Water conductivity measures the water’s ability to transmit an electrical current. This ability is directly proportional to the concentration of Total Dissolved Solids (TDS), which are primarily charged ions like salts and minerals. High concentrations of dissolved ions result in high conductivity, typically measured in microSiemens per centimeter (\(\mu\)S/cm). Reducing conductivity requires actively removing these dissolved ions to achieve the desired purity.
Removing Ions Through Reverse Osmosis
Reverse Osmosis (RO) is a physical separation process using a semi-permeable membrane to remove dissolved ions and contaminants. The membrane acts as a molecular filter, allowing water molecules to pass through while rejecting substances with a larger molecular weight. External pressure is applied to overcome natural osmotic pressure, forcing water through the membrane against the concentration gradient.
RO systems include pre-filters to protect the membrane from sediment and chlorine, ensuring system longevity. Water passes through the membrane, and the purified product (permeate) is collected, while the concentrated waste stream is sent to a drain. Modern membranes achieve salt rejection rates ranging from 95% to over 99% for most dissolved salts and inorganic compounds. Rejection effectiveness depends on factors like feedwater temperature, pressure, and the specific composition of dissolved solids.
The Deionization Process
Deionization (DI) is a chemical purification method using specialized ion exchange resins to remove nearly all remaining charged ions. This process is often employed after Reverse Osmosis to create ultra-pure water, frequently referred to as RO/DI water. DI systems utilize two types of synthetic resins—cation resins and anion resins—which are typically contained in separate vessels or mixed together.
Cation resins exchange positive ions (like \(\text{Na}^+\), \(\text{Ca}^{2+}\), and \(\text{Mg}^{2+}\)) for hydrogen ions (\(\text{H}^+\)). Anion resins then exchange negative ions (such as \(\text{Cl}^-\) and \(\text{SO}_4^{2-}\)) for hydroxyl ions (\(\text{OH}^-\)). The resulting hydrogen and hydroxyl ions combine to form pure water (\(\text{H}_2\text{O}\)). DI can produce water with extremely low conductivity, sometimes reaching 18.3 megaohms per centimeter of electrical resistance. However, the resins have a finite capacity and eventually become saturated, requiring replacement or chemical regeneration using strong acids and bases.
Reducing Conductivity Using Distillation
Distillation is a thermal process that mimics the natural hydrologic cycle to produce low-conductivity water. The process involves heating water to its boiling point, converting it into steam. This phase change separates water molecules from non-volatile contaminants, such as dissolved solids and salts, which are left behind in the boiling chamber.
The pure steam is directed into a separate section where it is cooled and condensed back into liquid. This collected liquid is highly purified water, often achieving over 99.9% removal of dissolved materials. While effective at removing ions, distillation is an energy-intensive and slow process, making it less practical than RO or DI for producing large volumes. Distilled water typically exhibits very low conductivity, often in the range of 1 to 5 \(\mu\)S/cm.
Measuring and Applying Low Conductivity Water
Monitoring water purity requires a conductivity meter, which measures the water’s ability to carry an electrical current in microSiemens per centimeter (\(\mu\)S/cm). Many modern devices are called TDS meters, which estimate Total Dissolved Solids content by measuring conductivity and applying a conversion factor (typically 0.5 to 0.7). Since the meter directly measures conductivity, the TDS reading is an estimation relying on the assumption that dissolved solids are primarily ionic.
Low conductivity water is a prerequisite for many specialized applications.
Applications of Low Conductivity Water
- Laboratories and pharmaceutical manufacturing require ultra-pure water for media preparation and testing, often demanding conductivity levels below 1 \(\mu\)S/cm.
- Maintaining low conductivity is crucial for specialized aquariums, such as reef tanks.
- Low conductivity water is used for mixing coolants in high-performance engines to prevent corrosion and scale formation.
- In hydroponics, pure water is used as a base to which specific nutrients are added, and the final conductivity must be carefully monitored.