Potable water is safe to drink, but it is not chemically pure water. Rivers, lakes, and groundwater must pass through several treatment stages — sedimentation, filtration, and chlorination — before meeting the standards required for homes in England and Wales.
What is potable water?
Potable water is water that is safe to drink. It is not pure water (which contains only H₂O molecules) — potable water is a solution containing dissolved minerals that are harmless or even beneficial (calcium and magnesium salts, fluoride in some areas) and is treated to remove harmful microorganisms and pollutants.
The distinction between pure and potable is important for GCSE:
- Pure water (as defined in chemistry) has a fixed boiling point of 100 °C and melts at 0 °C.
- Potable water contains dissolved substances and has a slightly elevated boiling point; it does not qualify as chemically pure.
In the UK, the vast majority of potable water comes from fresh water sources: rivers, lakes (reservoirs), and underground aquifers. A small amount is produced by desalination of sea water in water-scarce regions.
What are the stages of water treatment?
Water treatment works in England typically process water through three main stages before it enters the mains supply:
Stage 1: Sedimentation (settling)
Raw water from a river or reservoir is collected and passed into large settling tanks. Heavy suspended particles — soil, silt, and organic debris — sink to the bottom under gravity and are removed as sludge.
To assist this process, a coagulant (typically aluminium sulfate, Al₂(SO₄)₃) is added to the water. When dissolved, it forms positively charged aluminium ions that attract negatively charged colloidal particles (very fine clay and organic matter that would remain suspended for weeks if left alone). These particles clump together into larger flocs that are heavy enough to settle. The process is called flocculation.
Stage 2: Filtration
After settling, the water passes through sand and gravel filter beds, which remove fine particles that were too small to settle even after flocculation, including some microorganisms. Slow sand filters also contain a layer of bacteria at the top (the schmutzdecke or "dirty skin") that biologically degrades organic matter, improving water quality further.
Stage 3: Chlorination
Finally, a carefully controlled dose of chlorine gas (Cl₂) or sodium hypochlorite (NaOCl) is added to kill remaining bacteria, viruses, and other pathogens. This is called disinfection.
The concentration of chlorine used (typically 0.5 mg/L in the UK) is carefully chosen: high enough to kill microorganisms effectively but low enough that it poses no health risk to consumers and does not give the water an unpleasant taste or smell.
In some areas, the pH of the water is also adjusted using lime (calcium hydroxide) to reduce corrosion of pipes, and fluoride may be added at 1 mg/L to improve dental health.
| Stage | Process | What it removes |
|---|---|---|
| Sedimentation | Settling tanks + flocculation with Al₂(SO₄)₃ | Coarse and fine suspended particles; colloidal matter |
| Filtration | Sand and gravel beds | Fine particles; some microorganisms; organic matter |
| Chlorination | Chlorine gas or sodium hypochlorite | Bacteria, viruses, other pathogens |
| pH adjustment (if needed) | Lime addition | Acidity that would corrode pipes |
What is desalination and when is it used?
Desalination is the removal of dissolved salts from seawater or brackish water to produce potable water. It is energy-intensive and expensive, so it is used mainly in water-scarce regions (the Middle East, parts of Australia and Spain) rather than in the UK, where fresh water is generally plentiful.
Two main methods are used at GCSE:
1. Distillation
Seawater is heated until the water evaporates, leaving the dissolved salts behind. The steam is then condensed to produce pure water. This requires a large energy input (to heat the water to boiling point) and is therefore expensive to run.
2. Reverse osmosis
Water is forced through a semi-permeable membrane under very high pressure. The membrane allows water molecules to pass through but blocks dissolved salt ions. The pure water collects on the other side; the concentrated brine is discharged. Reverse osmosis is more energy-efficient than distillation but still requires significantly more energy than conventional fresh water treatment.
How is waste water treated before return to rivers?
Used water from homes and industry (sewage and trade effluent) must be treated before being released into rivers or the sea. Sewage treatment involves:
- Screening — large solid debris (rags, plastic) is filtered out on a wire screen.
- Sedimentation — organic solids settle as primary sludge; the liquid passes on.
- Biological treatment (aerobic) — bacteria in aeration tanks or trickling filters break down dissolved organic matter using oxygen.
- Sludge digestion (anaerobic) — primary sludge is digested by anaerobic bacteria in enclosed tanks, producing biogas (mainly methane, which can generate electricity) and digestate (used as fertiliser).
- Effluent discharge — the treated water, now meeting legal standards for BOD (biological oxygen demand) and suspended solids, is released to a river or the sea.
Frequently asked questions
Why is potable water not chemically pure?
Completely pure water (containing only H₂O molecules) is impractical and unnecessary for safe drinking. The dissolved minerals in potable water (calcium, magnesium, sodium, potassium salts; fluoride) are present at concentrations that are harmless or beneficial — calcium and magnesium contribute to dietary mineral intake, and fluoride at 1 mg/L has been shown to reduce tooth decay. Furthermore, the treatment processes (particularly chlorination) deliberately introduce small amounts of chemicals. The aim of water treatment is to make water safe, not to make it chemically pure.
Why is chlorine used to disinfect water even though it is a toxic gas?
Chlorine is effective at killing a broad spectrum of bacteria, viruses, and protozoa at very low concentrations. When dissolved in water, it forms hypochlorous acid (HOCl), which penetrates microbial cell walls and inactivates them chemically. The key is the dose: the concentration used in water treatment (~0.5 mg/L) is far too low to cause harm to humans — the lethal dose of chlorine to humans is in the thousands of mg/L range. Chlorine also provides a residual disinfectant effect: it remains dissolved in the water as it travels through pipes, killing any microorganisms that might enter the distribution system, whereas UV disinfection (an alternative) leaves no residual protection.
What is the difference between filtration and sedimentation in water treatment?
Sedimentation relies on gravity: particles denser than water sink to the bottom of a settling tank, separating from the water above. It removes coarser, heavier particles efficiently but cannot remove fine colloidal particles (which are too small and light to settle quickly — they may stay suspended for months). Flocculation is used to assist sedimentation by clumping fine particles together. Filtration uses a physical barrier — a sand and gravel bed — through which water percolates; particles too large to pass through the spaces in the filter medium are retained. Together, the two stages remove particles across a wide range of sizes.
How does reverse osmosis work at the particle level?
Osmosis is the movement of water molecules through a semi-permeable membrane from a region of lower solute concentration to a region of higher solute concentration. In seawater desalination, the natural osmotic pressure would drive pure water from the product side into the seawater side — the opposite of what is wanted. Reverse osmosis applies external pressure (typically 55–80 bar) to the seawater side, exceeding the osmotic pressure and forcing water molecules through the membrane against the concentration gradient. Salt ions (Na⁺, Cl⁻, SO₄²⁻, Mg²⁺) are too large to pass through the membrane's pores and are retained on the seawater side, which becomes increasingly concentrated brine.
For particle-model-first GCSE chemistry with Professor Curie — following a water molecule from reservoir to tap, one treatment stage at a time — visit aitutors.me.