Total Dissolved Solids (TDS) is a measure of the sum of all inorganic and organic substances in a liquid in molecular, ionized or micro-granular colloidal  suspended form. The solids must be small enough to survive filtration through a sieve the size of two micrometer.   TDS solids are normally found in all freshwater systems.  Major applications of TDS are: at low levels the study of water quality for streams, rivers, lakes, potable water taste, and at high levels the control of scale in  recirculating water systems. It is a quality indicator for drinking water and is a general measure of the presence of chemicals. In seawater RO desalination, TDS provides a way of comparing the feed with the permeate quality that is relevant to drinking water taste standards.

Total Suspended Solids (TSS), are different from TDS as they cannot pass through a sieve of two micrometers and are suspended in solution. Flocculation and sand filtration can remove settleable solids and suspended solids from the water but dissolved TDS solids are not removed.

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