Total Dissolved Solids (TDS) calculation quantifies the combined weight of all organic and inorganic substances dissolved in a given volume of water. This measurement is typically expressed in milligrams per liter ($\text{mg/L}$) or parts per million ($\text{ppm}$). Knowing the TDS level offers a fundamental assessment of water quality, indicating the level of mineralization and the potential for aesthetic issues like taste or scaling. TDS determination is accomplished through two primary methods: a rapid estimation technique and a precise laboratory measurement.
What Total Dissolved Solids Represents
Total Dissolved Solids is a collective measure of substances small enough to pass through a filter with pores around two micrometers. These dissolved materials primarily consist of inorganic salts, including mineral cations (calcium, magnesium, sodium, and potassium) and anions (chlorides, sulfates, and bicarbonates). Small quantities of organic matter from natural sources or human activity can also contribute to the final TDS value.
Monitoring TDS is important for water use and quality. High TDS levels affect taste, often making water salty or metallic, and can lead to excessive scaling in pipes, water heaters, and household appliances. Conversely, very low TDS water can be corrosive to plumbing and may have an undesirable flat taste. Monitoring TDS provides a simple, aggregate indicator for managing both the aesthetic quality and the utility of a water supply.
Using Electrical Conductivity to Estimate TDS
The most common and practical way to estimate TDS is by measuring the water’s electrical conductivity (EC). Dissolved inorganic solids exist as ions, which are charged particles that allow water to conduct an electrical current. The more ions present in the water, the higher its conductivity, establishing a direct correlation between EC and TDS. This relationship allows a handheld TDS meter, which is actually an EC meter, to provide a rapid, non-gravimetric estimate.
The meter first measures the electrical conductivity in units like microsiemens per centimeter ($\mu\text{S/cm}$). It then converts this value to an estimated TDS reading in $\text{mg/L}$ using a predetermined conversion factor. This factor, often called the K-factor, is an approximation, with $0.65$ being a widely accepted average for natural waters. The calculation is expressed as $\text{TDS} (\text{mg/L}) = \text{EC} (\mu\text{S/cm}) \times \text{Conversion Factor}$. For accurate field results, the meter must be properly calibrated against a standard solution, and the reading is automatically temperature-compensated because electrical conductivity increases as water temperature rises. This method offers a fast, on-site approximation, though its accuracy is limited because the conversion factor varies based on the specific mixture of ions present in the sample.
The Gravimetric Method: Laboratory Standard
The gravimetric method represents the precise, scientifically accepted technique for determining the true TDS concentration of a water sample. This calculation is based on the physical mass of the residue left after the water itself has been removed. The process begins by filtering a carefully measured volume of water through a standard glass-fiber filter to separate dissolved solids from any suspended particles.
The resulting liquid, known as the filtrate, is then transferred to a pre-weighed dish and placed into a laboratory oven. The water is evaporated completely at a high temperature, typically $180^\circ\text{C}$, to ensure that only the non-volatile dissolved solids remain. Once the sample has been dried to a constant weight, the dish is cooled in a desiccator and reweighed using a precision analytical balance. The final TDS is calculated by subtracting the initial dish weight from the weight of the dish plus the dried residue, then dividing this mass of solids by the initial volume of the water sample.
Interpreting and Managing TDS Levels
Interpreting a TDS reading requires understanding the context of the water’s intended use, though drinking water guidelines provide a solid reference point. The United States Environmental Protection Agency (EPA) suggests a secondary standard of $500\text{ mg/L}$ for TDS in drinking water. This non-enforceable guideline is established primarily for aesthetic concerns like taste and appearance. Readings significantly above this level often indicate an elevated presence of minerals that can lead to taste complaints and cause scaling or deposits in household plumbing and appliances.
To manage high TDS levels, several treatment methods can be employed, depending on the water source and specific dissolved components. Reverse Osmosis (RO) is a highly effective method that forces water through a semi-permeable membrane, rejecting up to 99% of dissolved inorganic material. Other techniques include distillation, where water is boiled and condensed, leaving the solids behind, and deionization, which uses ion-exchange resins to replace undesirable ions. These processes reduce high TDS concentrations, making the water more palatable and protecting water-using equipment from mineral buildup.