Objective
To investigate the disinfecting properties of sunlight.
Introduction
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, Concepts and Questions to Start Background Research
Pathogen
Bacteria
Cholera
Dysentery
Parasite
Giardia
Virus
Polio
Disinfect
Solar disinfection (SODIS)
Polyethylene terephthalate (PET)
Turbid
UVA
Sterilize
Reactive oxygen species
Questions
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?
Bibliography
The Swiss Federal Institute of Aquatic Science and Technology. (n.d.). Solar Water Disinfection—The Method. Retrieved February 22, 2009, from http://www.sodis.ch/Text2002/T-TheMethod.htm
Wikipedia Contributors. (2009, February 16). Solar Water Disinfection. Wikipedia: The Free Encyclopedia. Retrieved February 22, 2009, from http://en.wikipedia.org/w/index.php?title=Solar_water_disinfection&oldid=271168524
Materials and Equipment
Metal cookie sheet
Aluminum foil (1 roll)
Pot, 8-quart (qt.)
Pipet [dropper, flint glass, 105-millimeter (mm), 2-milliliter (mL)], (3 packages of 12); available at sciencekit.com, SKU # WW6240512. The pipets come with bulbs.
Glass stirring rods (6); available at sciencekit.com, SKU # WW6731074
Clean glass jar with lid (1), should be large enough to hold the pipets
Stove top
Oven
Tongs, metal
Oven mitt
Plastic jug with cap, 1-gallon (gal), clean (1)
Disposable gloves; available at your local drugstore
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.
Clamp lamp, approximately 8 1/2-inch. This item (or something similar to it) can be purchased at your local pet store.
Daylight Blue Reptile Bulb, 100-watt (W). This item (or something similar to it) can be purchased at your local pet store.
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.
Optional: Buret support stand; available from sciencekit.com, SKU # WW60832M01
Dark metal sheet, should be large enough to hold two 16-oz. bottles on their sides.
Liquid measuring cup
Pot with lid, 1-qt.
Tryptic soy agar plates, prepared (4 packages of 10); available from sciencekit.com, SKU # WW6732303. If you will not be using the plates immediately after delivery, store them in your refrigerator. Take the plates out of the refrigerator, as needed.
Clock
Permanent marker
Lab notebook
Experimental Procedure
Caution: Do not drink any of the water during or after you have completed the experiment, or you might risk getting sick. The SODIS process is a disinfection process and not a sterilization process. It also doesn't remove chemicals from the water. The resulting water may not fulfill EPA standards for drinking water.
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
The first step is to sterilize all of your tools prior to collecting the water sample. Sterilization kills all of the bacteria on the tools. Using sterilized tools prevents you from adding bacteria to the samples.
Preheat the oven to 225°F.
Cover the cookie sheet completely with aluminum foil.
Fill the pot with tap water. Put all of the pipets, the glass rods, and the jar and lid into the pot.
Place the pot on the stove and bring the water to a boil. Boil all of the tools for 5 minutes and then turn off the heat.
Using the metal tongs, lift each item from the pot and place it onto the cookie sheet. Once all of the items are on the sheet, place it into the pre-heated oven to dry. Let the items dry completely for 10–15 minutes. Carefully remove the cookie sheet from the oven with an oven mitt when the time is up.
Use the tongs to place each pipet into the glass jar. Put the lid on the jar so that the pipets remain clean. Completely wrap all of the glass rods in a sheet of aluminum foil to keep them clean for the duration of the project. Store all of the sterilized tools in an area that will not be disturbed.
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.
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 detailed in the Introduction when filling the 16-oz. water bottles, as follows. 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. In your lab notebook, note the time on the clock and the date on which you placed the samples under the lamp. Warn others in your household to be careful around the lamp and bottles on the counter.
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 a clean pipet from the glass jar and attach a bulb to the top. Get out three tryptic soy agar plates and a glass stirring rod from your aluminum foil package. Suck some of the boiled (but now room-temperature) water into the pipet.
Remove the cover of one of the plates. Apply three drops of the boiled water to the soy agar. Use the glass rod and smear the water drops in a zigzag pattern on the surface of the soy 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 tryptic soy agar plates. You can use the same pipet and bulb. 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). Discard the pipet and the bulb. You can reuse the glass rod later, but you will have to sterilize it again. If you don't sterilize right away, then keep it separate from the clean glass rods and use a clean glass rod for the next trials with the untreated water.
Keep your tryptic soy 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. You should have six agar plates.
Let the boiled water and untreated water plates 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. 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.
Repeating the Experiment
Repeat the entire experiment two additional times, with fresh materials. Remember to record all your observations in your lab notebook.
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?
Variations
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.
For more science project ideas in this area of science, see Microbiology Project Ideas.
Credits
Michelle Maranowski, PhD, Science Buddies
To investigate the disinfecting properties of sunlight.
Introduction
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, Concepts and Questions to Start Background Research
Pathogen
Bacteria
Cholera
Dysentery
Parasite
Giardia
Virus
Polio
Disinfect
Solar disinfection (SODIS)
Polyethylene terephthalate (PET)
Turbid
UVA
Sterilize
Reactive oxygen species
Questions
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?
Bibliography
The Swiss Federal Institute of Aquatic Science and Technology. (n.d.). Solar Water Disinfection—The Method. Retrieved February 22, 2009, from http://www.sodis.ch/Text2002/T-TheMethod.htm
Wikipedia Contributors. (2009, February 16). Solar Water Disinfection. Wikipedia: The Free Encyclopedia. Retrieved February 22, 2009, from http://en.wikipedia.org/w/index.php?title=Solar_water_disinfection&oldid=271168524
Materials and Equipment
Metal cookie sheet
Aluminum foil (1 roll)
Pot, 8-quart (qt.)
Pipet [dropper, flint glass, 105-millimeter (mm), 2-milliliter (mL)], (3 packages of 12); available at sciencekit.com, SKU # WW6240512. The pipets come with bulbs.
Glass stirring rods (6); available at sciencekit.com, SKU # WW6731074
Clean glass jar with lid (1), should be large enough to hold the pipets
Stove top
Oven
Tongs, metal
Oven mitt
Plastic jug with cap, 1-gallon (gal), clean (1)
Disposable gloves; available at your local drugstore
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.
Clamp lamp, approximately 8 1/2-inch. This item (or something similar to it) can be purchased at your local pet store.
Daylight Blue Reptile Bulb, 100-watt (W). This item (or something similar to it) can be purchased at your local pet store.
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.
Optional: Buret support stand; available from sciencekit.com, SKU # WW60832M01
Dark metal sheet, should be large enough to hold two 16-oz. bottles on their sides.
Liquid measuring cup
Pot with lid, 1-qt.
Tryptic soy agar plates, prepared (4 packages of 10); available from sciencekit.com, SKU # WW6732303. If you will not be using the plates immediately after delivery, store them in your refrigerator. Take the plates out of the refrigerator, as needed.
Clock
Permanent marker
Lab notebook
Experimental Procedure
Caution: Do not drink any of the water during or after you have completed the experiment, or you might risk getting sick. The SODIS process is a disinfection process and not a sterilization process. It also doesn't remove chemicals from the water. The resulting water may not fulfill EPA standards for drinking water.
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
The first step is to sterilize all of your tools prior to collecting the water sample. Sterilization kills all of the bacteria on the tools. Using sterilized tools prevents you from adding bacteria to the samples.
Preheat the oven to 225°F.
Cover the cookie sheet completely with aluminum foil.
Fill the pot with tap water. Put all of the pipets, the glass rods, and the jar and lid into the pot.
Place the pot on the stove and bring the water to a boil. Boil all of the tools for 5 minutes and then turn off the heat.
Using the metal tongs, lift each item from the pot and place it onto the cookie sheet. Once all of the items are on the sheet, place it into the pre-heated oven to dry. Let the items dry completely for 10–15 minutes. Carefully remove the cookie sheet from the oven with an oven mitt when the time is up.
Use the tongs to place each pipet into the glass jar. Put the lid on the jar so that the pipets remain clean. Completely wrap all of the glass rods in a sheet of aluminum foil to keep them clean for the duration of the project. Store all of the sterilized tools in an area that will not be disturbed.
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.
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 detailed in the Introduction when filling the 16-oz. water bottles, as follows. 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. In your lab notebook, note the time on the clock and the date on which you placed the samples under the lamp. Warn others in your household to be careful around the lamp and bottles on the counter.
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 a clean pipet from the glass jar and attach a bulb to the top. Get out three tryptic soy agar plates and a glass stirring rod from your aluminum foil package. Suck some of the boiled (but now room-temperature) water into the pipet.
Remove the cover of one of the plates. Apply three drops of the boiled water to the soy agar. Use the glass rod and smear the water drops in a zigzag pattern on the surface of the soy 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 tryptic soy agar plates. You can use the same pipet and bulb. 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). Discard the pipet and the bulb. You can reuse the glass rod later, but you will have to sterilize it again. If you don't sterilize right away, then keep it separate from the clean glass rods and use a clean glass rod for the next trials with the untreated water.
Keep your tryptic soy 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. You should have six agar plates.
Let the boiled water and untreated water plates 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. 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.
Repeating the Experiment
Repeat the entire experiment two additional times, with fresh materials. Remember to record all your observations in your lab notebook.
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?
Variations
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.
For more science project ideas in this area of science, see Microbiology Project Ideas.
Credits
Michelle Maranowski, PhD, Science Buddies
You are supposed to have better Bibliography; where did you get your information
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