This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
While flocculation doesn't kill pathogens, it will reduce their levels
along with removing particles that could shield the pathogens from
chemical or thermal destruction, and organic matter that could tie up
chlorine added for purification. 60-98% of coliform
bacteria, 65-99% of viruses, and 60-90% of giardia will be removed
form the water, along with organic matter and heavy metals.
Some of the advantages of coagulation/flocculation can be obtained by
allowing the particles to settle out of the water with time
(sedimentation), but it will take a while for them to do so. Adding
coagulation chemicals such as alum will increase the rate at
which the suspended particles settle out by combining many smaller
particles into larger
floc which will settle out faster. The usual dose for Alum is 10-30
mg/liter of water. This dose must the rapidly mixed with the water,
then the water must be agitated for 5 minutes to encourage the
particles to form flocs. After this at least 30 minutes of
settling time is need for the flocs to fall to the bottom, and them
the clear water above the flocs may be poured off. Most of the
flocculation agent is removed with the floc, Nevertheless some
question the safety of using alum due to the toxicity of the aluminum
in it. There is little to know scientific evidence to back this up.
Virtually all municipal plants in the US dose the water with alum.
In bulk water treatment, the alum dose can be varied until the idea
dose is found. The need dose varies with the pH of the water and the
size of the particles. Increase turbidity makes the flocs easier to
produce not harder, due to the increased number of collisions