Common Water Quality Indicators in Pure Water Treatment

In our pure/ultrapure water treatment process, many parameters often appear, most of which are water quality indicators. So what do these indicators mean? Today, we'll summarize them.

What is Total Organic Carbon (TOC)?

The content of organic matter in water, expressed as the amount of carbon—the main element in organic matter—is called total organic carbon. TOC measurement is similar to TOD measurement. At a high temperature of 950℃, the organic matter in the water sample is vaporized and burned to produce CO2. The amount of CO2 produced is measured using an infrared analyzer, thus determining the total organic carbon content. During the measurement process, inorganic carbon compounds in the water, such as carbonates and bicarbonates, will also generate CO2, which should be measured separately and deducted. If the water sample is filtered through a 0.2μm microporous membrane, the carbon content measured is dissolved organic carbon (DOC). TOC and DOC are commonly used water quality indicators.

What is water resistivity?

When measuring the electrical conductivity of water, it is related to the water's resistance. A high resistance indicates poor conductivity, while a low resistance indicates good conductivity. According to Ohm's Law, at a constant water temperature, the resistance R of water is inversely proportional to the cross-sectional area F of the electrodes and directly proportional to the distance L between the electrodes, as shown in the following formula: R = ρ, where ρ is resistivity, or specific resistance. The unit of resistance is the ohm (Ω), or microohm (μΩ), where 1 Ω equals 10⁶ μΩ; the SI unit of resistivity is the ohmmeter (Ω•m). If the cross-sectional area F of the electrodes is 1 cm², the distance L between the electrodes is 1 cm, and the unit of resistivity is Ω•cm, then the resistance value is equal to the resistivity value. The resistivity of water is related to the amount of salt in the water, the ion content, the charge of the ions, and the velocity of the ions. Therefore, pure water has a high resistivity, and ultrapure water has an even higher resistivity. The purer the water, the greater its resistivity.

What are total solids, dissolved solids, and suspended solids in water?

All impurities in water, excluding dissolved gases, are called solids. Solids in water can be further divided into dissolved solids and suspended solids. The sum of these two is called total solids (TSS). Dissolved solids refer to the various inorganic salts and organic matter that remain dissolved in water after filtration. Suspended solids refer to insoluble substances such as mud, sand, clay, organic matter, and microorganisms that can be filtered out. TSS is determined by evaporating the water to dryness and then weighing it. Therefore, the temperature at which the water is evaporated is crucial, generally controlled between 105 and 110℃.

What are the effects of the main anions and cations in water on water quality?

The main anions in water include Cl-, SO42-, HCO3-, CO32-, and OH-. Among them, HCO3-, CO32-, and OH- often combine with cations such as K+, Na+, Mg2+, and Ca2+ to form hardness and alkalinity. Changes in their amounts affect the pH value of the water, indicating whether the water is corrosive or scale-forming. Therefore, these are the main ions affecting the properties of water. Cl- is the most common anion in water and a catalyst for water corrosion. It strongly promotes and accelerates the electron exchange reaction on metal surfaces, especially for stainless steel materials in water systems. Stress concentration points (such as thermal stress and vibration stress) will cause Cl- to accumulate, accelerating the electrochemical corrosion process. SO42- is also a relatively common corrosive anion in water, increasing the conductivity of water. At the same time, it can react with cations such as Ca2+ to form CaSO4 precipitate, resulting in scaling. It is not a nutrient source for sulfate-reducing bacteria in water. The main cations in water include K+, Na+, Ca2+, Mg2+, Fe3+, and Mn2+, among which Na+ is the most common cation in water. The presence of Na+ and K+ increases the conductivity of water and increases its instability. Ca2+ and Mg2+ are the main ions that make up the hardness of water. Under certain conditions, they often form scale on the surface of heated equipment, affecting heat transfer. Fe3+ and Mn2+ readily form Fe(OH)3 and Mn(OH)2 precipitates, which form scale and cause under-scale corrosion. They also act as promoters for the growth of iron bacteria.