Sewage Treatment Essay

820 WordsFeb 3, 20144 Pages
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 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. 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: 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

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