Total Dissolved Solids (TDS)

Definition - What does Total Dissolved Solids (TDS) mean?

Total dissolved solids (TDS) refers to the amount of minerals, metals, organic material and salts that are dissolved in a certain water volume that is expressed in mg/L. It is directly associated with the quality and purity of water, particularly in water purification systems.

Total dissolved solids may be suspended in forms such as:

  • Molecular
  • Ionized
  • Micro-granular (colloidal sol)

The levels of total dissolved solids affect all that lives in, drinks or uses water. Thus, it needs to be measured to ensure the quality of drinking water and the performance in industrial settings involving pipes, valves and other equipment.

Corrosionpedia explains Total Dissolved Solids (TDS)

Total dissolved solids are often caused by the following:

  • Industrial sewage and waste
  • Plankton
  • Silt
  • Urban runoff
  • Road salts during winter
  • Pesticides and fertilizers

In addition to these, TDS can also come from air that contains nitrogen, sulfur, calcium bicarbonate and other kinds of minerals, as well as rocks. Water is capable of picking up copper, lead and other metals as it passes through pipes used to deliver water to consumers. A water purification system's efficacy in removing TDS may become weaker over time, so the quality of membranes and filters should be monitored, and replacement performed when necessary.

High TDS levels may indicate the presence of harmful chemicals. It may also indicate hard water that causes buildup of scale in valves and pipes, which may hinder performance. In industrial and commercial settings, elevated TDS could slow down the function of cooling towers, boilers and other machinery.

Hence, when inappropriate TDS levels are detected in certain applications, treatments such as water softening and reverse osmosis should be implemented immediately.

Total dissolved solids are measured as parts per million (ppm), and standard drinking water recommends a limit of 500 ppm. The lower the TDS, the better the water quality. For drinking water and fountain beverages, a TDS of up to 500 is acceptable, but for boiler-based steam ovens, TDS should be kept very low—less than 100 ppm.

High TDS levels indicate hard water. This can:

  • Cause scale buildup in pipes, valves and filters
  • Reduce performance
  • Increase system maintenance costs

Typically, in industrial applications, total dissolved solids are tested frequently, and filtration membranes are checked in order to prevent adverse effects. Common water filter and water purification systems include:

  • Carbon filtration
  • Reverse osmosis (RO)
  • Distillation
  • Deionization (DI)

The effectiveness of water purification systems in removing total dissolved solids is diminished over time, so it is highly suggested to monitor the quality of a filter or membrane and replace when required.

The main application of TDS is in the study of water quality for streams/rivers and lakes. It is used as an indication of characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants.

TDS is differentiated from total suspended solids (TSS). TSS cannot pass through a sieve of two micrometers, yet are indefinitely suspended in solution.

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