The confusion between Total Dissolved Solids (TDS) and Parts Per Million (PPM) is common when testing water quality. Consumers often wonder if these two terms refer to the same measurement on a handheld meter or lab report. This misunderstanding stems from the fact that TDS is a property of the water, while PPM is the standard unit of concentration used to measure it. Clarifying this relationship is necessary for accurately assessing water purity and filtration effectiveness.
Understanding Total Dissolved Solids (TDS)
Total Dissolved Solids (TDS) is a measure of the combined concentration of all inorganic and organic substances dissolved in a liquid. This collective measure encompasses any substance that can pass through a filter with pores typically sized around two micrometers or less. TDS is not a unit of measurement but rather a physical property or characteristic of the water itself.
The composition of TDS is primarily made up of inorganic salts, including positively charged ions (cations) like calcium, magnesium, sodium, and potassium. It also includes negatively charged ions (anions) such as chlorides, sulfates, bicarbonates, and nitrates. These dissolved substances originate from natural sources like the weathering of rocks and soils, as well as human activities such as agricultural runoff and industrial wastewater discharge.
A high TDS level can lead to hard water, which causes scaling and buildup in pipes, fixtures, and appliances like kettles and water heaters. Conversely, water with very low TDS, such as distilled water, may be considered aggressive because it lacks minerals and can potentially leach contaminants from the pipes it flows through.
Parts Per Million as the Standard Unit
Parts Per Million (PPM) is the standard unit of concentration used to express the quantity of Total Dissolved Solids. PPM is a ratio denoting how many parts of a specific substance are present per one million parts of the total solution. In water testing, this small measurement is used for trace concentrations.
For practical purposes in water quality testing, one PPM is considered equivalent to one milligram of dissolved solids per liter of water (1 mg/L). Due to this close equivalence, the terms TDS and PPM are often used interchangeably by manufacturers and consumers. When a digital meter displays a single number, it is providing the concentration of the TDS, expressed in PPM.
Standard handheld TDS meters do not directly weigh the solids but instead measure the water’s electrical conductivity. Dissolved inorganic salts and metals conduct an electrical current, so the meter uses this measurement to estimate the total concentration of ions present. The meter then applies a conversion factor, typically ranging from 0.5 to 0.7, to translate the conductivity reading into the final estimated TDS value displayed in PPM.
Interpreting Water Quality Results
The numerical value, expressed in PPM, provides practical insights into the water’s aesthetic qualities and the efficiency of filtration systems. Water that registers below 50 PPM is typically very pure, such as water purified by reverse osmosis or distillation. However, some may find its taste flat due to the lack of minerals.
The optimal range for drinking water is between 50 and 150 PPM, offering a pleasant taste while containing trace amounts of natural minerals. Readings between 150 and 300 PPM are safe but may impart a noticeable taste that affects beverages. The US Environmental Protection Agency (EPA) established a secondary maximum limit of 500 PPM, primarily because levels above this can cause undesirable aesthetic effects such as a salty or metallic taste, staining, and excessive scaling.
High TDS readings serve as an important indicator that harmful contaminants could be present. For example, if tap water measures 400 PPM and water after a new reverse osmosis system measures 40 PPM, this indicates the system is removing 90% of the dissolved solids. Regular monitoring of the PPM value is an effective way to assess the performance and lifespan of a water filter or purification system.