3.1 - Potable Water

Drinking water, what it carries, and its supply

Potable water (water safe to drink) must meet three conditions:

1. The level of dissolved substances is low enough to do no harm
2. The pH is between 6.5 and 8.5
3. It carries no harmful microbes

Safe to drink is not the same as pure

• Pure water contains only H₂O molecules.
• Potable water is not pure because it contains low levels of dissolved substances.
• Tap water carries dissolved ions that can interfere with chemical reactions, so experiments use deionised water (or distilled water).

What natural water carries

• Dissolved gases - Carbon dioxide and oxygen absorbed from the air. Dissolved carbon dioxide makes water slightly acidic.
• Dissolved ions - Minerals absorbed as water passes through rocks.
• Microorganisms - Naturally occurring bacteria and viruses.
• Pollutants - Agricultural runoff (fertilisers and pesticides) and industrial or domestic waste.

Managing water resources

• Fresh water is used extensively in agriculture, industry, and homes, not just for drinking.
• Supplies are limited because rainfall is unevenly distributed and demand increases with population growth.
• Extracting, treating, and piping water uses large amounts of energy, so sustainable supply relies on reducing water consumption and recycling waste water.

Treating water so it is safe to drink

• Untreated fresh water contains suspended solids and disease-causing microbes.
• It is first passed through a wire mesh to screen out large debris such as twigs and leaves.

Sedimentation

• The water is held in a large sedimentation tank where denser solid particles sink to the bottom under gravity.
• A coagulant chemical (such as aluminium sulfate) is added to make tiny particles clump together into larger lumps that settle out.

Filtration

• Clearer water from the top of the sedimentation tank is passed through beds of sand and gravel.
• These trap the remaining fine suspended solids.

Chlorination

• Chlorine gas is bubbled through the filtered water to kill harmful bacteria and other microbes.
• This step sterilises the water, making it safe to drink.

What these stages do not remove

Dissolved substances remain in the water after sedimentation, filtration, and chlorination.

Desalination of sea water

Desalination removes dissolved salts from sea water to produce potable water.

Distillation

1. Sea water is heated until it boils.
2. Water evaporates into steam, leaving dissolved salts behind.
3. The steam is cooled and condensed back into liquid water.

Reverse osmosis

• Sea water is forced under high pressure through a semi-permeable membrane.
• The membrane pores allow water molecules through but block dissolved ions and larger molecules.

Limitations of desalination

• Both distillation and reverse osmosis require large amounts of energy (to heat the water or generate high pressure).
• This makes desalination expensive and causes high carbon emissions if fossil fuels provide the energy.