Producing and Testing Potable Water

Safe drinking water is not necessarily pure

Potable water is water that is safe to drink. It must not contain harmful levels of dissolved substances or harmful microorganisms. However, it can contain small amounts of dissolved substances, such as mineral salts.

Chemically pure water contains only water molecules. This makes purity different from safety: drinking water does not need to be chemically pure to be potable. A clear glass of water may contain dissolved salts that cannot be seen, and its appearance alone cannot show whether it is safe.

Producing potable water from fresh water

The best treatment method depends on the available water supplies and local conditions. In the UK, rainfall supplies fresh water with low levels of dissolved substances. This water collects in rivers, lakes and the ground.

Most UK potable water is produced in three main stages:

1. Choose an appropriate source of fresh water. Starting with water containing relatively few contaminants reduces the treatment needed.
2. Pass the water through filters. These remove suspended, insoluble particles, such as soil. Dissolved salts normally pass through ordinary filters with the water.
3. Sterilise the water. Chlorine, ozone or ultraviolet light can be used to kill harmful microorganisms.

Filtration and sterilisation therefore have different jobs. Removing visible dirt does not necessarily remove microorganisms, while killing microorganisms does not remove dissolved salts.

Obtaining water where fresh water is scarce

Where rainfall and groundwater supplies are limited, salty water or sea water may provide an alternative supply. Removing its dissolved salts is called desalination. Two methods are distillation and reverse osmosis.

In reverse osmosis, pressure forces water through a membrane that allows water to pass while retaining dissolved salts. Energy is needed to generate this pressure.

Distillation instead separates water from dissolved salts using changes of state. It needs energy to heat the water. Both methods can increase drinking-water availability, but their energy requirements add to their cost.

How simple distillation separates water

Suppose salt water is heated. Water changes from liquid to vapour, but the dissolved salt remains in the heated container. The water vapour travels into a cooler part of the apparatus, loses energy and condenses back into liquid water. This collected liquid is the distillate.

The sequence is:

Heat the water → form water vapour → cool the vapour → collect liquid water.

Evaporating salt water without collecting and condensing the vapour would leave the salt behind, but would not recover the water. Distillation does both: it leaves the dissolved solid behind and collects the separated water. Desalinating sea water and purifying a school laboratory salt solution are examples of this process.

Comparing ground, waste and salt water

Groundwater may need relatively little treatment if it contains few contaminants. However, some groundwater contains harmful dissolved substances, so the source must be assessed. Boiling can kill harmful microorganisms, but it does not remove dissolved minerals.

Waste water can contain organic matter, harmful microorganisms and harmful chemicals. Producing potable water from it requires several treatment stages to remove these contaminants. Reusing it can increase water availability, particularly where fresh water is scarce, but ordinary sewage treatment for environmental release does not automatically make it safe to drink.

Salt water contains substantial amounts of dissolved salts, which ordinary filtration and sterilisation cannot remove. Desalination is therefore necessary and requires energy. Waste-water treatment is more complex than treating a suitable fresh-water source, but can use less energy than desalination.

There is no single best source everywhere. A suitable fresh-water source is usually easier to treat, whereas desalination or waste-water reuse can be valuable where that supply is insufficient. The decision depends on the contaminants present, available supplies and the resources needed for treatment.

Required practical: testing pH and dissolved solids

Required practical activity 11 involves analysing water samples from different sources and purifying water by distillation. The tests investigate different properties: pH indicates how acidic or alkaline a sample is, while evaporation reveals the solid material dissolved in it.

Measure each sample’s pH using universal indicator paper and compare its colour with the supplied pH chart. Use a separate portion for this test so that indicator does not contaminate the portion used to measure dissolved solids.

To investigate dissolved solids, use a measuring cylinder, balance, evaporating basin, tripod, gauze, Bunsen burner and heatproof mat:

1. Weigh a clean, dry evaporating basin and record its mass to two decimal places in grams.
2. Measure 10 cm³ of a water sample and pour it into the basin.
3. Heat gently on the tripod and gauze. As most of the water evaporates, solid residue begins to appear.
4. Avoid strong heating as the sample becomes nearly dry. Allow any remaining water to evaporate gently, then let the basin cool before weighing it again.
5. Subtract the empty basin’s mass from the mass of the basin and dry residue.

$$\text{mass of dissolved solids} = \text{mass of basin and dry residue} - \text{mass of empty basin}$$

Record the sample identity, pH and mass of solids in 10 cm³, with the mass in grams. If comparing residues from different sources, repeat using equal volumes and clean apparatus. A greater residue mass then indicates more dissolved solid per equal volume.

Wear eye protection, work on a heatproof mat and avoid splashes from boiling water. Handle hot apparatus with suitable tongs and allow it to cool before weighing. Remaining water would add to the measured mass, so the residue must be dry for a valid comparison.

These measurements give useful information about water quality, but do not test for every harmful chemical or microorganism. They cannot, by themselves, establish that water is potable.

Required practical: collecting water by distillation

A simple school setup uses a conical flask containing 10 cm³ of a water sample. A bung and delivery tube carry vapour from the flask into an open boiling tube held by a clamp in an ice-and-water bath. The flask is heated over a Bunsen burner on a tripod and gauze, with a heatproof mat underneath.

Heat the sample until it boils, then reduce the heat so that it boils gently. Vapour travels along the delivery tube and condenses in the cooled receiver. Drops of liquid collecting in the boiling tube show that distillation is working. Collect about 1 cm depth of water, then stop heating rather than boiling the flask dry.

Wear eye protection, secure the receiving tube and keep the apparatus open to the atmosphere: heating a completely sealed system could cause a dangerous pressure build-up. Let hot glassware cool before dismantling it. Do not drink laboratory samples or the collected distillate.

Analyse the distillate by determining its boiling point. Heat a separate portion carefully and use a thermometer with its bulb in the liquid, not touching the container. Record the temperature while it boils steadily. Pure water boils at about 100°C at normal atmospheric pressure; a boiling point close to this supports the conclusion that water has been collected, although it is not a complete drinking-water safety test.

Potable water

• Potable: safe to drink; may contain dissolved substances.
• Chemically pure: contains only water molecules.

Treatment routes

• UK fresh water: choose source → filter → sterilise.
• Filtration removes suspended solids; chlorine, ozone or UV kills harmful microorganisms.
• Groundwater may need little treatment, but harmful dissolved substances may be present.
• Waste water needs several stages to remove contaminants; treatment for environmental release does not automatically make it potable.
• Salt water needs desalination, using distillation or . Both require energy.