Total Dissolved Solids (TDS) in water refers to the combined content of inorganic salts and small amounts of organic matter present in a dissolved state. Understanding TDS levels is a common way to assess general water quality, as it can influence various characteristics of water.
Understanding Total Dissolved Solids
TDS consists of inorganic salts and some organic materials, including minerals like calcium, magnesium, potassium, and sodium, and anions such as bicarbonates, chlorides, and sulfates. These substances enter water from both natural and human-made sources. Naturally, water dissolves minerals as it flows through geological formations, rocks, and soils. Human activities, such as agricultural and urban runoff, industrial wastewater discharges, and road de-icing salts, also contribute to TDS levels.
TDS is typically measured in parts per million (ppm) or milligrams per liter (mg/L), which are equivalent units. The most common method for measuring TDS involves using a digital TDS meter, which assesses the electrical conductivity of the water. Since dissolved ionized solids conduct electricity, conductivity measurements can be converted to an approximate TDS value. More precise measurements can be obtained through gravimetric analysis, which involves evaporating the water and weighing the remaining solids.
What Constitutes High TDS
Defining “high” TDS can be relative, as there is no universally enforced legal standard for TDS in drinking water. However, guidelines exist to provide a reference. The U.S. Environmental Protection Agency (EPA) has established a Secondary Drinking Water Standard of 500 mg/L for TDS, which is a guideline based on aesthetic qualities rather than direct health impacts. Levels above this guideline may affect taste, odor, or appearance.
Typical TDS ranges vary significantly depending on the water source. Distilled water has very low TDS, often near zero, as purification removes nearly all dissolved solids. Tap water in the United States often ranges around 350 ppm, while bottled mineral water can naturally contain higher TDS levels due to its mineral content. Water with TDS exceeding 1000 mg/L is generally considered unfit for human consumption due to taste issues and potential harmful substances. Water classification ranges from fresh water (less than 1,000 ppm TDS) to hypersaline water (exceeding 35,000 ppm).
Impacts of Elevated TDS Levels
One of the most common impacts is on the taste and odor of water. High concentrations of minerals can impart a bitter, salty, or metallic taste, making the water less palatable. The overall “mouthfeel” of the water can also be affected, making it seem heavier.
High TDS can also cause issues with household appliances and plumbing. When water contains elevated levels of minerals like calcium and magnesium, it can lead to scale buildup in pipes, water heaters, and other appliances. This mineral accumulation can reduce the efficiency and lifespan of these systems. While less common, certain dissolved solids can also contribute to corrosion in plumbing.
Regarding health, high TDS itself is generally not considered a direct health hazard. However, an elevated TDS reading can indicate the presence of other potentially harmful substances, such as heavy metals, nitrates, or sulfates, requiring further testing. Water with very high TDS might also be less effective for rehydration or specific medical applications like dialysis. While some beneficial minerals contribute to TDS, excessively high levels may interfere with their absorption or lead to digestive issues.
Methods for Reducing High TDS
Reverse osmosis (RO) is a widely used and highly effective process. RO systems force water through a semi-permeable membrane, allowing water molecules to pass while blocking larger dissolved solids, including minerals, salts, and other contaminants. RO systems can remove a significant percentage of TDS, often between 95-99%.
Distillation is another method that works by mimicking the natural water cycle. Water is boiled, turning into steam, and leaving behind impurities and dissolved solids. The steam is then collected and condensed back into liquid water, resulting in highly purified water. While effective, distillation can be a slower process and may require more energy compared to other methods.
Deionization (DI) uses ion-exchange resins to remove ionized impurities from water. In this process, water passes through resin beds that exchange positively charged ions (cations) for hydrogen ions and negatively charged ions (anions) for hydroxide ions, which then combine to form pure water. DI systems are capable of producing ultra-pure water with near-zero TDS, making them common in laboratory and industrial settings. Common carbon filters or sediment filters are not designed to remove dissolved solids and are therefore not effective for reducing TDS. These filters primarily target suspended particles, chlorine, and organic compounds that affect taste and odor.