Sources Of Potable Water (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Potable water

What is potable water?

• Potable water is water that has been processed and is safe for human consumption and daily use

• Pure water contains only H₂O molecules

  • Potable water may also contain dissolved minerals and salts in small quantities

• Potable water should:

  • Have a pH between 6.5 and 8.5

  • Contain dissolved substances (e.g. salts) in very small, regulated quantities

  • Be free of bacteria and harmful microbes

Sources of fresh water

• Water is considered fresh when it is relatively free from dissolved substances, e.g. rainwater

• Surface water collects in reservoirs, lakes and rivers

• Groundwater collects in aquifers

  • Aquifers are porous rocks that store water underground

Treatment methods

• To obtain potable water, a suitable fresh water source is chosen

• The method used depends on the local climate and geology

  • In warmer areas such as south-east England, water collects primarily underground

• Despite being relatively low in dissolved substances, fresh water still requires treatment before it is safe to use

Filtration

• Debris such as soil, grit and twigs are removed by a wire mesh screen

• Water is then passed through sand and gravel filter beds to remove finer particles

Sterilisation

• Ultraviolet light, ozone or chlorine gas can be used to kill any remaining bacteria or harmful microbes

• This makes the water safe to drink

Desalination

What is desalination?

• Where fresh water supplies are limited, desalination is used to produce potable water from seawater or salty water

• Desalination can be done by distillation or reverse osmosis

• Both methods require significant energy, making desalination expensive

Distillation

• Salt water is heated until the water evaporates, leaving the dissolved salts behind

• The water vapour rises and is cooled, condensing back into liquid water

• The pure liquid water is collected and the salts remain in the original container

• This is used in regions with a very hot climate such as Saudi Arabia

Reverse osmosis

• Salt water is forced through a semi-permeable membrane under pressure

• The membrane allows water molecules to pass through but does not allow dissolved substances (salts and ions) to pass through

• This separates pure water from the salt

Waste water treatment

Sources of waste water

• Domestic water use (washing, cooking, bathing) produces waste water that passes through sewers to treatment plants

• Agricultural waste from animal farms and nutrient run-off from fields also requires treatment

• Industrial processes (such as the Haber process) produce waste water containing harmful chemicals and organic matter

• If not treated, waste water poses serious health risks and causes environmental damage

What happens at a sewage treatment plant?

Screening and grit removal

• The first stage uses screening to remove large materials such as:

  • Plastic

  • Twigs

  • Grit

Sedimentation

• Water stands still in a settlement tank

  • Heavier solids sink to form sewage sludge

  • Lighter matter (effluent) floats to the top

Aerobic digestion

• The effluent is treated by aerobic biological digestion

  • This is where air is pumped in to encourage aerobic bacteria to break down organic matter and harmful microbes

Anaerobic digestion

• The sewage sludge is placed in sealed tanks where anaerobic bacteria break it down

• This releases methane gas, which can be used as an energy source

• The remaining digested waste can be used as fertiliser

Removal of toxic substances

• Industrial waste water containing toxic substances requires additional treatment:

  • Membranes

  • Chemical precipitation of metals

  • UV radiation

• Treating waste water uses less energy than desalinating salt water