to keep the TDS levels as low as possible, in order to avoid any kind of scaling or fouling in their system.

Another very practical use is to see how efficient a filtering system is working. If one like to see the perfomance of a reverse osmosis system or an ultrafiltration system, one can compared the TDS before and after the unit. This way one can see if the filter is still performing well.

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