2.1 Theoretical Concept

TDS in water represent the inorganic salts and small amounts of organic matter present in water. The inorganic salts are in the form of cations (usually calcium, magnesium, sodium and potassium) and anions (usually carbonate, hydrogen-carbonate, chloride, sulfate and nitrate). TDS in the water supply originate from natural sources, sewage, urban and agricultural run-off and industrial wastewater. The TDS is measured with the unit milligrams per liter (mg/L), also known as parts per million (ppm).

Higher TDS levels in water may affect its taste. When the TDS value is less than 300 mg/L, the water is considered good for drinking and when the TDS value is 300 to 600 mg/L, the water is considered acceptable for drinking. Certain components of TDS, such as chlorides, sulfates, magnesium, calcium and carbonates cause corrosion and/or scaling (layers of salts) in water distribution systems. High TDS levels (>500 mg/L) result in excessive scaling in water pipes, water heaters, boilers and household appliances, such as kettles and steam irons. Such scaling may shorten the service life of these appliances unless necessary actions are taken to limit the TDS levels in water. To ensure acceptable TDS levels and water quality, water purifiers and softeners are available. In addition, a water TDS meter may be used to determine TDS levels.

The presence of cations and anions in water makes it conductive. Therefore, TDS measurement is done by measuring conductivity in water. The conductivity of water depends upon concentration of ions present and temperature of the water. An increase in temperature increases the mobility of ions, which means TDS levels increase slightly as water temperature increases. Conductivity measurements are converted into TDS values by means of a factor, which is based on the type of salts present in water. The calibration of TDS meters is usually done using salts such as KCl (Potassium Chloride), NaCl (Sodium Chloride) or 442 (a combination of mixed salts with deionized water).

KCl - This potassium chloride solution is a stable salt and is an international calibration standard for conductivity measurements. It has a conversion factor of ~0.55. This factor may vary from 0.5048 to 0.6521.

NaCl - This sodium chloride solution best represents sea water, brackish water, or water with a high saline solution. It has a conversion factor of ~0.5.This factor may vary from 0.4755 to 0.6048.

442 - This refers to the combination of salts mixed with deionized water to comprise this standard: 40% sodium sulfate, 40% sodium bicarbonate, 20% sodium chloride. This solution best represents natural fresh water. It has a default conversion factor of ~0.7. This factor may vary from 0.6563 to 0.9961.

The conductivity equation with temperature compensation is as follows:

Where σ(T) is the solution conductivity at any temperature T; σ(T0) is the solution conductivity at a reference temperature T0, usually 25°C; and α is the temperature compensation gradient in %/°С.

The temperature compensation gradient for most naturally occurring samples of water is approximately 2%/°C; however, it may range between 1% and 3%/°C.