You may take clean water for granted, but you shouldn't. Purifying water is one of the most important things we do. It is as important as growing food, for without purified water many of us would die from water-borne diseases.

When you look at natural water from a river, you may think it is a bit cloudy and think that filtering it to get the brown bits out is all that is needed. In fact, the brown bits (that scientists call sediment) do you no harm at all, they just look odd. What does you harm is what you can't see, such as disease-producing microbes and chemical poisons. These have to be removed by complicated chemical means.

Keep in mind that we use huge volumes of water, so water purifying is a massive task.