it is important to ...

it is important to "coagulate" or agglomerate smaller particles into larger ones so that they can settle under gravity quickly (settling rate in the clarifier) and be removed as sludge. This causes the clear water devoid of most of the suspended particles to flow through to the next step of purification. the coagulation is done effectively with inorganic coagulants like alum or ferric chloride or one can also use organic coagulants manufactured and sold by specialty chemical companies. Generally the organic coagulants have many advantages over the inorganic ones.