Sewage treatment plants combine mechanical, chemical and biological processes to purify our wastewater. First of all, mechanical processes are used to filter large components out of the water. The next steps are biological processes in which degradable water components are processed by microorganisms. Processes such as "nitrification" and "denitrification" take place. Well-known parts of the biological process are the sludge basin and the digestion tower, in which aerobic or anaerobic decomposition of water components takes place. Finally, there are chemical processes, where phosphorus compounds are removed by precipitation, for example.
Explanation for children: Many different techniques are combined in sewage treatment plants in order to clean our wastewater. First of all all the big components is taken out of the water, this is done with a huge rake. Then microorganisms extract degradable compounds from the water, e.g. by "ammonification" and "deammonification". However, some compounds are even too difficult for the microorganisms to break down in smaller components, which is why a chemical process is used in the end. When all this is done, the water is clean again.