Groundwater: How can it be cleaned?
Surface water — water in lakes, rivers, and wetlands — often contains impurities that make it look and smell bad. It may also contain bacteria and other organisms that can cause disease. Consequently, this water must be "cleaned" before it can be used. Water treatment plants typically clean water by taking it through these processes:
- aeration, which allows foul-smelling gases to escape and adds oxygen from the air
- coagulation, which causes solid particles to stick together
- sedimentation, which allows gravity to pull the solid particles out of a liquid
- filtration, which removes more impurities with a filter
- disinfection, which uses chemicals to kill harmful organisms
This project will demonstrate the procedures that municipal water plants use to purify water. It's important to maintain a clean water supply, as this water often affects the quality of the groundwater used by people who depend on wells.
Level of Difficulty
- 10.5 pints (5 liters) of "swamp water" (or add 2.5 cups of dirt or mud to 10.5 pints of water)
- 3 large clear plastic soft-drink bottles: 1 with a cap; 1 with its top removed; 1 with its bottom removed
- 1.5-quart (1.5-liter) or larger beaker (or another clear plastic soft-drink bottle bottom)
- 2 tablespoons (20 grams) alum (potassium aluminum sulfate; available from biological supply houses or ask your teacher for a source.)
- 1.5 pounds (0.7 kilograms) fine sand
- 1.5 pounds (0.7 kilograms) coarse sand
- 1 pound (0.5 kilograms) small pebbles (natural color aquarium rocks, washed)
- large (500 milliliter or larger) beaker or jar
- coffee filter
- rubber band
$10 for sand, pebbles, and alum.
1 to 2 hours.
How to Work Safely
Do not drink the water you are using in this project. Be careful using the scissors when you cut the tops and bottoms off the soda bottles.
- Pour about 1.5 quart (1.5 liter) of the swamp water into the uncut soft-drink bottle. On a data sheet, describe the look and smell of the water.
- To aerate the water, place the cap on the bottle and shake it vigorously for 30 seconds. The shaking allows gases trapped in the water to escape and adds oxygen to the water. Then pour the water back and forth between the bottle with the cap and the cut-off bottle ten times. Describe any changes in the water. Pour the aerated water into the large beaker or bottle bottom.
- To coagulate solid impurities in the water so they can be removed, add the alum crystals to the water. Slowly stir for 5 minutes.
- To allow sedimentation, let the water stand undisturbed for 20 minutes. Observe it at 5-minute intervals and write your observations about the changes in the water's appearance.
- Construct a filter from the bottle with its bottom removed. First, attach the coffee filter to the outside of the neck of the bottle with a rubber band. Turn the bottle top upside down and pour in a layer of pebbles. The filter will prevent the pebbles from falling out. Pour the coarse sand on top of the pebbles. Pour the fine sand on top of the coarse sand. Clean the filter by slowly and carefully pouring through 10.5 pints (5 liters), or more, of clean tap water. Try not to disturb the top layer of sand as you pour.
- To filter the swamp water, wait until a large amount of sediment has settled on the bottom of the bottle of swamp water. Then carefully—without disturbing the sediment—pour the top two-thirds of the swamp water through the filter. Collect the filtered water in a beaker or other container.
- Compare the smell and appearance of the treated and untreated water.
Note: The final step in water treatment is disinfection by adding chemicals to kill any harmful organisms. Because disinfectants must be handled carefully, this process is not included here. Do remember that the water you have treated is NOT safe to drink.
Summary of Results
Write a report of your observations of the smell and look of the water before and after treatment. Include the amount of time that it took for the sediments to form.
Source: Experiment Central. U·X·L, 2000.