Water Purification and Drinking Water Treatment
Water purification is the set of processes that remove solids, dissolved chemicals and disease-causing microbes from raw water so that it is safe to drink. A typical municipal treatment plant uses screening, coagulation, sedimentation, filtration and disinfection in that order. In brief: the water is first cleared of large debris and fine particles, then filtered through sand, and finally disinfected, usually with chlorine, so that it can be safely delivered to taps.
Safe drinking water is one of the most important public health achievements. Many of the techniques in water treatment are the separation methods introduced earlier in chemistry: filtration, sedimentation, distillation and adsorption. This article follows water through a treatment plant, explains each step, compares home purification methods, works through a disinfection dose example and gives the history of water treatment.
Where does drinking water come from?
Drinking water comes from surface water (rivers, lakes and reservoirs) or from groundwater taken from wells. Groundwater is often cleaner because soil and rock filter it, but it can contain dissolved minerals or contaminants such as arsenic or nitrates. Surface water usually contains more suspended particles and microbes, and so it needs more treatment. In coastal regions without fresh water, seawater can be desalinated by distillation or by reverse osmosis.
What are the steps in a treatment plant?
- Screening. Bars and screens remove leaves, branches and fish.
- Coagulation and flocculation. Chemicals such as aluminum sulfate (alum) are added. They neutralize the charge on tiny suspended particles, which then clump together into larger flakes called flocs during gentle stirring.
- Sedimentation. The water flows slowly through a tank, and the heavy flocs settle to the bottom as sludge.
- Filtration. The water passes through layers of sand, gravel and often activated carbon. These trap remaining fine particles, and the carbon adsorbs organic chemicals, tastes and odors.
- Disinfection. Chlorine, chloramine, ozone or ultraviolet light kills bacteria, viruses and protozoa. A small amount of chlorine is left in the water to protect it as it travels through pipes.
- pH adjustment and fluoridation (in some places). The pH is adjusted to reduce corrosion of pipes, and fluoride is added in some areas to protect teeth.
Treatment stages and what they remove
| Stage | Method | What it removes |
|---|---|---|
| Screening | Physical barrier | Large debris |
| Coagulation | Chemical addition | Makes fine particles stick together |
| Sedimentation | Settling | Flocs and heavy particles |
| Filtration | Sand, gravel, carbon | Fine particles, some microbes, organic chemicals |
| Disinfection | Chlorine, ozone, UV | Bacteria, viruses, protozoa |
| Reverse osmosis (special cases) | Membrane under pressure | Dissolved salts and many chemicals |
How can water be purified at home?
| Method | Kills or removes microbes? | Removes dissolved chemicals? | Note |
|---|---|---|---|
| Boiling (rolling boil for at least 1 minute) | Yes | No | Very reliable; use longer at high altitude |
| Chlorine or iodine tablets | Most | No | Contact time is needed |
| Ceramic or membrane filter | Bacteria and protozoa, sometimes viruses | Limited | Check the pore size |
| Activated carbon filter | Not reliably | Some organic chemicals, chlorine taste | Replace cartridges on time |
| Distillation | Yes | Most | Slow and uses energy |
| UV light device | Yes, in clear water | No | Water must not be cloudy |
Worked example: dosing chlorine
A small treatment plant must treat 50,000 m3 of water per day. The chlorine demand of the water (the amount used up by reacting with impurities) is 1.2 mg/L, and the plant wants a residual of 0.5 mg/L at the outlet.
- Dose required. 1.2 + 0.5 = 1.7 mg/L.
- Volume in liters. 50,000 m3 × 1,000 L/m3 = 50,000,000 L.
- Mass of chlorine per day. 1.7 mg/L × 50,000,000 L = 85,000,000 mg = 85,000 g = 85 kg.
- Per hour. 85 kg ÷ 24 = 3.54 kg per hour.
Careful dosing is essential. Too little leaves microbes alive, and too much gives an unpleasant taste and may form unwanted by-products when chlorine reacts with natural organic matter.
How is drinking water quality checked?
Water suppliers test samples regularly for microbes, usually by looking for indicator bacteria such as Escherichia coli, and for chemical contaminants. National and international guidelines, such as those from the World Health Organization, set maximum values. Because tests cannot cover every possible contaminant, the best approach combines protection of the source, good treatment and safe distribution through intact pipes.
How did water treatment develop?
Ancient civilizations boiled, filtered and let water settle, and Sanskrit and Greek texts describe such methods. In the 19th century, John Snow’s work on cholera and the germ theory of Louis Pasteur and Robert Koch showed the need for clean water. Continuous chlorination of public water began in the early 20th century, for instance in Jersey City, New Jersey, in 1908, and the large fall in typhoid and cholera deaths in cities that adopted filtration and chlorination is regarded as one of the greatest public health successes.
Which mistakes are common?
Mistake 1: Assuming that clear water is safe to drink
Disease-causing microbes and dissolved chemicals are invisible.
Mistake 2: Believing that filtering alone makes any water safe
Many simple filters do not remove viruses or dissolved chemicals. Disinfection is also needed.
Mistake 3: Thinking that boiling removes all pollutants
Boiling kills microbes but does not remove metals, nitrates or most chemicals, and may concentrate them slightly as water evaporates.
Mistake 4: Not changing filter cartridges
An old cartridge can stop working, and bacteria can grow in it.