Pre-treatment processes include removing the surface water's debris using a screen filter, natural biological purification, pre-conditioning with soda ash or lime to raise the pH of hard salts and making the water a little alkaline ensures coagulation and flocculation to lessen lead being dissolved from lead pipes.
Flocculation and coagulation are methods that clump or glue the floating particles together so they can settle onto the granules in a granular media filter. Larger particles are clumped by chemicals of flocculation and coagulation eleminate the natural electrical charges to they attract to each other because many particles have negative electrical charages. The coagulant used most of the time--aluminum sulfate--forms flocs of aluminium hydroxide mixed in water. Sedimentation uses "dissolved air flotation" to inject it into the bottom of the settling tank which then "small air bubblers are formed to attach themselves to the floc particles" and bring them to the surface.
Filtration is the last step in "removing suspended particles and unsettled floc" usually with the "rapid sand filter." Vertically, the water goes through the top layer of organic compounds, then through layers of activated carbon--anthracite coal--taking weeks to months time because of the graded layers of sand from coarsest to finest, top to bottom, respectively. Active coal can be used to "ultrafiltrate" the water.
The final step for water purification is disinfection which exterminates pathogens, viruses and bacteria that pass through filters. Many disinfecant angents stay in there for days before reaching the consumer. Chlorine or chloramines or chlorine dioxide is a strong oxidant that kills many harmful organisms, microorganisms. But since it is toxic, sodium hypochlorite can release chlorine when in water. There are no byproducts when using chloramines, which cause to be more popular and effective. Ozone, created by passing through UV rays or "cold" electrical discharge, kills harmful protozoan that form cysts and also prevents pathogens. The highly useful method is using ultraviolet radiation light to pass through water without being taken in by the low level of color of the water, should that be the case, and does not leave residue.
Nonetheless, additional treatments are fluoridation, water conditioning, plumbosolvency redution, fluoride removal, and radium removal.
The treatment of water purification can take a few or many months until the water is potable for human consumption which takes pre-treatments, disinfectants, and etc.
Reference: Wikipedia. http://en.wikipedia.org/wiki/Water_purification.