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 explain the term “flocculation” to an
audience of beginning civil engineering students. It will be written as though it is part of
a larger work in an entry level environmental engineering textbook, and will go into
detail to hopefully help the reader 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 confusion 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 subject.
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 this 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 that
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 the goal of removing suspended
particles from the colloid, making it into pure water. A colloid 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 by 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 flocculation 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 then 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 between 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. With 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
oninWaterandWastewaterTreatment>.
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>.