Using resources: water, life-cycle assessment and sustainability — AQA GCSE Chemistry revision

Potable water production

• Potable water is water that is safe to drink; it is not pure in the chemical sense as it contains low levels of dissolved minerals and salts.
• Fresh water from rivers, lakes or groundwater usually needs only filtration to remove solids and sterilisation (e.g. with chlorine, ozone or ultraviolet light) to kill microbes.
• In areas with limited fresh water, desalination of seawater can be used, either by distillation or by reverse osmosis through a partially permeable membrane.
• Desalination requires large amounts of energy, making it expensive compared with treating existing fresh water sources.

Treating waste water

• Waste water from homes and industry must be treated before being released back into the environment to avoid pollution.
• Screening removes large solid objects, followed by sedimentation where suspended solids settle out as sludge.
• Aerobic biological treatment uses bacteria to break down organic matter in the remaining effluent.
• Anaerobic digestion of the sludge by bacteria breaks down organic material further and can produce biogas, which can be used as a fuel.

Key terms

Potable water

Water that is safe to drink, though not chemically pure.

Desalination

Removing dissolved salts from seawater to produce fresh water, e.g. by distillation or reverse osmosis.

Reverse osmosis

Forcing water through a partially permeable membrane to remove dissolved ions.

Sedimentation

Waste water treatment step where suspended solids settle out under gravity forming sludge.

Phytomining

Using plants to absorb metal compounds from soil, then burning them to obtain a metal-rich ash.

Bioleaching

Using bacteria to extract metal ions from ore, producing a leachate solution.

Common exam traps

• Potable water is not the same as pure water — pure water has nothing dissolved in it at all.
• Desalination uses a lot of energy, so it's more expensive than simply treating fresh water — don't say it's the cheap default option.
• Phytomining and bioleaching are slower than traditional mining but are useful for low-grade ores and reduce the need for traditional mining.

Practice questions with answers

1. 1. What does 'potable' mean when describing water?

   • • Chemically pure
   • • Safe to drink
   • • Free from all dissolved solids
   • • Distilled

   Answer: Safe to drink

   Potable water is safe to drink but still contains low levels of dissolved minerals and salts.

2. 2. Name two methods used to sterilise fresh water to make it potable.

   Answer: Chlorination and use of ozone or ultraviolet light.

   These methods kill microorganisms that could otherwise cause illness.

3. 3. Which process forces water through a partially permeable membrane to remove salts?

   • • Distillation
   • • Reverse osmosis
   • • Sedimentation
   • • Electrolysis

   Answer: Reverse osmosis

   Reverse osmosis pushes water through a membrane, leaving dissolved ions behind.

4. 4. Why is desalination more expensive than treating fresh water?

   Answer: It requires much more energy input to remove dissolved salts.

   Both distillation and reverse osmosis are energy-intensive compared with simple filtration and sterilisation.

5. 5. What is the first step in treating waste water?

   • • Anaerobic digestion
   • • Screening
   • • Electrolysis
   • • Distillation

   Answer: Screening

   Screening removes large solid objects before further treatment steps.

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