Coagulation and flocculation are two steps in water treatment that involve chemical and physical conditioning of water. Coagulation uses chemicals to reduce repulsion between particles, allowing them to come together. Flocculation then brings these destabilized particles together through slow mixing so they aggregate into flocs large enough to settle by gravity. Coagulant aids can help improve the coagulation process in waters where coagulants alone do not produce satisfactory flocs, and help coagulation work over a broader pH range. Proper coagulant dosing is important - overdosing increases costs and public health risks while underdosing reduces contaminant removal effectiveness.

1. Water Treatment
(Coagulation and Flocculation)

5. Coagulation and Flocculation
Coagulation (or Destabilization): Chemical conditioning
❏ Reduce the forces acting to keep the particles apart after they contact each
other (i.e., lower repulsion forces).
❏ Chemical Addition, Rapid Mixing, “Pin‐point” Floc Formation
Flocculation: Physical conditioning
❏ Process of bringing destabilized colloidal particles together to allow them to
aggregate to a size where they will settle by gravity.
❏ Slow Mixing, Floc Growth, Increased Diameter

13. Coagulant Aids
● In some waters, even large doses of primary coagulant fails to produce a satisfactory floc. In such
cases, the coagulation process is often enhanced through the use of coagulant aids.
● Coagulant aids also help to create satisfactory coagulation over a broader pH range.
● Insoluble particulate materials such as clay, sodium silicate, pure precipitated calcium
carbonate,diatomite, and activated carbon are common coagulant aids.
● They are used in waters that have low concentrations of particles (few nucleating sites). Because
their density is higher than most floc particles, floc settling velocity is increased by the addition of
such coagulant aids.
● Polymeric coagulant aid those help in bridging small floc to agglomerate rapidly into larger and
denser floc are also used to reduce the amount of primary coagulant required.
● These are usually slightly anionic polyacrylamides with very high‐molecular weights. In some
studies, non‐ionic or cationic types have also been proven effective. Synthetic organics such as
anionic polyelectrolyte, and natural organics such as starch, starch derivatives, proteins, and tannins
have been used as coagulant aids.

14. Coagulation Practices based on Colloids and Alkalinity Levels
1. High Colloid, low alkalinity: The
strategy here is to add coagulant without
worrying about pH. The lower pH is better
because destabilizing is by charge
neutralization. Generally, there is no
concern with overdosing because the
colloidal surface area is too large.
2. High colloid concentration, high
alkalinity: The choices are to destabilize
byadsorption/charge neutralization at
neutral pH (a larger dose at higher pH),
or add acid tolower pH. Economics
dictate choices.
3. Low colloid concentration , high
alkalinity: For this case we can either
destabilize by high dosage to give sweep
floc or we can add coagulant aid such as
bentonite (aluminium phyllosilicate clay) to
get destabilization at lower dosage.
4. Low colloid concentration, low
alkalinity: This is the most difficult case
and generally requires added alkalinity or
collides. Sweep floc is difficult to form as
pH drops and it’s easy to overdose at low
pH and low colloid concentration.

17. Importance of Optimum Coagulant Dosing
Coagulant over‐dosing may leads to
o Increased treatment costs
o Restabilization of colloids
o Increases sludge mass
o Public health concerns
Coagulant under‐dosing may leads
o Lesser degree of removal
o Failure to meet the water quality targets