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Sewage treatment is the process of dealing with sewage so that it does not cause harm to people or to rivers. Sewage flows in sewer pipes from houses and factories. When it arrives at a sewage treatment plant it passes through many stages. Larger treatment plants often have more stages than smaller ones. A house that doesn't have access to a city's sewage treatment will have its own septic system.
Large objects and rags are removed using screens. The sewage then flows through a tank so sand and grit can fall out. This protects the machines in later stages. Fats and solids are removed in the first treatment stage. Sewage enters a tank where solids fall to the bottom and fat floats to the top. Fats and solids stay in the tank and the water continues to the next stage. This is called primary treatment.
Modern sewage treatment started in London in about 1850. The first treatment plants for sewage were designed by Sir Joseph Bazalgette.