Learn How to Disinfect Contaminated Water
AbstractHave you ever thought about how fortunate you are to have safe and clean water coming out of your faucet? Many people in undeveloped nations don't have this luxury. But does that mean they can't have clean water at all? Is there an inexpensive way they could use to make their own clean water? In this microbiology science fair project, you will investigate whether or not sunlight can disinfect contaminated water.
Recommended Project Supplies
Investigate the disinfecting properties of sunlight.
Water is a precious commodity. It keeps us healthy and clean. Water is necessary for growing plants, many of which produce food for us. You probably go about your day without giving your abundant and clean water a second thought. Our towns and cities provide us with clean and safe drinking water because they have the means and infrastructures to clean and disinfect water. However, there are people that don't have the luxury of clean and safe water. The United Nations International Children's Emergency Fund (UNICEF) and the World Health Organization (WHO) estimate that 1 billion people do not have access to safe drinking water. The lack of safe water can be due to drought, war, or perhaps a town doesn't have the money or infrastructure to provide clean water to its citizens.
Water can be contaminated with several types of water-borne pathogens. These include Vibrio cholerae (a bacteria that results in cholera), Shigella dysenteriae (a bacteria that causes dysentery), Giardia lamblia (a parasite that results in giardia), and viruses like polio. Often, when a person who doesn't have access to good health care contracts one of these diseases, it can be fatal. In fact, more than 1.5 million children under the age of 5 years die each year of water-borne diseases in Africa and Asia. These pathogens can be present in water anywhere. According to the Environmental Protection Agency (EPA), the best way to disinfect water is to first filter the water and then boil it vigorously for 1–3 minutes. Let the water cool, and then transfer it to a clean container. To treat the water chemically, first filter the water and then use regular, unscented household bleach. Add two drops of bleach for every quart or liter of water, stir or shake the water, and then let it stand for at least 30 minutes. If the water is cloudy before you've chemically treated it, then double the number of bleach drops and double the amount of time that it stands. Both iodine tinctures and calcium hypochlorite are better at treating contaminated water than bleach is. However, there are significant safety issues when using these chemicals.
But what if you live in an undeveloped nation? You might not have the financial resources available to purchase chemicals. Fuel for boiling water might not even be available. In this situation, there is a water-disinfection procedure that is available, solar disinfection, also known as SODIS. This process takes advantage of sunlight, which is free and readily available, and plastic polyethylene terephthalate (PET) bottles, which are available around the world.
The SODIS process is easy to follow. The first step is to find a PET bottle with a lid and clean it well. PET bottles are recyclable and have a "1" surrounded by a triangle symbol on the bottom. Then fill the bottle ¾ full with water. The water should not be too turbid. Highly turbid water protects the pathogens from the Sun's radiation. Shake the bottle for 20 seconds to aerate it. Now fill the bottle fully and close the lid. Place the bottle on a black iron sheet or on your roof, where it will have access to sunlight. Leave it in the sunlight. After a certain amount of time (which you'll be investigating in this science fair project), the water will be ready to drink.
So what exactly does the sunlight do to the organisms in the water, making it safe to drink? Light from the Sun can be separated into several sections, three of which are as follows: visible light (400–700 nanometers (nm)), ultraviolet (UV) light (10 nm–400nm), and infrared (IR) light (700 nm–1 millimeter (mm)). The UV spectrum is also separated into several portions, one of which is UVA, which has a spectrum of 320 nm–400 nm. The UVA reacts with the oxygen dissolved in the water and produces highly reactive forms of oxygen that are thought to kill pathogens in the water by causing fatal DNA damage and destroying the cell walls of bacteria. The IR portion of the spectrum heats the water. If the temperature of the water rises above 50°C, the disinfection process proceeds three times faster.
In this microbiology science fair project, you will investigate the exposure time to UVA light that is required for a bottle of water to be disinfected. You will test different exposure times and compare the results to boiling water. One of the missions of science research is to help improve human health and life. In doing this science fair project, you can become a part of that mission.
Terms and Concepts
- Solar disinfection (SODIS)
- Polyethylene terephthalate (PET)
- Reactive oxygen species
- In which parts of the world is SODIS used?
- What does UV light do to pathogens?
- Is SODIS just as effective with glass bottles as it is with clear plastic PET bottles?
- What is the technical difference between sterilization and disinfection? Is SODIS a sterilization or a disinfection procedure?
- What kinds of reactive oxygen species are created during the irradiation of oxygenated water with UVA? How do these reactive oxygen species contribute to the disinfection process?
- Bacteria Discovery Kit, available from our partner
Home Science Tools. Needed from the kit:
- Nutrient agar plates (24)
- Disposable gloves (4 pairs)
- Medicine droppers (2)
- Sterile swabs (24)
- You will also need to gather these items, not included in the kit:
- Buret support stand, available from our partners at Home Science Tools. Alternatively, you could use something else to clip the bulb onto, such as the bottom of a cabinet door or over a counter.
- Isopropyl rubbing alcohol 70% or higher; available in the first aid aisle of grocery stores and pharmacies
- Access to a local creek or stream
- Plastic jug with cap, 1-gal, clean
- Clamp lamp, approximately 8 1/2-inch. This item (or something similar to it) can be purchased at your local pet store or on Amazon.com
- Daylight Blue Reptile Bulb, 100-watt (W). This item (or something similar to it) can be purchased at your local pet store or on
- Note: The Daylight Blue Reptile Bulb mimics sunlight and emits UVA. In order to get similar results for each trial (versus actual sunlight), you will use the blue reptile bulb to act as the Sun.
- Plastic water bottles with caps, 16-oz., clean (6). The bottles must be clear polyethylene terephthalate (PET) bottles. PET bottles are recyclable and have a "1" surrounded by a triangle on the bottom.
- Dark metal sheet or pan, should be large enough to hold two 16-oz. bottles on their sides.
- Liquid measuring cup
- Pot with lid, 1-qt.
- Stove top
- Permanent marker
- Lab notebook
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For health and safety reasons, science fairs regulate what kinds of biological materials can be used in science fair projects. You should check with your science fair's Scientific Review Committee before starting this experiment to make sure your science fair project complies with all local rules. Many science fairs follow Intel® International Science and Engineering Fair (ISEF) regulations. For more information, visit these Science Buddies pages: Project Involving Potentially Hazardous Biological Agents and Scientific Review Committee. You can also visit the webpage ISEF Rules & Guidelines directly.
Note: There will be many steps going on at the same time with this experiment, so be sure to read through the procedure carefully beforehand, and keep careful track of samples, upcoming steps, dates, and times as you perform the experiment.
Preparing the Setup
- Go to your local creek or stream with the clean, plastic 1-gal jug and a pair of disposable gloves. Place the jug into the water and fill it up. Avoid trapping any large particles or foreign objects in the jug. Once the jug is full, replace the cap. Take the jug back to where you are conducting your testing.
- Now put the clamp-lamp and the Daylight Blue Reptile Bulb together, following all instructions that came with the lamp. Find a quiet location near an electrical outlet. Clip the lamp onto something, such as the bottom of a cabinet door over a counter, allowing the lamp to face downward. You could also clip the lamp assembly to the top of the rod of a buret stand. Figure 1 shows this configuration with a homemade buret stand.
- Sterilize the medicine droppers. Sterilization kills all of the bacteria on the tools. Using sterilized tools prevents you from adding bacteria to the samples.
- Wash the glass part of the medicine droppers with soap and water.
- Dip the outside of the medicine droppers in isopropyl alcohol. Suck up a dropper full of the alcohol with each dropper then squirt it out. Repeat twice.
- Let the droppers dry completely before using.
Figure 1. This image shows the experimental setup, with the UV light shining on the test samples (Note: the samples in the picture have not been placed on the dark, metal sheet yet, but yours should be.)
Preparing the SODIS Samples
Carefully transfer some of the water from the jug into two of the clean 16-oz. clear plastic water bottles. Follow the SODIS procedure:
- Fill the bottles ¾ full with the creek or stream water and screw on the lid.
- Shake the bottles for 20 seconds each in order to aerate them.
- Now fill the bottles fully and screw on the lids tightly.
- Keep the rest of the water in the jug in a cool, dark place.
When the sample bottles are prepared, plug the lamp into the outlet and turn on the lamp. The lamp should be about 6 inches away from the counter, or from the bottom of the buret stand. Place both samples onto the dark metal sheet beneath the lamp, directly in the path of the light, to mimic the SODIS process as much as possible. Based on your background research, what does the metal sheet represent, and why it is important? Leave the light shining on the samples. Do not disturb the light or the samples until you are ready to test their bacterial content, 12 hours later.
- Note: When the SODIS process is applied in a real situation, the minimum time that the bottle of water sits in direct sunlight is 6 hours (longer if the water is very turbid). In this science fair project, you are using the lamp as a substitute for the sun. Since the lamp doesn't produce the same amount of UVA or heat as the Sun does, you will need to keep your bottles of creek or stream water under the lamp for longer than 6 hours.
Preparing and Testing the Boiling and Untreated Water Samples
Note: It is important that you observe the following agar plates at the same time. You will apply the boiled water and untreated water samples to the agar plates while the SODIS samples are still under the UV lamp. You must keep track of the time when you start and stop tests so that the observations can be compared.
- While the SODIS samples continue resting under the UV light, measure 1 cup of the creek or stream water in the liquid measuring cup and pour it into the 1-qt. pot. Place the pot onto the stove top and bring the water to a rolling boil. Boil the sample for 5 minutes. Remove the pot from the burner, cover the pot, and let the water cool to room temperature.
- Put on a pair of disposable gloves. Get out three nutrient agar plates. Suck some of the boiled (but now room-temperature) water into a sterilized medicine dropper.
- Remove the cover of one of the plates. Apply three drops of the boiled water to the soy agar. Use a sterile cotton swab to smear the water drops in a zigzag pattern on the surface of the nutrient agar, starting in the center—smear the water sample from the center of the plate to the edge of the plate. Replace the cover.
- Repeat steps 2–3 with the two other nutrient agar plates. Reuse the same medicine dropper. Using the permanent marker, note down the time, the date, and the treatment process on the bottom of the plates (in this set of trials, it is boiling; for the next set of trials it will be untreated). Set aside the medicine dropper to sterilize again for future use.
- Keep your nutrient agar plates in a warm location in your house that will not be disturbed.
- Repeat steps 2–5 with untreated water from the 1-gal jug. Use a freshly sterilized medical dropper for the untreated water sample. You can reuse the same dropper for all three agar plates.
- Let the boiled water and untreated water plates (6 in total) sit undisturbed for 24 hours. Do you observe any growth on the plates? Record the time, date, and your observations in your lab notebook. Check again in, 48 hours, 72 hours, and 96 hours. Record your observations each time you check. If you see any growth, count the number of bacterial colonies and record the number in your lab notebook.
Testing the SODIS Samples
- In the meantime, you will need to also keep track of the time the water samples spend underneath the UV lamp. After 12 hours, remove one of the bottles from underneath the lamp. Repeat steps 2–5 of the previous section with this SODIS sample. Use a freshly sterilized medical dropper. Again, you can reuse the same one for all three agar plates. Then perform step 7 of the previous section, counting the number of bacterial colonies, and recording the time and date in your lab notebook.
- After 48 hours have elapsed, remove the second SODIS sample from under the lamp. Repeat steps 2–5 and step 7 (all from the previous section) with this sample, using one freshly sterilized medicine dropper for all three agar plates.
Repeating the Experiment
- Repeat the entire experiment one additional times, with fresh materials. Remember to record all your observations in your lab notebook. Repeating the experiment will help you determine how reliable your data is.
- As you finish obtaining your sets of data, follow the procedure detailed in Microorganisms Safety Guide to safely dispose of your agar plates.
Analyzing Your Data
- Now analyze your data. Plot the data on a scatter plot. For the first plot, label the x-axis Treatment and the y-axis Bacterial Count at 96 Hours. For the second plot, label the x-axis Observation Time and the y-axis Bacterial Count. For this second plot, you can plot all of your data on one plot or you can make a plot for each treatment.
- How does the bacterial count change with each treatment? Is SODIS a viable treatment process? Do bacteria grow at different rates for different treatments?
Bacteria are all around us in our daily lives and the vast majority of them are not harmful. However, for maximum safety, all bacterial cultures should always be treated as potential hazards. This means that proper handling, cleanup, and disposal are necessary. Below are a few important safety reminders.
- Keep your nose and mouth away from tubes, pipettes, or other tools that come in contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.
- Make sure to wash your hands thoroughly after handling bacteria.
- Proper Disposal of Bacterial Cultures
- Bacterial cultures, plates, and disposables that are used to manipulate the bacteria should be soaked in a 10% bleach solution (1 part bleach to 9 parts water) for 1–2 hours.
- Use caution when handling the bleach, as it can ruin your clothes if spilled, and any disinfectant can be harmful if splashed in your eyes.
- After bleach treatment is completed, these items can be placed in your normal household garbage.
- Cleaning Your Work Area
- At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly clean any surfaces you have used.
- Be aware of the possible hazards of disinfectants and use them carefully.
Ask an Expert
- Prepare samples and place them outside in direct sunlight. Follow the SODIS process. Does sunlight kill bacteria? What is the difference in latitude between where you live and Africa? Does this difference affect the efficacy of SODIS?
- Try using different-colored PET bottles. Does the color of the bottle affect the efficacy of the SODIS process?
- Does changing the orientation of the bottles affect the efficacy of the SODIS process? Try changing the orientation and find out.
- Does increasing the potential for reactive oxygen species increase the efficiency of SODIS? You'll need to do some background reading about how reactive oxygen species are formed and think about what additives could be introduced to the water to promote reactive oxygen species production.
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