During the purification of water for city supply, the raw water from the river lake is allowed to stand undisturbed in large tanks. What happened here?

During the purification of water for city supply, the raw water from the river lake is allowed to stand undisturbed in large tanks. A process called sedimentation takes place here. All the dirt, mud, sand and other suspended materials settle down at the bottom of the tank. And clean water is removed leaving behind the impurities.