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What Are Total Dissolved Solids?


  • "Dissolved solids" refer to any minerals, salts, metals, cations or anions dissolved in water. This includes anything present in water other than the pure water (H20) molecule and suspended solids. (Suspended solids are any particles/substances that are neither dissolved nor settled in the water, such as wood pulp.)


  • In general, the total dissolved solids concentration is the sum of the cations (positively charged) and anions (negatively charged) ions in the water.


  • Parts per Million (ppm) is the weight-to-weight ratio of any ion to water.


  • A TDS meter is based on the electrical conductivity (EC) of water. Pure H20 has virtually zero conductivity. Conductivity is usually about 100 times the total cations or anions expressed as equivalents. TDS is calculated by converting the EC by a factor of 0.5 to 1.0 times the EC, depending upon the levels. Typically, the higher the level of EC, the higher the conversion factor to determine the TDS. NOTE - While a TDS meter is based on conductivity, TDS and conductivity are not the same thing. For more information on this topic, please see our FAQ page.


Where Do Dissolved Solids Come From?


  • Some dissolved solids come from organic sources such as leaves, silt, plankton, and industrial waste and sewage. Other sources come from runoff from urban areas, road salts used on street during the winter, and fertilizers and pesticides used on lawns and farms.


  • Dissolved solids also come from inorganic materials such as rocks and air that may contain calcium bicarbonate, nitrogen, iron phosphorous, sulfur, and other minerals. Many of these materials form salts, which are compounds that contain both a metal and a nonmetal. Salts usually dissolve in water forming ions. Ions are particles that have a positive or negative charge.


  • Water may also pick up metals such as lead or copper as they travel through pipes used to distribute water to consumers.


  • Note that the efficacy of water purifications systems in removing total dissolved solids will be reduced over time, so it is highly recommended to monitor the quality of a filter or membrane and replace them when required.


Why Should You Measure the TDS Level in Your Water?

The EPA Secondary Regulations advise a maximum contamination level (MCL) of 500mg/liter (500 parts per million (ppm)) for TDS. Numerous water supplies exceed this level. When TDS levels exceed 1000mg/L it is generally considered unfit for human consumption. A high level of TDS is an indicator of potential concerns, and warrants further investigation. Most often, high levels of TDS are caused by the presence of potassium, chlorides and sodium. These ions have little or no short-term effects, but toxic ions (lead arsenic, cadmium, nitrate and others) may also be dissolved in the water.

Even the best water purification systems on the market require monitoring for TDS to ensure the filters and/or membranes are effectively removing unwanted particles and bacteria from your water.


How can water with high TDS be undesirable or harmful?


  • It may taste bitter, salty, or metallic and may have unpleasant odors


  • High TDS water is less thirst quenching.


  • High TDS interferes with the taste of foods and beverages, and makes them less desirable to consume.


  • Some of the individual mineral salts that make up TDS pose a variety of health hazards. The most problematic are Nitrates, Sodium, Sulfates, Barium, Cadmium, Copper, and Fluoride.


  • If a person drinks 2 pints of water a day, this will total 4500 gallons of water passing through his body over a 70 year span. If the water is not totally pure, this 4500 gallons will include 200-300 pounds of rock that the body cannot utilize. Most will be eliminated through excretory channels. But some of this will stay in the body, causing stiffness in the joints, hardening of the arteries, kidney stones, gall stones and blockages of arteries, microscopic capillaries and other passages in which liquids flow through our entire body.


Why is it especially important for children to consume pure water?


  • A child’s immune system and detoxification system are still developing throughout early childhood and teen years. Exposure to even very low levels of toxic chemicals or lead in drinking water at a young age can lead to increased risks of degenerative diseases and learning disorders in later years. Since many of the crucial defense systems that help protect adults from disease and environmental pollutants are not fully developed in children, they are much more sensitive to contaminants. A child consumes 3 times as much water per pound of body weight than an adult does, so they get a much bigger dose of the contaminants in our water. Their developing bodies are simply much more sensitive.²


  • Currently, the health standards that determine how much and what levels of contaminants we are permitted to consume in our drinking water are all based on the potential effects on adults. 


What should the TDS level of my water be?


There is no specific level nor 'good or bad' answer to this question.  Generally speaking, for drinking water, a lower level of TDS (purer water) is preferred.  The U.S. EPA, all U.S. states, the World Health Organization (WHO) and most nations put maximum limitations on TDS allowed in drinking water.  These limitations are typically 500 or 1000 ppm, but they do vary.  There is no known minimum for drinking water.
Besides drinking water, a TDS level is specific for each application and particular usage.  Though humans generally prefer purer water for their health, fish and plants, for example, require water with widely varying TDS levels, most of which are higher than healthy human drinking water.  If you are using a meter to test the water pertaining to a particular device, object or operation, contact the manufacturer of that object. For example, if you are using the meter to test the efficacy of a water filtration system, contact the manufacturer of that system for preferred TDS levels. If you are testing the water for a pool, plants, fish, etc. contact a specialist for your specific application, or the manufacturer of additives or nutrients. 

How Do You Reduce or Remove the TDS in Your Water?


Common water filter and water purification systems:


Carbon filtration - Charcoal, a form of carbon with a high surface area, adsorbs (or sticks to) many compounds, including some toxic compounds. 


Reverse osmosis (R.O.) - Reverse osmosis works by forcing water under great pressure against a semi-permeable membrane that allows water molecules to pass through while excluding most contaminants.


Distillation - Distillation involves boiling the water to produce water vapor. The water vapor then rises to a cooled surface where it can condense back into a liquid and be collected.


Deionization (DI) - Water is passed between a positive electrode and a negative electrode. Ion selective membranes allow the positive ions to separate from the water toward the negative electrode and the negative ions toward the positive electrode. High purity de-ionized water results. The water is usually passed through a reverse osmosis unit first to remove nonionic organic contaminants.


Total Dissolved Solids (TDS) are the total amount of mobile charged ions, including minerals, salts or metals dissolved in a given volume of water, expressed in units of mg per unit volume of water (mg/L), also referred to as parts per million (ppm). TDS is directly related to the purity of water and the quality of water purification systems and affects everything that consumes, lives in, or uses water, whether organic or inorganic, whether for better or for worse.

*Chart values represent national U.S. averages.  Actual TDS levels for geographic regions within the U.S. and other countries may vary.

Click here for the U.S. EPA's list of National Secondary Drinking Water Regulations.