Coagulant aids are chemicals which are added to water during coagulation to:

1)      Improve coagulation;

2)      Build a stronger, more settleable floc;

3)      Overcome slow floc formation in cold water;

4)      Reduce the amount of coagulant required;

5)      Reduce the amount of sludge produced.


The key reason coagulant aids are used 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.




Activated Silica

1)      Increase the coagulation rate;

2)      Reduce the amount of coagulant needed;

3)      Widen the pH range for effective coagulation;

4)      Chief advantage-strengthens floc

5)      Usually added after coagulant, never directly with alum.



Weighting Agents

1)      Provide additional particles that can enhance floc formation.

2)      Used to treat water that is high in color;

3)      Used to treat water that is low in turbidity;

4)      Used to treat water that is low in mineral content

5)      Different types include: Bentonite Clay, Powdered Limestone, and Powdered Silicia




Extremely large molecules which produce highly charged ions when dissolved in water.  These are also called polymers.



Types of Polyelectrolytes

Cationic (+)

1)      Have a positive (+) charge.

2)      Allows reduced coagulant dose;

3)      Floc settles better;

4)      Less sensitivity to pH;

5)      Improved flocculation of organisms such as bacteria and algae.



Anionic (-)

1)      Have a negative (-) charge

2)      Used primarily as a coagulant aid

3)      Increases floc size;

4)      Improve settling;

5)      Produce a stronger floc;

6)      Not materially affected by pH, alkalinity, hardness, or turbidity




1)      Balanced or neutral charge;

2)      Used as a primary coagulant or coagulant aid.



Factors which affect how well a coagulant aid works

1)      Mixing conditions

2)      pH

3)      Alkalinity

4)      Water temperature

5)      Turbidity