Flocculants are substances added to a suspension to enhance aggregation of the suspended particles. In wastewater treatment, flocculants are often used to enhance the clarification process after coagulants have destabilized the colloidal suspensions.

Flocculation is caused by addition of minute quantities of chemicals known as flocculants.

In a solution, fine particles are suspended in water, stabilized by electrostatic charges (usually negative charge). This electrostatic charge prevents the fine particles to combine with each other to form clusters. Since they cannot form clusters, the fine particles cannot settle. Therefore coagulating and flocculating agents are applied to promote the aggregation of the suspended solids into large enough particles so that they can settle and be removed.

Coagulation: Coagulating agents neutralize the negative electrostatic charge on the fine suspended particles in solutions. Since the barrier for the particles to combine has been broken down, the fine particles can now collide and combine with each other to form larger and heavier particles colloids. These large particles are called flocs.

Flocculation: Flocculating agents are used to bridge flocs together to form even larger clumps or agglomerates. Flocculants help by having long polymeric chains adsorbing the different particles and help the particles to aggregate. According to studies, polymeric flocculants act by extensive salvation. When polymeric flocculants applied in a solution, it will spread across the whole solution, forming a salvation domain (Figure 1). The polymer will also pushed the suspended particles out of the salvation domain so that now, the particles will accumulate in a smaller area and can combine with each other easier and better.

Inorganic flocculants are substances with multivalent cations such as aluminium, iron, calcium or magnesium. Examples of inorganic flocculants are polyaluminium chloride (PAC), which is the most common in use, aluminium sulfate, calcium hydroxide, iron (II) sulfate, iron (III) sulfate and etc.

Sludge removal: Removing and disposal of sludge with heavy metals is not easy. In normal wastewater treatment with heavy metals, sludge first needs to be pumped out from sedimentation basins, and then dewatered to reduce the volume and finally dispose off. However, dewatering of heavy metal hydroxide sludges is difficult to achieve due to the poor thickening and dewatering characteristic. This difficulty is compounded by the large amount of water incorporated into the metal sludge floc matrix during precipitation. Even after vacuum filtration or centrifugal dewatering, the volumes are not significantly reduced. Companies often spend a large amount of money to seek for aids on disposal of heavy metal sludges.

Organic flocculants can be divided into two categories:
  • Organic polymers
      o Examples: Chitosan, Gelatin, guar gums and alginates
  • Synthetic polymers
      o Consist of anionic, cationic and non-ionic
      o Non-ionic: Polyacrylamide and poly(ethylene oxide)
      o Cationic: Polyelectrolytes derived by introducing quarternary ammonium
         groups onto polymer backbones. Most common: PolyDADMAC
      o Anionic: Polymers containing carboxyl groups and sulfonic acid groups