10.1 - 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.

Treating the domestic water supply

Sedimentation and filtration

• Water sits in a tank so heavy suspended solids sink to the bottom under gravity (sedimentation).
• The water is then passed through beds of sand and gravel to trap smaller solid particles (filtration).

Carbon treatment

• The water passes through a carbon filter.
• The activated carbon adsorbs dissolved substances that cause unpleasant tastes and odours.

Chlorination

Chlorine is added to kill bacteria and other disease-causing microbes.