1.11 - Potable Water

What drinking water is, and where it comes from

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 (such as dissolved salts).
• Tap water carries dissolved ions (such as calcium, iron, and copper) that can interfere with chemical reactions, so experiments use distilled water (or deionised water).

Where water comes from

• Surface water - Collects in lakes, rivers, and reservoirs. Easy to reach, but exposed to the open air so it picks up solids and microbes and needs more treatment.
• Ground water - Trapped in permeable underground rock (aquifers). The rock acts as a natural filter, so it needs the least treatment.
• Waste water - Water contaminated by human activity that must undergo multiple treatment stages before reuse.
• Sea water - Contains very high concentrations of dissolved salts that cannot be removed by filtering, making it the most energy-intensive to treat.

Filtering, aerating and chlorinating water

Filtration and membrane filtration

• Filtration removes suspended solids by passing water through progressively finer filters.
• Membrane filtration forces water through a membrane with very small pores, trapping microscopic particles that pass through standard filter beds.

Aeration

Air is mixed into the water to increase dissolved oxygen:

• It drives out other dissolved gases
• It reacts with certain dissolved ions to form insoluble solid oxides that can be filtered out

Using bacteria

• For water with high levels of organic matter, air is pumped in to support aerobic bacteria.
• The bacteria feed on and break down the organic matter.

Chlorination

• Chlorine gas is bubbled through the water to kill harmful bacteria and other microbes.
• Although chlorine is toxic and can react with organic compounds to form trace by-products, the benefit of preventing waterborne diseases far outweighs the risk.

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.