Water purification removes various impurities from water sources through advanced processes. This multi-stage treatment transforms raw water into a highly refined product, distinguishing it from standard tap water or basic filtered water. This article details the comprehensive journey of water purification, from initial preparation to the final quality checks.
What is Purified Water?
Purified water undergoes extensive treatment to remove impurities, chemicals, and contaminants to a high standard. It removes bacteria, algae, fungi, parasites, metals (like copper and lead), and chemical pollutants. The goal is to eliminate almost all dissolved solids and organic compounds. Purified water differs from tap water, which receives municipal treatment for safety but retains many dissolved minerals. Filtered water removes some impurities (like sediment or chlorine), but not the same high level of contaminants as purified water.
Distilled water is a type of purified water, produced by boiling water into steam and condensing it back into liquid, leaving impurities behind. While distillation is effective, modern purification often combines methods to achieve similar or higher purity. Purified water finds extensive use in laboratories, medical facilities, various industrial processes, and as a component in some bottled drinking water.
Pre-Treatment: Preparing the Water
Before advanced purification, raw water undergoes pre-treatment to remove larger particles and initial contaminants. This phase protects sensitive purification technologies that follow. These steps often include coagulation, flocculation, sedimentation, initial filtration, and basic disinfection.
Coagulation adds chemicals (e.g., alum or ferric chloride) to the water. These coagulants carry a positive charge that neutralizes the negative charges on suspended particles like clay, silt, and organic matter. This causes tiny, dispersed particles to clump into larger aggregates.
Following coagulation, flocculation gently mixes water, encouraging these small clumps, known as microflocs, to collide and grow into larger, visible particles called flocs. This process creates aggregates substantial enough for subsequent removal. After flocculation, water enters a sedimentation basin where heavier flocs settle by gravity, removing significant suspended solids and turbidity.
Clearer water then moves to initial filtration, passing through sand and gravel layers. These filters trap remaining suspended solids and unsettled flocs. Finally, basic disinfection (e.g., pre-chlorination) may reduce the initial microbial load. This minimizes microorganism growth in the system.
Core Purification Technologies
Following pre-treatment, water undergoes core purification processes to achieve high purity by removing dissolved substances and microorganisms. These advanced technologies are central to producing purified water. Each method targets different types of contaminants, and often, multiple technologies are used in combination.
Reverse Osmosis (RO)
Reverse Osmosis (RO) employs a semi-permeable membrane. Pressure forces water molecules through the membrane while leaving behind dissolved salts, ions, organic molecules, and larger particles like bacteria and viruses. The membrane acts as a selective barrier, allowing pure water to pass through and rejecting up to 99% of dissolved solids and many other contaminants. This process effectively removes a broad range of impurities, including chemicals, heavy metals, and some pharmaceuticals.
Deionization (DI)
Deionization (DI) uses ion-exchange resins to remove dissolved mineral salts (ions) from water. Water passes through two types of resin beds: cation exchange resins that replace positively charged ions (cations) with hydrogen ions, and anion exchange resins that replace negatively charged ions (anions) with hydroxyl ions. The hydrogen and hydroxyl ions then combine to form pure water. Deionization effectively removes mineral impurities, yielding water comparable in purity to distilled water, but it generally does not remove uncharged organic molecules, bacteria, or viruses.
Distillation
Distillation involves heating water to its boiling point, converting it into steam, and then cooling the steam to condense it back into liquid water in a separate container. Impurities with higher boiling points, such as inorganic compounds, heavy metals, and non-volatile organic molecules, are left behind in the boiling chamber. The boiling process also kills microorganisms like bacteria and some viruses. While distillation produces very pure water, it is an energy-intensive process and some volatile organic compounds with lower boiling points than water may vaporize and re-contaminate the distilled product if not properly vented.
Post-Treatment and Quality Assurance
After the core purification stages, additional treatments ensure water meets specific purity standards and remains contaminant-free. These steps further refine water quality, followed by rigorous testing to verify compliance.
Final Disinfection (UV Light)
Final disinfection often involves Ultraviolet (UV) light, which inactivates remaining microorganisms (e.g., bacteria, viruses, parasites). UV light works by damaging the genetic material of these pathogens, preventing them from reproducing without adding chemicals to the water. This method preserves water’s taste and avoids disinfection byproducts.
Activated Carbon Filtration
Activated carbon filtration removes residual organic compounds, chlorine, and improves taste and odor. Activated carbon has a porous structure that adsorbs contaminants through physical and chemical interactions. It is especially effective at removing chlorine and chloramines that might have been used in earlier disinfection steps, protecting downstream components like RO membranes from oxidation damage.
Remineralization
For purified water intended for drinking, an optional remineralization step may be included. This process adds back beneficial minerals like calcium, magnesium, and potassium, typically removed during rigorous purification (e.g., RO, distillation). Remineralization can improve the taste of the water, which can otherwise be described as flat, and contributes to better hydration and balanced pH.
Quality Testing and Monitoring
Continuous quality testing and monitoring are integral throughout the purification process, especially after final treatment. Parameters such as electrical conductivity (to measure dissolved solids), total organic carbon (TOC) for organic compounds, and microbial tests are regularly performed. This rigorous testing ensures the purified water consistently meets established purity standards, such as those set by organizations like ASTM, USP, or local regulatory bodies.