Flocculation: Making Clean Water
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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