Total Dissolved Solids (TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid in molecular, ionized or micro-granular (colloidalsol) suspended form. Generally the operational definition is that the solids must be small enough to survive filtration through a filter with two-micrometer (nominal size or smaller) pores. TDS are normally discussed only for fresh water systems, as salinity includes some of the ions constituting the definition of TDS. The principle application of TDS is the study of water quality for streams, rivers, lakes, although TDS is not generally considered a primary pollutant it is used as an indication of aesthetic characteristics of drinking water and as an aggregate indicator of the presence of a broad array of chemical contaminants.
Primary sources for TDS in receiving waters are agricultural and residential runoff, clay rich mountain waters, leaching of soil contamination and point source water pollution discharge from industrial or sewage treatment plants. The most common chemical constituents are calcium, phosphates, nitrates, sodium, potassium, and Chloride, which are found in nutrient runoff, general storm Water runoff and runoff from snowy climates where road de-icing salts are applied. The chemicals may be cations, anions, molecules or agglomerations on the order of one thousand or fewer molecules,so long as a soluble micro-granule is formed. More exotic and harmful elements of TDS are pesticides arising from surface runoff. Certain naturally occurring TDS arise from the weathering and dissolution of rocks and soils. The USA has established a secondary water quality standard of 500 – 1000 mg/l to provide a palatability of drinking water.
TDS are differentiated from total suspended solids (TSS) in that the latter cannot pass through a sieve of two micrometers and yet are indefinitely suspended in solution. The term “settleable solids” refers to material of any size that will not remain suspended or dissolved in a holding tank not subject to to motion, and excludes both TDS and TSS. Settleable solids may include larger particulate matter or insoluble molecules.