TDS or Total Dissolved Solids is used to specify the amount of substances in a liquid, mainly in water. These solids can be both organic or inorganic and are by definition smaller than 2 micrometer. It is meassured in PPM parts per million or in miligrams per per unit.
We concentrate here, on TDS in water used in industrial applications, such as cooling water or feed water for boilers or similar systems.
In such water calcium, maganese, nitrat, phosphate or sodium are of interest. These have the tendency to precipitate and form scale in pipes and in machines. An absolute figure, from which amount of TDS the scaling starts, can not be given as the pH of the liquid is influencing 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.
Practically TDS is used for instance to check the quality of cooling water. There are limits set by the user of cooling water systems. If the TDS reaches a certain value, blow down has to be done.
Users try to keep the TDS as low as possible to avoid any kind of scaling or fouling in the system.
TDS is not a pollutant, it is just a value to descibe the tendency, if a water will start fouling or a heat exchanger will start to scale and block.