The coagulants  

Trivalent cations : the neutralization of the negative surface of the colloid is accomplished by the addition of cations in the case of inorganic coagulants. The trivalent ions are ten times more effective than the divalent ion. Trivalent iron and aluminum salts keep on the be be widely used in all the water coagulation treatments.

The influence of the pH : Inorganic coagulant because of their hydrolysis change the physical-chemical characteristics of water to be treated (pH, conductivity, …) :

 M ³⁺ + 3 H₂O    <=>       M(OH)₃ + 3 H⁺

 The pH necessary for coagulation may be adjusted by addition of an acid or a base.

Sludge production : The formation of metallic hydroxide causes the production of a substantial amount of sludge. This sludge should be removed in the final solids-liquids separation process.

Organic coagulants may also be used. The advantage of those cationic polyelectrolytes is because they directly neutralize the negative colloids. Consequently to this direct action the amount of sludge is considerably reduced. 

The flocculant 

Inorganic polymers (activated silicia) and natural polymers (starches, alginate) were the first to be used. But the use of synthetic flocculants often results in a minimum amount of sludge. Combined with modern separation techniques can allow to produce very dense sludge that can be directly treated in a dewatering unit.  

Resource: Water treatment Handbook Degrémont