Making Water Safe to Drink

Introduction

Water from rivers, reservoirs and wells may look clean, but it can contain mud, leaves, dissolved chemicals and — most dangerously — bacteria and viruses that cause diseases such as cholera and typhoid. Before it reaches our taps, water is treated in several stages, each designed to remove a particular type of impurity. The goal is potable water: water that is safe to drink. Potable water is not chemically pure; it still contains small amounts of dissolved minerals, which are harmless and give it taste.

Core explanation

1. Screening. Water from a river or reservoir first passes through metal screens or grids. These remove large floating objects such as branches, leaves, plastic litter and fish.

2. Sedimentation (settling), often helped by coagulation. The water is held in large, still tanks. Heavy particles such as sand and grit sink to the bottom. Very fine particles of clay stay suspended, so a coagulant — often aluminium sulfate or an iron(III) salt — is added. It makes the tiny particles stick together into larger clumps (flocs), which settle more quickly. The sludge at the bottom is removed.

3. Filtration. The water then flows slowly through deep beds of sand and gravel. The narrow gaps between sand grains trap the remaining small particles. Some filters also contain activated carbon, which adsorbs substances that cause unpleasant tastes and smells. Filtration removes insoluble particles but not dissolved substances such as salt.

4. Chlorination (sterilisation). Filtration cannot be relied on to remove every microorganism, so a small, carefully controlled amount of chlorine is added. Chlorine kills bacteria and other microbes. Enough is added that a little remains in the water as it travels through the pipes, protecting it from contamination on the way. Some plants use ozone or ultraviolet light as well as, or instead of, chlorine at this stage.

5. Final adjustments. The pH may be adjusted so that the water does not corrode pipes. In some regions fluoride is added in tiny amounts to help reduce tooth decay.

Stage What it removes or does --- --- Screening large floating objects Sedimentation (with coagulant) sand, grit, fine clay particles Filtration smaller insoluble particles; tastes and smells with carbon Chlorination kills microorganisms

Step-by-step reasoning

To match a stage to an impurity:

1. Is the impurity large and floating? Screening. 2. Is it heavy, insoluble particles? Sedimentation. 3. Is it small insoluble particles? Filtration. 4. Is it living — bacteria or viruses? Chlorination. 5. Is it dissolved, such as salt? None of these stages removes it; that needs distillation or reverse osmosis.

Visual explanation

Draw the treatment works as a line of connected boxes from left to right: river → screen (a grid) → settling tank (particles drifting to the floor) → sand filter (layers of sand over gravel) → chlorine dosing (a small green cylinder symbol) → storage reservoir → homes. Label under each box the type of impurity it removes.

Real-world analogy

Water treatment is like preparing vegetables from a garden. First you pull off the big leaves and stalks (screening), then let the soil wash off in a bowl of water (sedimentation), rinse them in a sieve (filtration) and finally cook them to kill any germs (chlorination).

Real-world example

Before chlorination became widespread in the early twentieth century, waterborne diseases such as cholera and typhoid killed huge numbers of people in cities. Adding chlorine to public water supplies is regarded as one of the greatest public-health advances in history, and it still saves millions of lives worldwide.

Why?

Why is chlorine added after filtration rather than before? Filtration removes particles that could shelter microbes from the chlorine and organic matter that would react with and use up the chlorine. Adding it last means a smaller dose can do the job, and some chlorine remains to protect the water in the pipes.

Common misconception

"Tap water is pure water." Tap water is potable — safe to drink — but it contains dissolved minerals and traces of chlorine. Its boiling point is very slightly above 100 °C, and evaporating it leaves a small residue.

Worked example

Question: A reservoir sample contains leaves, fine clay, dissolved calcium ions and bacteria. Which treatment stage deals with each, and which impurity is left in the tap water?

Reasoning: Leaves are removed by screening; fine clay by coagulation and sedimentation, then filtration; bacteria are killed by chlorination. Dissolved calcium ions pass through all these stages.

Answer: Screening — leaves; sedimentation and filtration — clay; chlorination — bacteria. Dissolved calcium ions remain.

Quick check

1. Which stage of water treatment kills bacteria? Answer: Chlorination (sterilisation).

Exam focus

Learn the stages in order and state what each removes. Examiners often ask why filtration alone does not make water safe (it does not reliably remove microbes or any dissolved substances) and why chlorine is added (to kill microorganisms). Use the word potable and explain that it does not mean pure.

Advanced insight

Chlorine reacts with water to form hypochlorous acid, HOCl, which is the main disinfecting species. It can also react with natural organic matter to form small amounts of by-products, so treatment works carefully balance enough chlorine to kill pathogens against keeping by-products low. This is one reason why some plants use ozone or ultraviolet light as extra disinfection steps.

Summary

Drinking water is made safe by screening out large objects, allowing particles to settle (helped by coagulants), filtering through sand and gravel, and chlorinating to kill microorganisms. Small adjustments to pH, and sometimes fluoride, follow. The product is potable water — safe to drink but not pure, because it still contains dissolved minerals.

Practice questions

1. List the four main stages of water treatment in the correct order. Answer: Screening, sedimentation, filtration, chlorination. 2. Why is a coagulant such as aluminium sulfate added during sedimentation? Answer: It makes tiny suspended particles clump together into larger flocs that settle out more quickly. 3. Explain the difference between potable water and pure water. Answer: Potable water is safe to drink but contains dissolved substances; pure water contains only H₂O molecules. 4. Why does some chlorine remain in the water as it leaves the treatment works? Answer: The remaining chlorine continues to kill microbes that might enter the water in the pipes before it reaches homes.