Potable water — AQA GCSE Combined Science

Potable water is water that is safe to drink: it has sufficiently low levels of dissolved salts and microbes. It is not the same as pure water, which contains no dissolved substances at all. The method chosen to make water potable is not fixed; it is selected to suit the water available and the local situation. If the local supply is fresh water with low levels of dissolved substances, such as UK rain collected in the ground, lakes and rivers, then relatively simple treatment is enough. If the only supply is seawater, which contains a high concentration of dissolved salts, then desalination is needed, for example by distillation or by processes using membranes. Local conditions such as energy costs, distance from the sea, rainfall and the scale of demand all influence the choice.

In the UK, rainfall provides fresh water because rain contains low levels of dissolved substances. This water collects in the ground, including aquifers, and in lakes and rivers. Most potable water in the UK is produced from these fresh water sources by a sequence of steps. First, water is passed through filter beds to remove insoluble particles such as grit, soil and small pieces of debris. Second, the water is sterilised to kill microbes, for example by chlorination, by exposure to ultraviolet light or by ozonation. The result is potable water that is safe to drink. The exact sequence and the sterilising agent chosen depend on the source and local conditions, but the principles of removing insoluble material and then killing microbes are common to most UK supplies.

Fresh water is water with low levels of dissolved salts, and it is obtained from sources such as rivers, lakes, reservoirs and groundwater held in aquifers. Choosing a source depends on local availability, the volume of water needed, the distance to the treatment works and the level of contamination. In the UK, rainfall collected in reservoirs and abstracted river water are common choices because they need relatively little treatment. In dry coastal regions, seawater is not fresh water, so desalination by distillation or reverse osmosis may be needed, which uses large amounts of energy. Groundwater from aquifers is often cleaner because soil and rock filter it, but it can still contain dissolved minerals and microbes. A sensible choice balances water quality, reliability of supply, cost and environmental impact, and the chosen source must be treated to make it potable.

After a suitable fresh water source has been chosen, the water is treated to make it potable. One key stage is passing the water through filter beds, which are layers of sand and gravel, sometimes with other materials, that trap solid particles suspended in the water. As water percolates through the beds, insoluble grit, soil, plant fragments and some microorganisms are removed, producing clearer water. Filter beds do not remove dissolved salts or all microbes, so the filtered water is then sterilised, for example by chlorination, ozonation or ultraviolet light, to kill remaining microorganisms. In the laboratory, filtration through filter paper separates an insoluble solid from a liquid, and the same principle applies on a larger scale in water treatment. Understanding filter beds helps explain why the order of treatment matters and why filtration alone cannot make water potable.

Sterilising is the final stage of producing potable water: killing or inactivating microorganisms so the water is safe to drink. Water is first screened to remove large debris, then filtered through beds of sand and gravel to remove smaller suspended particles, and finally sterilised. Sterilising does not remove dissolved substances, so it is not a purification step in the chemical sense. In the UK, chlorine is commonly added; ozone or ultraviolet light are alternatives. For example, chlorine added at around 0.5 mg per dm³ of water kills bacteria such as those causing cholera and typhoid. The method chosen must leave no harmful residue and must reach all of the water, so the dose and contact time matter.

Potable water must be free of disease-causing microorganisms, so treatment works use a sterilising agent. Chlorine gas or a chlorine compound is added to water and kills bacteria by disrupting their cell processes; it is widely used in the UK because it is effective, relatively cheap and leaves a residual that protects water as it travels through pipes. Ozone, O₃, is a strong oxidising agent that destroys microorganisms and leaves no chemical residual, but it must be generated on site and does not protect the water afterwards. Ultraviolet light damages the genetic material of microorganisms so they cannot reproduce; it adds no chemicals but only works where the water is clear enough for the light to penetrate. All three agents kill or inactivate microorganisms; none removes dissolved substances.

Fresh water is scarce in some regions, so salty water or sea water must be treated to make it potable. Desalination removes dissolved salts. Distillation heats the salty water until it boils, condenses the steam and collects the distillate, leaving salts behind. Reverse osmosis forces salty water at high pressure through a partially permeable membrane that retains dissolved ions while water molecules pass through. Both methods demand large amounts of energy: distillation mainly for heating and cooling, reverse osmosis mainly for pumping water at high pressure. Energy use makes desalination expensive and can release carbon dioxide if the electricity comes from burning fossil fuels. For example, a coastal town with little rainfall may rely on a reverse osmosis plant, whereas a small-scale distillation unit can be used where a reliable heat source exists.

Potable water is water that is safe to drink: it has acceptable levels of dissolved substances and microbes, but it is not necessarily pure in the chemical sense. Pure water, in chemistry, contains only H₂O molecules and nothing else, so it has a fixed boiling point of 100 °C at standard pressure and no dissolved ions. Potable water may contain dissolved salts, such as sodium chloride, and traces of other minerals, so its boiling point can differ slightly from 100 °C and it conducts electricity weakly. For example, distilled water is pure, while tap water is potable but contains chloride ions, calcium ions and dissolved gases. To distinguish them, test boiling point or test for dissolved ions; a sample that boils exactly at 100 °C and leaves no residue on evaporation is pure, whereas a sample that leaves a residue or boils over a range is potable but impure.

Ground water is usually treated by screening to remove large objects, then filtration to remove insoluble particles, followed by sterilisation using chlorine, ozone or ultraviolet light to kill microorganisms. Salty water, such as sea water, needs desalination because dissolved sodium chloride cannot be removed by filtration or sterilisation. Desalination is carried out by distillation or by reverse osmosis, both of which require significant energy. Distillation involves heating the salty water so that pure water evaporates and is then condensed, leaving dissolved salts behind. Reverse osmosis forces water through a membrane that retains dissolved ions. In both cases the product is potable water, but the processes differ because ground water mainly needs removal of microbes and insoluble solids, while salty water needs removal of dissolved salts.

Potable water is water that is safe to drink. It is not pure water in the chemical sense because it contains dissolved substances, but the levels of microbes and contaminants are low enough not to harm health. The steps used depend on the source. For most UK sources, water is passed through filter beds of sand and gravel to remove insoluble particles such as grit and plant material. Chlorine is then added to kill microorganisms. If the water is very salty, for example sea water, simple filtration and chlorination are not enough because dissolved salts remain. Desalination by distillation or reverse osmosis removes dissolved salts, but it needs large amounts of energy, so it is expensive. Students should link each step to the impurity it removes and explain why that step is needed for the water to be safe.

In this required practical, you analyse water samples from different sources, such as tap water, rain water, sea water or pond water. You measure pH using universal indicator or a pH meter to compare acidity or alkalinity. You test for dissolved solids by evaporating a measured volume of each sample to dryness and comparing the mass of residue left. You then purify a sample by distillation: heat the water so it boils, condense the steam and collect the distillate. The distillate should have a pH closer to 7 and leave little or no residue on evaporation, showing that dissolved solids have been removed. You should record observations clearly, compare samples and explain how the results show the effectiveness of distillation.