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University
of Idaho

MEMORANDUM


To:

Ariadne Rooney

From:

Marliese Breuer

Date:

September 23, 2012

Subject:

Extended Definition of “flocculation”


Problem, Purpose and Placement

The purpose of this extended definition
is to expla
in the term “flocculation” to an
audience of
beginning civil engineering student
s
. It will be written as though it is part of
a larger work in a
n entry level environmental engineering

textbook, and will go into
detail to hopefully help the reade
r learn more about this term, which is important to
civil and environmental engineering. It is my hope that this definition would help the
student understand more about a term that they haven’t heard before. This would
hopefully save them time and
confusio
n

later on in their studies. In my experience,
learning something correctly the first time saves a lot of frustration, so the purpose of
the definition is to ensure there is no confusion, even if the reader is starting with little
or no knowledge of the su
bject.


Definition

Flocculation

is
a method
used
to get suspended particles out of a liquid
. This
process
is
done by adding specific ingredients to the mixture that will help the suspended
particles stick together
, and

settle out of the solution. Though th
is process is used in
everything from beer brewing, to what causes milk to spoil, to medical fluids testing, it
also plays a large role in civil engineering. For the purposes of this definition,
civil/environmental engineering uses
in water treatment
will
be discussed.


In wastewater and drinking water treatment, there are many steps involved before the
final
purified water

and
waste
sludge can leave the treatment plant. These include
filters, sedimentation tanks,
lime
-
soda softening,
flocculation, and more

depending on
the result desired.
In high
-
turbidity

(or hard to see through)

water,

flocculation is very
important.
It also plays a large role in removing pathogens like
Giardia

and
Cryptosporidium
, and some types of natural organic material.
This means th
at
flocculation is usually part of the treatment for surface waters
, since surface waters
usually have high turbidity, color (from organic material), and microbial activity.
Ground water on the other hand is naturally filtered

by the soil

as it seeps down,

and
rarely has

these problems
.


University
of Idaho

MEMORANDUM


Flocculation is
usually seen as part
of three
essential
elements in a water treatment
plant. These three are
coagulation, flocculation and sedimentation.
When put in
together in this order, these three processes
achieve th
e goal of removing su
spended
particles from the colloid, making it into pure water
.
A c
olloid

is a solution

that looks
uniform, but is actually a mix of different particles floating suspended in water. Usually
these particles are so lightweight that no amount of sitting in a sedimentation tank will
cause them to fall out of solution. This problem is furthered b
y the fact that these
particles tend to have negative surface charges, which then act like similar poles on a
magnet and cause natural repulsions between particles. This prevents the particles
from ever combining to a larger, more settleable particle. This

is where coagulation,
flocculation and sedimentation come into play.
During this process, coagulants (which
will be discussed shortly) are injected into a rapid mixing tank. After rapid mixing to
ensure the solution is uniform, the solution enters the flo
cculation tank, where slow
mixing helps bring newly neutral surface charged particles together. This creates larger
particles, known as “floc”. The solution is then pumped into a sedimentation tank,
where the floc is finally allowed to settle out, and the
water can then go on to further
treatment.


Flocculation Chemistry

The chemistry behind this process is
actually quite simple

at a basic level. To start off,
there are millions of tiny particles floating in solution repelling one another. Coagulants
are th
en added. These coagulants are usually metals that fall into two general
categories: aluminums and irons. These are chosen because of their highly positive
surface charges. As you can assume, adding these coagulants to the mixture causes an
attraction betw
een the
positive
coagulants and the
negative
particles. Once these have
combined, the resulting particles have a neutral surface charge.
Once energy has been
added in the form of slow mixing within the flocculation tank, the neutral particles are
then

able

to combine

with other neutral particles
.
This process can be seen in Figure 1.











Figure
1
. The Effects of Coagulants

University
of Idaho

MEMORANDUM


After these particles combine, they are now known as “floc”, and the flocculation stage
of water
treatment is finished. The next step for the liquid is sedimentation.



Flocculation is an essential part of treating surface waters with high color, turbidity
and/or microbial activity. The three step process of coagulation, flocculation and
sedimentation

make a huge difference in the water we use every day,
and the basic
chemistry is simple to understand. W
ith further study of civil and environmental
engineering, you will

learn more about flocculation
and other water treatment
techniques.
















References:

Beddow, Victoria. "Coagulation and Flocculation in Water and Wastewater Treatment."
IWA Water Wiki
. N.p., 02 Feb. 2010. Web. 18 Sept. 2012.
<http://www.iwawaterwiki.org/xwiki/bin/view/Articles/CoagulationandFlocculati
oninWaterandWastewaterTrea
tment>.

Smith, S.E., and Bronwyn Harris. "What Is Flocculation?"
WiseGeek
. Conjecture, n.d.
Web. 18 Sept. 2012. <http://www.wisegeek.com/what
-
is
-
flocculation.htm>.