Chemical based Water Purifier
On addition of inorganic coagulants like aluminium sulfate(alum) or Iron salts causes several chemical and physical interaction take place among particles.  

This inorganic coagulants neutralize the negative charge on particles. Also metal hydroxide precipitates of the aluminum and iron salts ions begin to form.

This precipitates combine to form larger particles under natural processes such as Brownian motion 

Aluminum hydroxides are formed within a fairly narrow pH range, typically: 5.5 to about 7.7. Iron (III) hydroxides can form over a larger pH range including pH levels lower than are effective for alum, typically: 5.0 to 8.5



Source wikipedia