Cleaning used water, a model watershed and thirst by the sea

Nature recycles water in the water cycle, and people recycle it too. Water that we use for washing, cooking and cleaning does not disappear. It flows away through pipes as wastewater. If it were poured straight into a river, it would pollute the water for everyone downstream. So it goes to a treatment plant. First, screens catch large things such as rags and plastic. Then the water rests in big tanks, where heavy dirt sinks to the bottom. More steps remove smaller bits and harmful germs. At the end the water is tested. In Egypt, very large plants clean used water so that it can irrigate farms and green areas. Wastewater engineers plan the plants, choose where to build them, check every step and test the water. Some also design ways to protect a town from floods.