Coagulation and Coagulants

Coagulation is defined as the destabilization of charge (charge neutralization) on colloids and suspended solids, including bacteria and viruses, by a coagulant. The effect of charge neutralization is shown graphically below.

Charge neutralization

Loosely defined, a coagulant is a charged chemical that is added to wastewater (or water) to destabilize the substances contained in treated water so that they aggregate or adsorb to particles. The resulting floc can be removed by a solids-liquid separation process such as a dissolve air flotation unit or a clarifier. Some of the processes that cause particle and colloid destabilization include adsorption, charge neutralization, and enmeshment. Positively charged metal salts may work by neutralizing the negative charges associated with naturally occurring particles. The most commonly used coagulants include:

  • Aluminum sulfate (alum)
  • Ferric salts (ferric chloride and ferric sulfate)
  • Ferrous sulfate
  • Polymeric inorganic coagulants (partially neutralized metal salts such as polyaluminum chloride)
  • Synthetic polymers such as polydiallyldimethylammonium chloride (pDADMAC)
  • Natural cationic polymers such as chitosan (made from crustacean shells)

The difference between metal salt coagulants and cationic polymers is their hydrolytic reaction with water. Metal salts undergo hydrolysis when they are fed to the process water; yet all polymers do not. When cationic polymers are used as coagulants, instantaneous mixing is not critical because hydrolytic reactions do not occur; the rate of colloid adsorption is much slower because of their large physical size. Thus, from a practical point of view, the mixing time for polymers may be 2 to 5 seconds.