Fundamentals:-
In fundamentals of clarification technology we will
discussed about Coagulation, flocculation and sedimentation
phenomena.
Description:-
Clarification removes suspended matter from wastewater.
Surface waters require clarification because they
have moderate to high levels of suspended matter.
Well waters do not require clarification because they
have low levels of suspended matter.
The suspended matter in water includes two kinds of
particles:
o Settle-able Particles (macro-particles, typically
visible to the eye)
o Non-Settle-able Particles (micro-particles, normally
visible through a microscope)
Settle-able particles are particles in water that
settle out over time. The water itself is clear, indicating
an absence of suspended matter (turbidity). If non-settle-able
particles had been in the water, the water would not
be clear. This "turbidity" would have indicated the
presence of non-settle-able particles. Turbidity is
an indirect measurement of the amount of suspended
matter (settle-able particles and on-settle-able particles)
in water. Clarification uses chemicals and sedimentation
to remove suspended matter (settle-able particles
and non-settle-able particles). Several steps are
involved.
First, coagulation destabilizes the particle surface
charge that keeps the particles in solution. Once
destabilized, the particles no longer repel one another
and come together as floc.
Second, floc agglomerate into larger particles and
Polymers are used to enhance the flocculation process.
Third, sedimentation causes agglomerated floc to settle
out. The settled floc is collected and concentrated
for discharge to waste, called clarifier blowdown,
or recycled to the coagulation step, called sludge
recycle. Clarified water is collected and flows out
of the clarifier.
Coagulation:-
The first step of the clarification process in wastewater
treatment is coagulation. Particles in water have
a naturally occurring negative charge. This causes
them to repel each other and stay in suspension. When
this charge is destabilized, the particles no longer
repel one another, and can come together in closer
proximity. A chemical salt, called a coagulant, is
mixed with the inlet water to destabilize the charge.
Common coagulants are aluminum sulfate (alum), ferric
sulfate, ferric chloride and organic coagulant. The
coagulants provide a positive charge, in the form
of metallic cations, that destabilize the natural
negative charge of the particles. The metallic cations
combine with hydroxide in the water to form a metallic
hydroxide that is an insoluble compound. The destabilized
particles and metal hydroxide precipitates agglomerate
into small, visible particles called floc. Color,
organic matter and colloids, including colloidal silica,
are removed by becoming bound up in the floc. The
precise mechanism for removal- absorption, adsorption,
co-precipitation, or a combination-is not fully understood.
The addition of too much coagulant can cause the suspended
matter to be re-dispersed with the opposite charge.
The amount of removal is dependent upon the coagulant
dosage and the pH.
Alum (aluminum sulfate), ferric sulfate, ferric chloride
and organic coagulants are acidic salts and decrease
the pH of the influent water. Because of this, the
pH of the water must be adjusted with caustic (sodium
hydroxide) or another alkaline (high pH chemical).The
adjustment is to a pH of 5.5 to 6.5 and is done to
achieve the lowest residual of suspended matter. Lime
is used as the coagulant when the treatment objective
is hardness reduction. The dosage depends on the desired
operating pH of the clarifier. For the greatest removal
of hardness, the pH range is 9.5 - 10.5. Feed of coagulant
alone does not produce satisfactory floc in waters
having a low suspended matter concentration. In this
instance, bentonite clay is added. Bentonite clay
creates an artificial base of settleable macroparticles
that seed the growth of floc.
Polymers are added to reduce the amount of coagulant
required, broaden the working pH range and create
denser, heavier floc that settles out more easily.
Polymers are long-chain organic compounds of high
molecular weight that bridge floc particles together
or modify their surface charge.
In almost all cases, the water to be treated is disinfected
with either gaseous chlorine or sodium hypochlorite.
This oxidizes organic matter in the water that has
taste and odor and certain metals, such as manganese
and iron. When oxidized, these constituents are transformed
into a form that can be removed during clarification.
Their removal is important because they can cause
fouling of process components.
Coagulation is carried out in a fast mix chamber.
Fast mix is required because the coagulant and water
must be thoroughly mixed to allow the suspended matter
and coagulant to come into contact with each other.
If it is not fast mixed, some suspended matter may
not come into contact with coagulant, the surface
charge will not be destabilized and flocculation will
not occur. As flocculated water flows into the slow
mix chamber, polymer is added.
Flocculation:-
In the next step of clarification, the small floc
(microfloc) is allowed to grow into larger floc, called
macrofloc or agglomerated floc. This process, called
flocculation. Flocculation is accomplished by gently
stirring the coagulated water to assure contact between
microfloc particles and polymer. The polymer enhances
agglomerated floc formation. As the agglomerated floc
continues to grow, it becomes denser and heavier,
allowing it to settle. Mixing too rapidly can create
what is called floc shear. Shear is the breaking apart
of existing floc particles. The agglomerated floc,
or macrofloc, is sheared back into microfloc.
Mixing too rapidly can create what is called floc
shear. Shear is the breaking apart of existing floc
particles. The agglomerated floc, or macrofloc, is
sheared back into microfloc.
Sedimentation:-
The final step of the clarification process is sedimentation.
In this step, agglomerated floc settles out to form
sludge and the sludge is transported to the sludge
concentration chamber by the scraper. The sludge thickening
pickets concentrate the sludge. The sludge is discharged
to waste in a process called clarifier blowdown. Above
the sludge, clarified water is collected in the outlet
launder and flows forward for use or further treatment.
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