Water Treatment & Purification
Water is essential to all known life, yet natural water sources — rivers, lakes, groundwater, and reservoirs — are rarely pure. They contain dissolved mineral salts, suspended solids, microorganisms, and sometimes toxic contaminants. Water treatment is the set of physical, chemical, and biological processes used to remove these impurities and make water safe for drinking, industrial use, or discharge back into the environment. Understanding these processes requires knowledge of chemistry at the molecular level, including solubility, oxidation–reduction reactions, and colligative properties. Pure water (H₂O) has precisely defined physical properties: a melting point of 0 °C and a boiling point of 100 °C at standard atmospheric pressure (101.325 kPa). These values shift measurably when solutes are dissolved — a phenomenon called colligative properties (freezing-point depression and boiling-point elevation). Chemists and water engineers exploit these shifts to test the purity of a water sample: the closer the measured melting and boiling points are to the theoretical values, the purer the water. Natural water contains both beneficial substances — such as calcium and magnesium ions that support bone health, and fluoride ions at low concentrations that protect teeth — and harmful contaminants such as pathogenic bacteria, heavy-metal ions (lead Pb²⁺, arsenic As³⁺/As⁵⁺), nitrate ions (NO₃⁻) from agricultural runoff, and organic pollutants. The three principal treatment methods studied at this level are filtration, chlorination, and distillation, each targeting different categories of impurity.
- Sedimentation and Coagulation
- Filtration
- Chlorination
- Distillation
- Testing Water Purity by Melting and Boiling Points
- Beneficial and Harmful Dissolved Substances
Sedimentation and Coagulation
Before filtration, large suspended particles are allowed to settle by gravity (sedimentation). Fine colloidal particles — too small to settle — are aggregated by adding aluminium sulfate (alum, Al₂(SO₄)₃) or iron(III) sulfate (Fe₂(SO₄)₃). These salts hydrolyse in water to form gelatinous metal hydroxide flocs: Al³⁺(aq) + 3H₂O(l) → Al(OH)₃(s) + 3H⁺(aq). The positively charged floc particles attract negatively charged colloids, causing them to clump together and settle — a process called coagulation and flocculation.
Filtration
Filtration passes water through layers of sand and gravel (slow or rapid sand filters) to remove suspended particles, floc, and many microorganisms. Sand filters operate by both mechanical straining — particles larger than the pore size are physically blocked — and biological action in slow filters, where a biofilm of beneficial microorganisms (the 'Schmutzdecke') breaks down organic matter. Activated carbon (charcoal) filters are also used to adsorb dissolved organic compounds, chlorine by-products, and odour-causing chemicals through van der Waals forces at the vast internal surface area (up to 3000 m² g⁻¹).
Chlorination
Chlorination is the primary method of disinfecting water — killing bacteria, viruses, and protozoa. Chlorine gas (Cl₂) or sodium hypochlorite (NaOCl) is added to water. Cl₂ dissolves to form a mixture of hydrochloric acid and hypochlorous acid: Cl₂(g) + H₂O(l) → HCl(aq) + HOCl(aq). Hypochlorous acid (HOCl) is the active disinfecting agent; being uncharged, it diffuses through microbial cell membranes and oxidises key enzymes, disrupting cellular respiration. A residual chlorine concentration of 0.2–0.5 mg L⁻¹ is maintained to protect water throughout the distribution system. Over-chlorination is avoided because Cl₂ reacts with naturally occurring organic compounds to form trihalomethanes (THMs) such as chloroform (CHCl₃), which are potentially carcinogenic.
Distillation
Distillation produces the highest-purity water by boiling and condensing the vapour. Because water's boiling point (100 °C at 1 atm) is much lower than that of dissolved salts (e.g., NaCl decomposes above 800 °C), virtually all ionic solutes are left behind in the boiling flask. However, dissolved gases and volatile organic compounds (VOCs) with boiling points near 100 °C may co-distil. Double distillation, or distillation combined with ion-exchange chromatography, yields ultra-pure deionised water used in analytical chemistry and pharmaceuticals. Distillation is energy-intensive and not cost-effective for large-scale municipal supply but is vital industrially.
Testing Water Purity by Melting and Boiling Points
A solute dissolved in water lowers the freezing point below 0 °C and raises the boiling point above 100 °C. These are colligative properties — they depend on the number of solute particles (moles), not their identity. Freezing-point depression: ΔTf = Kf × m, where Kf for water = 1.86 °C kg mol⁻¹ and m is molality (mol of solute per kg of solvent). Boiling-point elevation: ΔTb = Kb × m, where Kb for water = 0.512 °C kg mol⁻¹. A 1 mol kg⁻¹ NaCl solution (which fully dissociates into Na⁺ + Cl⁻, giving i = 2) would depress the freezing point by approximately 2 × 1.86 = 3.72 °C. The closer a water sample's melting and boiling points are to exactly 0 °C and 100 °C respectively, the lower its total dissolved solids (TDS) and the purer it is.
Beneficial and Harmful Dissolved Substances
Natural water contains dissolved ions from rock weathering. Beneficial substances include: Ca²⁺ and Mg²⁺ (essential minerals; hard water reduces cardiovascular disease risk); F⁻ at 0.7–1.0 mg L⁻¹ (fluoride strengthens tooth enamel by converting hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ to fluorapatite Ca₁₀(PO₄)₆F₂, which is less soluble in acid); and HCO₃⁻ (bicarbonate acts as a pH buffer). Harmful substances include: NO₃⁻ above 50 mg L⁻¹ (causes methaemoglobinaemia — 'blue baby syndrome' — in infants); Pb²⁺ from old pipes (neurotoxin); As(III)/As(V) from geological sources (carcinogen); SO₄²⁻ at high concentrations (laxative effect); and pathogenic microorganisms including Escherichia coli, Vibrio cholerae, and Cryptosporidium.
The Key Equation
- \ce{Cl2} — Chlorine gas (mg L⁻¹): Diatomic halogen added as a disinfectant; oxidation state 0
- \ce{HCl} — Hydrochloric acid (—): Fully dissociates in water to H⁺ and Cl⁻; lowers pH slightly
- \ce{HOCl} — Hypochlorous acid (mg L⁻¹): Weak acid; active disinfecting agent; Cl in oxidation state +1; diffuses through cell membranes
Worked Example
Problem. A water sample is analysed and found to boil at 100.31 °C at 1 atm pressure. The boiling-point elevation constant for water is Kb = 0.512 °C kg mol⁻¹. Assuming the dissolved solute does not dissociate (van't Hoff factor i = 1), calculate the molality of dissolved solute in the sample. Then, determine the mass of NaCl (molar mass = 58.44 g mol⁻¹) that would need to be dissolved in 500 g of water to produce an equivalent boiling-point elevation. (NaCl fully dissociates: i = 2.)
Answer. The molality of the unknown solute is 0.605 mol kg⁻¹. To produce the same boiling-point elevation using NaCl (which dissociates), 8.85 g of NaCl must be dissolved in 500 g of water.
Origins · 1 minJohn Snow and how this was discovered
John Snow was a physician and anaesthetist who is widely regarded as the father of epidemiology and a pioneer of water treatment science. During the 1854 Broad Street cholera outbreak in London, Snow systematically mapped cases and demonstrated that cholera spread through contaminated water from a pump handle on Broad Street — not through 'miasma' (bad air) as previously believed. By persuading local authorities to remove the pump handle, he halted the epidemic. His work directly motivated the introduction of water filtration and later chlorination in municipal water supplies. Snow marked each cholera death on a hand-drawn street map of Soho, London — an early and groundbreaking example of data visualisation in science. His ghost pump (with a removed handle) on Broad Street (now Broadwick Street) still stands today as a historical monument.
Related Concepts
- Properties of Water — Understanding water treatment requires knowledge of water's unique physical properties — high polarity, hydrogen bonding, surface tension, and specific heat capacity — which determine how solutes dissolve and how separation methods work.
- Hard and Soft Water — Hard water contains dissolved Ca²⁺ and Mg²⁺ ions from limestone — directly related to the beneficial dissolved substances in natural water and the concept of total dissolved solids assessed during water treatment.
- Colligative Properties of Solutions — Freezing-point depression and boiling-point elevation — the quantitative tools used to test water purity — are colligative properties that depend on the concentration of dissolved solute particles.
- Chemistry of Chlorine and the Halogens — Chlorination relies on the oxidising power of Cl₂ and the formation of HOCl; this is directly explained by the reactivity, electronegativity, and oxidation states of the halogens group.
- Separation Techniques in Chemistry — Filtration and distillation are both core separation techniques. Understanding the principles of each — particle size exclusion vs. differences in boiling point — connects water treatment to broader analytical chemistry.