There is a documented history of treating water that dates back as far as 2000 BC in Greece. The general public knew then that water could be purified by heating it up, filtering it through sand or gravel, or filtering it in some other way. The goal was simply better tasting water as those cultures were still unaware of pathogens, microorganisms, and chemicals that might be present in their water supplies.
The Egyptians discovered coagulation around 1500 BC, and the healing powers of water were uncovered by Hippocrates in 500 BC. Just a short two hundred years later the Romans built the first aqueducts to move water over greater distances.
But it wasn’t until 1804 that municipal water treatment began. Robert Thom designed the first facility which was built in Scotland using sand as a filtering medium. The treated water was delivered using the trusty cart and horse. A few years later, piping was used for the first time.
Today, water treatment focuses on chemical processes in large part to remove the invisible nasties from drinking water. Physical processes are still part of the treatment but municipalities rely heavily on chemical agents to do the heavy lifting.
The process has become quite involved and based on large-scale scientific methods. The videos below, though commercial in nature, illustrate the process quite well. You should preview each video and present the one that is most suited to the age of your students.