TDS stands for Total Dissolved Solids. Water in nature contains dissolved solids picked up by water as it encounters inorganic minerals in rocks and soil. Water can also dissolve solids from contact with organic and manmade substances and equipment.
A current marketing trend has resulted in a common misconception that water is safe to drink and contaminant-free if your water has zero TDS, as measured by an inexpensive TDS metre. This is not accurate.
Zero TDS does not indicate contaminant free water. It only indicates that the water contains no dissolved solids.
Some dissolved solids borne in water may be desirable, such as the salts and minerals that give water distinctive taste and that charge water with potential healing properties.
On the other hand, Zero TDS water may be loaded with non-solids in the form of liquids, gases, and volatile organic compounds (VOCs). Chlorine, chloramine, THMs and 700 known, yet unregulated, by-products of chemical disinfection fall in this category. A TDS metre does not indicate the presence of these harmful compounds.
TDS levels in naturally sourced water vary greatly based on the mineral load in that water.
Some minerals, like magnesium and calcium, are beneficial to health. Other minerals found in water, like iron and manganese, are a nuisance with potential negative health effects. And still others, such as arsenic, are downright poisonous.
Ground water contains lots of dissolved minerals. The type and concentration of these minerals are unique to the location of each ground water source.
Surface water (from lakes, rivers and streams) typically has a lower mineral content but carries more dissolved organic matter introduced from surrounding vegetation plus from human and animal activity.
Rain water is naturally low in dissolved solids, unless substances have been added via atmospheric injection.
Reverse osmosis and water distillation remove all minerals and yield water with a TDS close to zero. These two water treatment methods also offer the most comprehensive removal of most (but not all) other types of water contaminants. Reverse osmosis systems and water distillers typically finish their purifying process by passing water through a final carbon filter to refine the finished taste of the purified water. Contact with this final carbon filter typically contributes a small amount of desirable dissolved solids back into the finished water.
NOTE Watch for a future Watermatters article which will discuss the pros and cons of organic vs inorganic minerals.
NOTE Metro Vancouver’s tap water comes from mountain reservoirs filled by rainfall and snow melt. Consequently, the TDS levels of Metro Vancouver’s source water are comparatively low. However, minerals are added to Metro Vancouver’s tap water before it leaves the filtration plant. These minerals may increase the TDS of water delivered to your tap. The purpose of these additives is to reduce the corrosion of water infrastructure and the leaching of heavy metals from water pipes. Their intended use contributes no direct benefit to our health.