Coagulant Aid

Definition - What does Coagulant Aid mean?

A coagulant aid is a chemical or material, which is not a coagulant, used to assist or modify coagulation. Coagulant aids add density to slow-settling flocs and add toughness to the flocs so that they do not break up during the mixing and settling processes.

A coagulant aid improves the effectiveness of a coagulant by:

  • Forming larger or heavier particles
  • Speeding reactions
  • Permitting reduced coagulant dosage

Coagulant aids are also known as flocculants.

Corrosionpedia explains Coagulant Aid

The primary reason to use coagulant aids is to reduce the amount of alum used, which, in turn, decreases the amount of alum sludge produced. Alum sludge is difficult to dewater and to dispose of.

Coagulant aids may be:

  • Nonionic, cationic or anionic polymers
  • Sodium aluminate
  • Activated silica
  • Clay
  • Acids
  • Alkalis

The most common problems associated with coagulation are weak flocs that do not stay together long enough to settle completely or flocs that settle poorly. Coagulant aides are added to reduce or eliminate these problems. The addition of a coagulant aid may also reduce the amount of coagulant that is required.

The coagulation process is often enhanced through the use of coagulant aids. Sometimes, excess primary coagulant is added to promote large floc sizes and rapid settling rates. However, in some waters, even large doses of primary coagulant do not produce a satisfactory floc. Generally, the most effective types of coagulant aids are slightly anionic polyacrylamides with very high molecular weights. In some clarification systems, nonionic or cationic types have proven effective.

Factors which affect how well a coagulant aid works include:

  • Mixing conditions
  • pH
  • Alkalinity
  • Water temperature
  • Turbidity

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