Sewage Treatment and Disposal

T E A C H E R S ’

G U I D E

T E A C H E R S ’

Sewage Treatment and Disposal

Sewage Treatment and Disposal

Providing fresh clean water for all our needs is only part of the water management story. The more clean water we use, the more dirty water is produced. Severn Trent treats more than 2,700 megalitres of dirty, waste water (sewage) every day. This includes the domestic sewage of more than eight million people, industrial and commercial waste, and large amounts of rainwater in wet weather.

Sludge Sludge can be defined as the solid waste which has settled out of the sewage in the sedimentation, humus and final settling tanks. Severn Trent’s sewage works produce 250,000 tonnes of sludge every year and use several methods of disposal:

On arrival at the treatment works sewage is souplike, dirty liquid containing food, detergents, human waste, oils, sand and sometimes harmful chemicals such as acids. All these pollutants have to be removed and the water cleaned before it can be returned to the natural water cycle. The sewage treatment process can be complex. However, the key elements of the process are:

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a) Sludge contains nitrogen, phosphorus and organic matter which makes it ideal for use as an agricultural fertiliser. 68 per cent of Severn Trent sludge is used in this way. b) Sludge from sewage works in heavy industrial areas may have a 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 landfill purposes and tipping, or it can be incinerated.

1. Screening 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. 4. Filtration Biological Filter 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. Activated Sludge 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.

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Sewage Treatment and Disposal Natural Purification 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 the bottom. • 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. Follow-Up Ideas • 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. Fascinating Facts 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 dirty water. 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.

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