total dissolved solids (tds)

the term total disolved solids (tds) refers to the amount of dissolved minerals, salts, metals and other organic substances in the water. TDS is essentiall a measure of anything dissolved in water that is not an h2o molecule.  

Symptoms of High TDS Include:

• ∙ mineral deposits despite having a water softener
• ∙ unpleasant taste/smell to the water
• ∙ health issues, if constituents are present

treatment to lower/remove TDS:

reverse osmosis systems are effective at reducing 95-99% of the TDS of water, depending on the specific contaminants.
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distilling units and deionizers are also effective at removing TDS from the water, depending on the specific contaminants.

further reading

TDS is classified as a Secondary "aesthetic" contaminant by the EPA. The recommended limit of <500 PPM is based on minimizing aesthetic issues like taste and odor. if the TDS is >500 ppm, a detailed analysis of the water chemistry is typically recommended to ensure there are no significant levels of harmful contaminants in the water.

TDS is frequently used as a measure of how "pure" the water is. Because water is "the universal solvent" and wants to have minerals dissolved in it, the purer the water is - the more aggressively it will try to dissolve anything it comes in contact with. as such, any materials that come in contact with water treated to remove the TDS must be compatible with corrosive, low TDS water.