What are TDS?
TDS stands for total dissolved solids, and represents the total concentration of dissolved substances in water. TDS is made up of inorganic salts, as well as a small amount of organic matter. Common inorganic salts that can be found in water include calcium, magnesium, potassium and sodium, which are all cations, and carbonates, nitrates, bicarbonates, chlorides and sulfates, which are all anions. Cations are positively charged ions and anions are negatively charged ions.
How do These Solids End Up Dissolved in Water?
These minerals can originate from a number of sources, both natural and as a result of human activities. Mineral springs contain water with high levels of dissolved solids, because the water has flowed through a region where the rocks have a high salt content. The water in the Prairie provinces tends to have high levels of dissolved solids, because of high amounts of calcium and magnesium in the ground.
These minerals can also come from human activities. Agricultural and urban runoff can carry excess minerals into water sources, as can wastewater discharges, industrial wastewater and salt that is used to de-ice roads.
What Happens to the Water When the TDS Level is High?
Alone, a high concentration of dissolved solids is usually not a health hazard. In fact, many people buy mineral water, which has naturally elevated levels of dissolved solids. The United States Environmental Protection Agency (EPA), which is responsible for drinking water regulations in the United States, includes TDS as a secondary standard, meaning that it is a voluntary guideline in the United States. While the United States set legal standards for many harmful substances, TDS, along with other contaminants that cause aesthetic, cosmetic and technical effects, has only a guideline.
Most people think of TDS as being an aesthetic factor. In a study by the World Health Organization, a panel of tasters came to the following conclusions about the preferable level of TDS in water:
However, a very low concentration of TDS has been found to give water a flat taste, which is undesirable to many people.
Increased concentrations of dissolved solids can also have technical effects. Dissolved solids can produce hard water, which leaves deposits and films on fixtures, and on the insides of hot water pipes and boilers. Soaps and detergents do not produce as much lather with hard water as with soft water. As well, high amounts of dissolved solids can stain household fixtures, corrode pipes, and have a metallic taste. Hard water causes water filters to wear out sooner, because of the amount of minerals in the water. The picture below was taken near the Mammoth Hot Springs, in Yellowstone National Park, and shows the effect that water with high concentrations of minerals can have on the landscape. The same minerals that are deposited on these rocks can cause problems when they build up in pipes and fixtures.
However, while TDS itself may be only an aesthetic and technical factor, a high concentration of TDS is an indicator that harmful contaminants, such as iron, manganese, sulfate, bromide and arsenic, can also be present in the water. This is especially true when the excessive dissolved solids are added to the water as human pollution, through runoff and wastewater discharges.
What are the Guidelines for TDS?
In Canada, substances that are considered to be dangerous in high amounts are listed as Maximum Acceptable Concentrations (MACs) in the Canadian Guidelines for Drinking Water Quality. However, substances that are not considered dangerous at their MAC, such as TDS, are given an aesthetic objective in the Guidelines. The Canadian guideline for TDS is less than 500 milligrams per litre (which is the same as 500 parts per million). However, since the Canadian guidelines are not enforceable, each province is free to choose whether or not they will follow the guidelines. Saskatchewan has water that naturally contains high concentrations of TDS, so the province has chosen to not follow the Canadian guideline of 500 parts per million, and to implement its own guideline of 1,500 parts per million.
In the United States, substances that are health-based have Maximum Contaminant Levels (MCLs), and are enforceable by law. However, TDS, and other substances that are considered aesthetic, are given Secondary Maximum Contaminant Levels (SMCLs), but are not enforced, because they do not pose as great a health risk as the primary contaminants do. The United States guideline for TDS is also 500 parts per million.
How Can Water Treatment Facilities Remove TDS?
Water treatment facilities can use reverse osmosis to remove the dissolved solids in the water that are responsible for elevated TDS levels. Reverse osmosis removes virtually all dissolved substances, including many harmful minerals, such as salt and lead. It also removes healthy minerals, such as calcium and magnesium, and ideally such water should be filtered through a magnesium and calcium mineral bed to add the minerals to the water. The mineral bed also increases the pH and decreases the corrosive potential of the water. For more information about reverse osmosis, see the Ultrafiltration, Nanofiltration and Reverse Osmosis fact sheet.