The water purification process used by a treatment facility varies. In order to explain water purification in its entirety, visual aids are helpful, but as with many other scientific procedures, you can think of it as a circle. Where to begin on that circle is difficult to decide.
Let's start with a brief explanation of why we need a water purification process. In old movies, you would see a cowboy fill his canteen from a mountain stream. The stream could have been fed by a glacier, a spring or other groundwater, as well as regular rains. There were far less people and far fewer environmental pollutants at that time, so as long as the stream was moving rapidly, he could count on rocks, gravel and gravity to "clean". You could think of it as the "natural" water purification process.
That stopped working for us a long time ago, because so many different bacteria, parasites, viruses and protozoa thrive in above ground water-supplies. They can cause waterborne diseases like cryptosporidiosis, Giardiasis, typhoid and dysentery, with symptoms ranging from a stomach ache to death.
For many years, groundwater, from wells or springs was safe, because the ground itself contributed to the water purification process, by filtering debris and other contaminants. Now, even they require testing before they can be considered safe for human consumption.
To explain water purification, as it currently exists, we begin with disinfection processes that remove the majority of the disease causing micro-organisms mentioned above. Typically, chlorination is used on raw sewage, before it is treated and returned to a river or ocean. It is also used as source-water enters a facility to be prepared for drinking purposes and again, at the end of the water purification process before it enters the pipes and tanks.
Now, take a step back and look at the circle we mentioned when we began to explain water purification. Rain hits the ground, some is absorbed by the soil, some enters streams and lakes, and some evaporates and returns to the atmosphere. Treatment facilities pump water from reservoirs and lakes. They clean it up some. It enters a home, where it may be further cleansed if the homeowner has his own filtration system. People drink it and use it for cooking. Some ends up in the sewer as wastewater.
From there, it goes to a wastewater treatment facility, where scum and sludge are removed and it is screened, aerated and disinfected. It is then sent back to a river or ocean, where some is used again by the public and wildlife, but some evaporates again and falls back to the ground as rain.
At any point on the circle, there is the possibility of contamination. Every chemical pollutant in the environment eventually ends up in our rivers, streams, oceans and groundwater. Advancements in the water purification process have helped, but many chemicals and other contaminants still end up in tap-water. So, there is a final warning when we explain water purification.
You are ultimately responsible for your own health. Without a good home water purification process, there are many things that come through the tap that can adversely affect your current and future health. Don't drink straight from the tap. Always filter first.