Key concepts Physics Chemistry Water treatment Drinking water
Introduction Do you know where your drinking water comes from? Sure, it comes out of your faucet. But how does it get there? Drinking water all over the world originates from either surface waters—such as lakes, reservoirs and rivers—or from underground sources, such as groundwater. But would you want to drink water straight from a river or lake? Probably not; they can be really dirty! To become clean and safe, this water first has to undergo several treatment processes; the first ones are called coagulation and flocculation. These processes help get rid of particles in the water and transform murky, dirty water into crystal clear water: See for yourself in this activity!
Background Surface waters often contain soil particles, or flocs—organic matter and dirt that make the water look murky and cloudy. The solids floating in the water are called total suspended solids (TSS). They are usually so small and light that it takes a long time until they settle to the bottom of the water body. This is a problem because potentially dangerous bacteria can stick to these tiny particles, which makes the water unsafe to drink.
Before becoming safe and clean drinking water, surface waters have to undergo several treatment steps. This cleanup happens in a drinking water treatment plant where one of the first steps is getting rid of the suspended solids with a combined process using coagulation and flocculation. In this step chemicals called coagulants and flocculants are added to the water; they help clump all the small particles into bigger ones so they settle more readily to the bottom of the water supply due to their increased size and weight. Most suspended solids in water have a negative surface charge and thus repel one another, which prevents them from sticking together and keeps them in suspension. If a chemical with an opposite (positive) charge is added to the water, however, the particles become neutralized and are able to cling together to form larger particles or so-called “microflocs”. These settle down much faster and the water becomes much clearer. How clean do you think your water will get after adding alum, which is commonly used as a coagulant? Get ready to find out in this science activity!
Dry, fine soil from your garden or another source
Fine-meshed sieve or colander
Mortar and pestle
Large mixing bowl
At least four transparent cups, glass or plastic
Alum, available at the spice section of most grocery stores
Paper and pen or pencil (optional)
Lemon juice (optional)
Kitchen towel to catch spills (optional)
Gather some dry soil from your garden or another source.
Use the fine-meshed sieve to filter out large materials, such as stones or sticks and leaves.
Transfer the sieved soil into a mortar and grind it really well until you have a very fine, homogeneous soil powder.
Take one teaspoon of the finely ground soil and put it in a bowl.
Mix the soil with about one liter of tap water to make a murky soil solution. The soil particles should not settle easily and the solution should stay turbid after mixing.
Prepare your coagulant–flocculant solution by adding one quarter teaspoon of alum powder to about three teaspoons of tap water. (You might have to heat the solution in the microwave to get the powder dissolved.) Stir the solution until all the alum powder is dissolved.
Perform the activity on a surface that can get a little wet and dirty; you might want to have a kitchen towel ready nearby in case of spills.
Fill two cups half full with the prepared soil solution. (Make sure to stir the prepared soil solution in the bowl before transferring it into the cups.)
One of the cups will serve as your control and the other for the alum treatment. (You can label them accordingly with “Control” and “Alum Treatment”.)
Set your timer for two minutes, and start it.
With a spoon in each cup, stir both soil solutions as fast as you can (without spilling) for two minutes. You can carefully stir both cups using one hand for each or ask a helper to stir one of the cups. How does the soil solution look after you have finished stirring? Is it very turbid?
Stop stirring and set your timer for five minutes.
Start the timer and observe both soil solutions for five minutes. What happens to the soil solution within five minutes after stirring? Does it stay the same the whole time? Do you see soil particles settle to the bottom?
After five minutes compare both cups. Do they look different? Is one of them more turbid then the other? If no, why do you think this is the case? If yes, what could be the reason for that?
Now, get your prepared alum solution and add about one quarter teaspoon to the cup you labeled Alum Treatment. Do not add anything to the Control cup.
Set your timer for two minutes and start it.
Again, take two spoons and stir both cups as fast as you can (without spilling) for two minutes.
Stop stirring and set your timer for five minutes.
Start your timer and observe both cups again for five minutes. What do you see this time? Does the water stay turbid in both cups or do you notice a difference in the soil solution that you treated with alum?
Observe both cups a little longer (up to 15 minutes) to follow the flocculation process. Do you see the water getting clearer in one of the cups? How long does it take to become fully clear?
Get two new cups and label them Control and Alum Treatment again.
Fill them both with the same soil solution you prepared in the beginning. (Make sure to stir the soil solution in the bowl before you transfer it into the two cups.)
Add one quarter teaspoon of alum solution to the cup you labeled Alum Treatment.
This time do not stir the solutions and just observe what happens for seven minutes. Do you see the same thing happening in the Alum Treatment cup as before when you were stirring? Does the soil solution in both cups look the same after seven minutes? Why do you think this is the case?
After you have waited for seven minutes, start stirring both solutions as fast as you can (without spilling) for two minutes and then observe the cup for five more minutes. What happens after you stir the solutions? How do you think stirring the solutions makes a difference? Is your water getting clearer now?
Extra: Explore how water properties can affect the flocculation process. Prepare two cups with turbid soil solution and add two teaspoons full of lemon juice to each of the cups. This changes the pH of the soil solution. Now add one quarter teaspoon of your alum solution to one of the cups and stir both of them for two minutes. Observe what happens after stirring. Do you see floc formation in the cup with alum treatment? How does the lemon juice change the flocculation process?
Extra: How does the amount of alum change the floc formation in the soil solution? Make an alum solution by adding 7.5 grams of alum to one liter of tap water. Then prepare three or four cups with turbid soil solution. Add different amounts of the alum solution to each of the cups (for example zero, one, three and five milliliters) and stir all of them for two minutes (with help, of course). Observe the cups after stirring. Do you see a difference in the three or four cups after stirring? Do all of the water solutions change from turbid to clear?
Observations and results In this science activity you simulated the first steps at a full-scale water treatment process in a cup. Were you able to clean up your water from its visible suspended particles? If you do not treat the water with coagulants and just stir it, no water cleanup occurs. The water stays turbid, as you should have noticed. But when you stirred the solutions and added alum powder as a coagulant, you should have observed that with alum treatment, the particles settled down and the water became much clearer than the control. The reason for this is that the alum powder—which is an aluminum salt or, more specifically, KAl(SO4)2—contains positively charged aluminum ions that are able to neutralize the negatively charged particles in the water. This way the particles can clump together and microflocs form that settle down to the bottom of the cup, leaving behind clear water.
If you do not stir the water after you added the alum powder, the lack of mixing does not allow the positive aluminum ions to come in contact with the soil particles. Thus, the particles cannot be neutralized and will not be able to settle down. Therefore, stirring is a crucial step in the coagulation and flocculation process. If you added lemon juice to the solution, changing its pH, the alum becomes less effective in neutralizing the soil particles. The solution should have stayed more turbid than without lemon juice. Adding different amounts of alum to the turbid water results in different cleanup results. In the beginning adding more alum results in a better water cleanup. Adding too much alum, however, can lead to a less efficient cleanup. In addition, coagulants and flocculants are expensive, which is why you want to avoid adding more than are necessary to the water; drinking water treatment plants use “jar tests” to identify the optimal coagulant and flocculant concentrations.
Cleanup Do not drink any of your water samples—even if they appear clear. In a real water treatment plant, additional steps are taken to ensure the water is safe to drink and not contaminated with dangerous things that we cannot see. Pour all your solutions down the drain.