T E A C H E R S ’ G U I D E T E A C H E R S ’ G U I D E
Sewage Treatment and Disposal Sewage Treatment and Disposal
Providing fresh clean water for all our needs is only part of the water Sludge
management story. The more clean water we use, the more dirty water is Sludge can be defined as the solid waste which has settled out of the sewage
produced. Severn Trent treats more than 2,700 megalitres of dirty, waste in the sedimentation, humus and final settling tanks. Severn Trent’s sewage
water (sewage) every day. This includes the domestic sewage of more than works produce 250,000 tonnes of sludge every year and use several methods
eight million people, industrial and commercial waste, and large amounts of disposal:
of rainwater in wet weather.
a) Sludge contains nitrogen, phosphorus and organic matter
On arrival at the treatment works sewage is souplike, dirty liquid which makes it ideal for use as an agricultural fertiliser.
containing food, detergents, human waste, oils, sand and sometimes 68 per cent of Severn Trent sludge is used in this way.
harmful chemicals such as acids. All these pollutants have to be removed b) Sludge from sewage works in heavy industrial areas may have a
and the water cleaned before it can be returned to the natural water cycle. high metallic content and therefore cannot be used on the land.
In these cases the sludge can be dried into a cake and used for
The sewage treatment process can be complex. However, the key elements landfill purposes and tipping, or it can be incinerated.
of the process are:
Metal screens sieve out large objects such as wood, cans and plastics.
2. Grit and Sand Channels
The sewage then flows along wide, deep channels where grit and sand sink
to the bottom.
3. Sedimentation Tank
Here, solid particles in the sewage settle at the bottom of the tank to form
a thick sludge.
The ‘settled sewage’ is then sprinkled onto large circular beds, about two
metres deep, filled with stones or clinker. Biological filtration is based on
the principle that where enough air is present cultures of bacteria will
form. Millions of bacteria and other tiny creatures live on the stones and
feed on the organic material in the sewage, converting it into carbon
dioxide, water and nitrogen compounds. They literally ‘eat’ the sewage,
removing harmful waste. This biological activity produces a humus
sludge which settles out in special humus tanks.
In this alternative to biological filtration, activated sludge containing
bacteria is mixed with the settled sewage in an aeration tank. The air the
bacteria need is provided by water in the tank.
5. Final Settling Tank
Small and fine particles settle out leaving the cleaned water, or effluent, to
flow back into rivers and streams.
T E A C H E R S ' G U I D E
Sewage Treatment and Disposal
96 per cent of pollution is removed by sewage treatment. However, once the
effluent has been returned to the water cycle it continues to be cleaned by
processes of natural purification.
• When water is stored in lakes and ponds, solid particles settle to
• Harmful bacteria gradually die out.
• Algae can reduce hardness.
• Tumbling over rocks or falling down waterfalls puts valuable
oxygen into the water. This encourages water life and oxidises
organic impurities which improves taste and smell.
• Water is filtered naturally as it seeps through soil and rocks.
• Make a mini settling tank
a) Put 2-3 spoonfuls of soil into a jam jar of water.
b) Stir well and leave to settle.
c) The sedimentation process will produce well defined,
graduated layers with coarse particles at the bottom and fine
ones at the top.
• Separating substances
a) Put a mixture of sand and salt into a beaker of water and mix
well. The soluble salt will dissolve in the water, the insoluble
sand will not.
b) The solids can be separated out by pouring the solution
through a filter paper. The salt will pass through in solution
and the sand will be left on the paper. The salt itself can be
removed from the solution by evaporation.
• Arrange a visit to your local sewage treatment works.
The Romans used five times as much water as Londoners do today. Their
sewage and drainage systems were as good as those of a modern city.
Water is one of the main carriers of disease. According to the United Nations,
25,000 people throughout the world die every day from diseases related to
The largest Severn Trent sewage treatment works is at Minworth, near
Birmingham, treating the sewage of more than a million people. Benton
Green works at Solihull treats the sewage of just 100 people.