Sewage treatment

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. 
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 about 1850. The first treatment plants for sewage were designed by Sir Joseph Bazalgette.
Importance of oxygen.
Bacteria and amoebae in the gut and bowels are totally anaerobic. Oxygen kills them. A key aspect of the treatment is not just to remove solids, but to "run the sewage through oxygenated water". It is this which is the most important part of treatment after solids are removed. The use of chemicals is only done exceptionally. Dumping sewage into the sea is disgusting, but has been done at times when the treatment plants are not adequate.
Small treatment plants may use a small pond, called a lagoon, to hold the sewage while oxygen destroys the bacteria. Larger treatment plants use machines to help process the sewage. Some treatment works have rocks or pieces of plastic for the bacteria to grow on. Water is pumped over the rocks or plastic.
Solids from the first and second treatment tanks can be used to make methane gas and fertilizer for farmers fields or may be dried and sent to a landfill.