WSO Water Treatment Grade 1: Coagulation & Flocculation, Ch. 8

- [Narrator] Coagulation is
the addition of chemicals

to destabilize the electrostatic forces

of nonsettleable solids.

Coagulant chemicals typically
have a positive charge,

which binds to the negatively
charged particles in water,

neutralizing them,

and allowing them to come together

to form small clumps of
particles called microfloc.

In most cases, the
chemical reaction occurs

in a fraction of a second,

so it's important the coagulant

is dispersed quickly and evenly

to encourage as many particle
collisions as possible.

Because this process relies
on chemical reactions,

several considerations must be made.

Water temperature, for example.

Colder water usually slows coagulation

and flocculation reactions,

which may require more time

to achieve optimal floc formation.

Extreme values of pH, high or low,

will also interfere with the process.

Coagulant effectiveness
is highly dependent on pH.

For some coagulants,
alkalinity is a factor.

Operators may need to raise alkalinity

for coagulation to be effective

when using iron or aluminum coagulants

that can consume
alkalinity and depressed pH

when they are added to
low-alkalinity waters.

Turbidity is also important.

The lower the turbidity,

the more difficult it
is to form proper floc.

The most common coagulant
chemical used is aluminum sulfate,

commonly called alum.

When added to water, alum
forms jelly-like particles

of aluminum hydroxide.

The trivalent aluminum ions neutralize

and attach to negatively
charged particles in water,

forming microfloc.

Alum works best in a
pH range of 5.5 to 7.5.

Polyaluminum chloride and iron salts,

such as ferric chloride
and ferric sulfate,

are also used.

They operate over a wider pH range.

Coagulant and flocculant aids
are added to improved binding,

build stronger, more settleable floc,

and effect flocculation by
increasing floc size, strength,

and/or floc density.

In addition, they help overcome the effect

of low temperatures,

reduce the amount of coagulant needed,

and reduce the amount of sludge produced.

Commonly used coagulant aids
include cationic polymers.

Polymers are defined
as a synthetic compound

with high molecular weight

and composed of repeating
chemical units or monomers.

Polymers may be polyelectrolytes,

such as the water-soluble flocculants,

water-insoluble ion-exchange resins,

or insoluble uncharged materials.

Polymers are long chains of molecules

that produce highly charged ions

and help bind microfloc together

into larger floc particles.

They are very popular with operators

because they are very effective

and can be positive, negative,
or neutrally charged,

providing a great deal of flexibility

in different circumstances.