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- [Narrator] Coagulation is
the addition of chemicals

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to destabilize the electrostatic forces

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of nonsettleable solids.

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Coagulant chemicals typically
have a positive charge,

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which binds to the negatively
charged particles in water,

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neutralizing them,

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and allowing them to come together

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to form small clumps of
particles called microfloc.

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In most cases, the
chemical reaction occurs

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in a fraction of a second,

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so it's important the coagulant

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is dispersed quickly and evenly

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to encourage as many particle
collisions as possible.

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Because this process relies
on chemical reactions,

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several considerations must be made.

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Water temperature, for example.

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Colder water usually slows coagulation

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and flocculation reactions,

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which may require more time

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to achieve optimal floc formation.

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Extreme values of pH, high or low,

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will also interfere with the process.

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Coagulant effectiveness
is highly dependent on pH.

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For some coagulants,
alkalinity is a factor.

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Operators may need to raise alkalinity

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for coagulation to be effective

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when using iron or aluminum coagulants

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that can consume
alkalinity and depressed pH

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when they are added to
low-alkalinity waters.

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Turbidity is also important.

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The lower the turbidity,

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the more difficult it
is to form proper floc.

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The most common coagulant
chemical used is aluminum sulfate,

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commonly called alum.

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When added to water, alum
forms jelly-like particles

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of aluminum hydroxide.

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The trivalent aluminum ions neutralize

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and attach to negatively
charged particles in water,

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forming microfloc.

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

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Polyaluminum chloride and iron salts,

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such as ferric chloride
and ferric sulfate,

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are also used.

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They operate over a wider pH range.

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Coagulant and flocculant aids
are added to improved binding,

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build stronger, more settleable floc,

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and effect flocculation by
increasing floc size, strength,

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and/or floc density.

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In addition, they help overcome the effect

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of low temperatures,

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reduce the amount of coagulant needed,

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and reduce the amount of sludge produced.

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Commonly used coagulant aids
include cationic polymers.

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Polymers are defined
as a synthetic compound

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with high molecular weight

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and composed of repeating
chemical units or monomers.

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Polymers may be polyelectrolytes,

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such as the water-soluble flocculants,

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water-insoluble ion-exchange resins,

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or insoluble uncharged materials.

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Polymers are long chains of molecules

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that produce highly charged ions

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and help bind microfloc together

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into larger floc particles.

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They are very popular with operators

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because they are very effective

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and can be positive, negative,
or neutrally charged,

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providing a great deal of flexibility

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in different circumstances.