Water quality is often assessed using several metrics, with Total Dissolved Solids (TDS) being a primary indicator. This measurement quantifies the total concentration of inorganic salts and a small amount of organic matter physically dissolved in the water. Monitoring this metric is standard practice because elevated levels affect the aesthetic qualities of water, influencing public acceptance of a drinking source. Understanding TDS helps consumers evaluate the effectiveness of their water treatment processes.
Understanding Total Dissolved Solids
TDS consists primarily of inorganic salts and minerals that have been completely ionized in the water. The most common constituents include positively charged ions like calcium, magnesium, sodium, and potassium, and negatively charged ions such as chlorides, bicarbonates, and sulfates. Trace amounts of organic substances may also be counted within the TDS total.
These dissolved particles enter the water supply from both natural and human-made sources. Water naturally dissolves minerals from geological formations, such as rocks and soil, adding to the TDS count. Human activities also contribute through sources like agricultural and urban runoff, industrial wastewater discharge, and the use of de-icing salts on roads.
TDS concentration is measured in units of parts per million (ppm) or milligrams per liter (mg/L). Since dissolved solids allow water to conduct an electrical current, the most common measurement tool is a portable TDS meter or conductivity meter. This device approximates the total solid content by measuring the water’s electrical conductivity. A higher reading indicates a greater concentration of dissolved ions, but the measurement does not identify the specific composition of the dissolved solids present.
Establishing Safe TDS Levels for Consumption
The regulatory guidelines for TDS in potable water focus mainly on its impact on the water’s appearance and taste rather than direct health risks. The U.S. Environmental Protection Agency (EPA) established a guideline for TDS under its Secondary Maximum Contaminant Levels (SMCLs) program. These secondary standards are non-enforceable federal guidelines intended to assist public water systems in managing contaminants that affect the aesthetic quality of drinking water.
The EPA’s recommended SMCL for Total Dissolved Solids is 500 mg/L. This standard is set because levels above this threshold can cause the water to have an unpleasant salty or metallic flavor, or lead to staining and deposits on fixtures. SMCLs differ from Primary Maximum Contaminant Levels, which are legally enforceable limits set to protect public health by controlling toxic substances.
For general consumer preference, a TDS level below 500 mg/L is considered acceptable in terms of taste and appearance. Many water quality experts consider water with a concentration in the range of 300 to 500 mg/L to be ideal, as it offers a pleasant balance of purity and desirable mineral content. Water with concentrations significantly above 1,000 mg/L is regarded as unsuitable for human consumption due to objectionable taste and potential technical issues.
The Effects of High and Low TDS
Water with a high TDS concentration, particularly above 500 mg/L, often presents noticeable problems in a household setting. The presence of excess dissolved salts, such as chlorides and sulfates, can impart a distinct salty, metallic, or bitter taste to the water. High levels of minerals like calcium and magnesium contribute to water hardness, which accelerates the formation of scale buildup in pipes, kettles, and water-using appliances. This scale reduces the efficiency and lifespan of household equipment.
Conversely, water with a very low TDS level, generally below 50 mg/L, presents different challenges. This type of water can have a flat or bland taste due to the lack of dissolved minerals that contribute to flavor. Low-TDS water can be corrosive because it actively seeks to dissolve minerals from the materials it contacts. This corrosivity can cause the leaching of metals, such as lead and copper, from plumbing fixtures and pipes into the drinking water supply, which creates a health hazard.
Methods for Reducing TDS in Water
For consumers concerned about elevated TDS, several specific water treatment methods are highly effective at reducing the concentration of dissolved solids.
Reverse Osmosis (RO)
Reverse Osmosis (RO) is one of the most widely used technologies, which physically forces water through an ultra-fine, semipermeable membrane. This membrane acts as a molecular filter, allowing water molecules to pass while rejecting up to 99% of the larger dissolved solid particles and ions.
Distillation
Distillation works by boiling the water until it turns into steam and then condensing the purified vapor back into a liquid. Since the dissolved solids have boiling points much higher than water, they are left behind in the boiling chamber, resulting in water with an extremely low TDS count.
Deionization
Deionization is a chemical process that uses ion-exchange resins to remove ionic impurities. The resins replace these impurities with hydrogen and hydroxyl ions to create highly pure water.
Common water treatment solutions, such as standard carbon block filters or sediment filters, are generally ineffective for substantially reducing TDS. These filters are designed to remove larger suspended particles, chlorine, and organic contaminants. They do not have the mechanism to capture the very small, dissolved inorganic ions that make up the majority of the TDS concentration. Therefore, specialized technologies like RO or distillation are required to achieve a significant reduction in total dissolved solids.