Dear Sir  Adding alum  to promote coagulation & forming floppy flocs (during C...

I think we are getting confused here of the Non-reactive nature of Suspended and colloidal (non-ionized) with negative electrostatic charges at the surface of these particles. Dissolved solids are reactive because they are present as ions in the water.

Most suspended solids smaller than 0.1 mm found in waters carry negative electrostatic charges. Since the particles have similar negative electrical charges and electrical forces to keep the individual particles separate, the colloids stay in suspension as small particles. The magnitude of the zeta potential (Zp) is usually used to indicate colloidal particle stability. The higher the zeta potential, the greater are the repulsion forces between the colloidal particles and, therefore, the more stable is the colloidal suspension. To remove colloids, small particles have to be destabilized first and then they will form larger and heavier flocks which can be removed by conventional physical treatment. This is done by the surface positive hydroxides that are formed by the Alum reaction as mentioned by you.