The diagram shows the steps involved in filteration of water. These involve the following:

1. Coarse filter: A screen prevents large items such as leaves from entering the water treatment plant.
2. Aeration: During aeration, air may be bubbled into the water or water may be sprayed into the air. Aeration removes substances from water, such as iron compounds, that give water an unpleasant taste.
3. Forming a colloid: Alum is added to the mixing tank. Alum causes small particles in the water to form a type of colloid called a gel. Most of the bacteria in the water are trapped in the gel.
4. Sedimentation: In the sedimentation tank, large lumps slowly settle to the bottom of the tank from where they can be removed.