Summary
Introduction
Do you know where your drinking water is coming 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 and muddy! 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 water into crystal clear water: see for yourself in this activity!
Background
Surface waters often contain soil particles, 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. One of the first steps in a treatment plant is getting rid of the total 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 together 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 each other, which prevents them from sticking together and makes them stay in suspension. However, if a chemical with an opposite (positive) charge is added to the water, 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!
Materials
- Dry, fine soil from your garden or another source
- Fine-meshed sieve or colander
- Mortar and pestle
- Large mixing bowl
- Tap water
- Teaspoon (2)
- Transparent cups, glass or plastic (at least 4)
- Timer
- Alum, available at the spice section of most grocery stores
- Microwave
- Permanent marker
- Optional: Lemon juice
- Optional: Kitchen towel to catch spills
Preparation
- 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 1 teaspoon of the finely ground soil and put it in a bowl.
- Mix the soil with about 1 liter of tap water to make a turbid, 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 ¼ teaspoon of alum powder to about 3 teaspoons of tap water. You might have to heat up the solution in the microwave to get the powder dissolved. Stir the solution until all the alum powder is dissolved.
- Perform the experiment 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.
Instructions
- 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 cup.
- One of the cups will serve as your control and the other one you will use for the alum treatment. Label them accordingly with “control” and “alum treatment.”
- Set your timer for 2 minutes and start it.
- With a spoon in each cup, stir both soil solutions as fast as you can (without spilling) for 2 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?
- Then stop stirring and set your timer for 5 minutes.
- Start the timer and observe both soil solutions for 5 minutes. What happens to the soil solution within 5 minutes after stirring? Does it stay the same the whole time? Do you see soil particles settle to the ground?
- After 5 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 ¼ teaspoon to the cup you labeled with “alum treatment.” Do not add anything to the control cup.
- Set your timer for 2 minutes and start it.
- Again, take two spoons and stir both cups as fast as you can (without spilling) for 2 minutes.
- Stop stirring and set your timer for 5 minutes.
- Start your timer and observe both cups again for 5 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 get clearer in one of the cups? How long does it take to become fully clear?
- Get two new cups and label them with “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 a ¼ teaspoon of alum solution to the cup you labeled “alum treatment.”
- This time, do not stir the solutions and just observe what happens for 7 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 7 minutes? Why do you think this is the case?
- After you waited for 7 minutes, start stirring both solutions as fast as you can (without spilling) for 2 minutes and then observe the cup for 5 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 2 teaspoons full of lemon juice to each of the cups. This changes the pH of the soil solution. Now add ¼ teaspoon of your alum solution to one of the cups and stir both of them for 2 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 (add 7.5 grams of alum to 1 liter of tap water). Then prepare 3 or 4 cups with turbid soil solution. Add different amounts of the alum solution to each of the cups (for example 0, 1, 3 and 5 milliliters) and stir all of them for 2 minutes. Observe the cups after stirring. Do you see a difference in the three/four cups after stirring? Do all of the water solutions change from turbid to clear?
Observations and Results
In this science activity, you simulated a full-scale water treatment process in a cup. Were you able to clean up your water? 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 noticed 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. When adding lemon juice to the water solution and changing the pH of the water, 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. However, adding too much of alum can lead to a less efficient water cleanup. In addition, coagulants and flocculants are expensive, which is why you want to avoid adding lots of them to the water, which is why so-called “jar tests” have been developed to identify the optimal coagulant and flocculant concentrations for each drinking water treatment plant.
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Cleanup
- Pour all your solutions down the drain and put the plastic cups into the recycling bin.
Additional Resources
- Where does your water come from?, from The Nature Conservancy
- Drinking Water, from The Centers for Disease Control and Prevention
- Coagulation-Flocculation, from Sustainable Sanitation and Water Management (SSWM)
- Flocculation: Making clean water, from Khan Academy
- Science Activity for All Ages!, from Science Buddie
