Potable Water
Water We Can Drink
Access to clean drinking water is one of the most important applications of chemistry in everyday life. Potable water is not the same as pure water. Pure water contains only water molecules, but potable water contains small amounts of dissolved minerals and trace substances — these are acceptable at low levels and some are even beneficial. What matters is that the levels of dissolved substances and microorganisms are low enough to be safe.
In the UK, most potable water comes from surface water sources — rivers and reservoirs — and from groundwater stored in underground rock layers called aquifers. Rainfall replenishes these sources, so the UK’s temperate climate means supply is generally adequate, though regional variations mean some areas face greater pressure on water resources than others. Where fresh water sources are available, the treatment process described below is well established. In other parts of the world, particularly in arid or semi-arid regions, or on small islands without large freshwater reserves, different and more energy-intensive approaches are needed.
Treating fresh water to make it potable involves three main stages: filtration (removing particles and debris), sedimentation (allowing fine particles to settle), and sterilisation (killing microorganisms). Potable water is safe to drink — not chemically pure.
The Three Stages of Fresh Water Treatment
Filtration passes water through layers of gravel and sand. These filter beds trap large particles, suspended solids and some microorganisms. The water passes through progressively finer material, emerging much clearer. However, filtration alone does not remove all bacteria or dissolved contaminants — further treatment is needed.
Sedimentation allows fine particles that are too small to be caught by filtration to settle out under gravity in large settlement tanks. In some treatment plants, a coagulant chemical (such as aluminium sulfate) is added to cause tiny particles to clump together into larger flocs that settle more easily. The settled sludge is removed, and the cleaner water above it passes to the sterilisation stage.
Sterilisation kills bacteria and other harmful microorganisms that have survived filtration. The most common method in the UK is adding small, carefully controlled amounts of chlorine gas or a chlorine compound to the water. Ozone or ultraviolet (UV) light can also be used. These methods kill bacteria without making the water unsafe to drink at the concentrations used.
Desalination — Water from the Sea
In regions where fresh water is scarce, desalination can convert sea water into potable water. The two main methods are:
- Distillation: Sea water is boiled; the steam is collected and condensed to give pure water, leaving the dissolved salts behind. This requires a large amount of energy to heat the water.
- Reverse osmosis: Sea water is forced under high pressure through a semi-permeable membrane that allows water molecules through but blocks dissolved ions. This also requires significant energy to maintain the pressure.
Both desalination methods — distillation and reverse osmosis — produce potable water from sea water, but both require large amounts of energy. This makes desalination expensive and potentially high in carbon emissions unless powered by renewable energy. The UK relies on fresh water sources rather than desalination for most of its supply.
Analysing Water Samples (Practical Activity)
This course does not use ‘required practicals’. Instead you cover eight practical activity groups. One of them is identifying unknown samples, and a practical for it is to analyse water samples and compare their purity. Students may test for the presence of dissolved ions using techniques such as flame tests (for metal ions) or precipitation reactions (e.g. adding silver nitrate to test for chloride ions). Comparing samples from different sources — such as tap water, bottled water and distilled water — illustrates how the composition of ‘potable’ water varies even within safe limits.
When planning such an investigation, the key variables are: the independent variable (which water sample is being tested), the dependent variable (the result of the chemical test), and the control variables (volume of sample used, concentration of reagents, temperature). Safety considerations include handling silver nitrate and chlorine-based solutions, which can irritate skin and eyes.
Waste Water Treatment
Potable water treatment is only one side of the equation. Water that has been used in homes, hospitals and industrial processes must also be treated before it can be safely returned to the environment. If untreated sewage or industrial waste water entered rivers and lakes directly, it would deplete oxygen levels, harm aquatic life and spread disease.
Waste water treatment typically involves several stages: screening (removing large solid objects), sedimentation (allowing solids to settle as sludge), biological treatment (using microorganisms to break down organic material in the water), and disinfection (to kill remaining pathogens before discharge). The sludge produced by sedimentation can sometimes be used as fertiliser after further treatment, illustrating how waste management and resource recovery can be combined.
Untreated waste water contains dissolved organic material, pathogens and sometimes toxic chemicals. Releasing it into natural water bodies depletes dissolved oxygen (harming aquatic life), spreads disease, and can contaminate drinking water sources. Treatment makes the water safe enough for environmental discharge.