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Let's do a small experiment
Would you rather drink this water, or this water?
Well, of course you would choose the water on the left
Unfortunately, some people in other parts of the world have no choice at all
Did you know that small floating particles in drinking water can
make you sick? Imagine we have a super powerful microscope and we can
zoom into the water. ZOOM!
What will we find? What are these small, floating particles,
and how do they float? These particles are of two types:
inorganic (like clay, silt, and mineral oxides) and
organic (such as algae, protozoa, and bacteria).
The bacteria, once ingested by humans, can sometimes be fatal.
All of these small particles are able to float, because they are not heavy enough
to settle to the bottom by gravity. Suspended particles that are too
light and small to settle are called colloids.
When looked at together, these colloids cause a state of cloudiness
or haziness, known as turbidity. The more cloudy a fluid looks, the more
turbid it is. Here we see four beakers of
water with increasing levels of turbidity, from left to right.
There is a relation between turbidity and the risk of getting a disease.
Science shows that the more turbid
the drinking water is, the higher the risk of getting sick is.
Now why is this? This is because toxic compounds can
adsorb, that is, stick to the surface of the suspended colloids.
The more colloids there are, the more toxic the water can become.
These toxic materials and bacteria can cause cholera,
salmonellosis, hepatitis A, dysentery,
and e-coli infection. These illnesses effect and kill
millions of people a year, and are especially dangerous to children, whose weak
immune systems cannot provide an adequate defense.
Fortunately, we can do something about this! One of the very
practical ways to clean this turbid water is called flocculation
Flocculation is the process in which colloids aggregate,
or come together to form larger particles called flocks, by the addition
of a chemical called a flocculant. Typical flocculants
include Alum and Ferrix, because they work well with high turbidity fluid mixtures
Now, let's demonstrate how flocculation works. First, we'll need
to go out and collect some muddy water from the Charles River
Here are two beakers filled with the same amount of muddy Charles River water
On the left is our control, which will remain untouched,
and on the right, we'll add 3mL of prepared flocculant solution
Then we'll stir for two minutes, and wait
Wow! What just happened?
The colloids in the turbid water on the left may never settle
whereas, with the addition of just a little bit of flocculant
the water on the right became clear.
In order to make this water potable, it will require skimming and filtration
and maybe some additional treatment
If you're wondering what's going on, let's explain how this flocculant business works.
Almost all colloids have negatively charged surfaces
This means that positive ions, or charged particles in water
will attract to the colloid surface, forming a first layer.
Recall how like poles of a magnet will repel, while opposite poles will attract.
The same occurs with colloids in water.
A diffuse layer, made up of a mix of positive and negative ions will then surround the first
forming what is called a double layer.
This double layer provides a repulsive force which prevents two colloids from sticking to each other.
Once the flocculant is added, it adheres to the surfaces of the particles,
compressing the double layer,
and allowing the colloids to stick to each other and form "flocks"
These flocks are now heavy enough to settle to the bottom by gravity.
Given how effective flocculation is, many countries around the world
use this method for cleaning their water supplies.
Did you know that Singapore, for instance, produces drinking water from sewer water
using a number of methods, including flocculation?
As the global population increases
and freshwater resources become more and more scarce
flocculation is one tool that can supply clean, healthy, and tasty drinking water



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李應振 2013 年 2 月 3 日 に公開
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