Potable water and waste water treatment

Potable water

Water that is safe to drink

What does rain provide

freshwater

what can freshwater be collected as

• surface water in lakes, rivers and reservoirs

• Groundwater in rocks that trap water underground

Treatment of freshwater

1) choose an appropriate source of freshwater

2) Filtration

3) Sterilisation

What is filtration

a wire mesh screens out large twigs, then gravel-sand beds filter out any solid parts

what is sterilisation

water is sterilised to kill any harmful bacteria/ microbes. This is done by bubbling chlorine gas through it or by using ozone or ultraviolet light

How is seawater treated to provide potable water

by desalination

How can desalination of seawater be done

• via distillation

• By processes that use membranes (reverse osmosis)

Distillation

• seawater is heated

• Water evaporates at 100 degrees

• Water is condensed and collected 

Reverse osmosis

• salty water is passed through a membrane that only allows water mollecules to pass through

• Ions and larger molecules are trapped by the membrane and so separated form the water

Disadvantages of distillation and reverse osmosis

• need lots of energy, so very expensive

• Not used if other sources are available

Screening and grit removal

removes large materials such as plastic bags and twigs and grit by screening

Sedimentation

Sedimentation comes next which occurs in a settlement tank. The water is allowed to stand still in the tank while heavier solids sink to the bottom creating sewage sludge, whilst lighter matter which is also known as effluent, floats to the top

aerobic digestion

The effluent is removed and treated by biological aerobic digestion

This involves pumping air into the water to encourage the breakdown of organic matter and other microbes by aerobic bacteria

Anaerobic digestion

Anaerobic digestion is then used to break down the sewage sludge from the bottom of the settlement tank. It is firstly removed and placed in large tanks where bacteria break it down

Anaerobic digestion releases methane gas as a by product from the organic matter in the sludge. Methane gas is used as a source of energy and the leftover, digested waste as a fertiliser

Describe how sewage is treated to remove organic matter

• screening or grit removal

• sedimentation (to produce sewage sludge and effluent)

• anaerobic digestion of (solid sewage) sludge

• aerobic biological treatment of (liquid) effluent