Total Dissolved Solids is a measure of all the dissolved solids in water. The measurement is typically stated in milligrams per liter (mg/L) or parts per million (ppm).
Greater than 99% of TDS in tap water is minerals. Many people have been scared into believing that ‘TDS is bad’ by companies selling water filtration technologies that remove minerals. It simply isn’t true. They claim that zero TDS is the standard for pure water. Technically, that’s true. It’s also the type of water demonstrated to cause side effects in hundreds of studies concerning demineralized water.
The World Health Organization has even issued a warning about drinking reverse osmosis water. Don't be duped by water filter companies claiming TDS is bad for health; the opposite has been proven true in hundreds of scientific studies.
Zero TDS water is the type of water used in laboratories. It’s not the type of water you find in nature available to drink.
Hardness is the particular variety of dissolved solids responsible for creating water deposits and scale. Many people have water softeners in their homes to reduce the hardness in their water. Hard water concentrations are often expressed in the unit GPG or Grain Per Gallon.