Using the Concepts Associated with Providing Clean Drinking Water to Teach Science: An Interdisciplinary Approach
Elizabeth Paine, PhD
Introducing students to the issues associated with safe drinking water is a great real-world, interdisciplinary way to teach science. In learning about water quality, students can also learn about microbiology, environmental science, engineering, and social science.
The transmission of disease by water was first demonstrated by the epidemiologist John Snow in the summer of 1854 during a cholera epidemic in London. Snow observed that the only common factor among people contracting cholera was that they all drank water from a pump on Broad Street. One of the more striking pieces of evidence was that a woman who lived in a different part of town, but who preferred the water from the Broad Street pump, died from cholera after consuming water specifically carried to her from the Broad Street pump. Her niece, who also drank the water, died from cholera as well.
Because of the practices that evolved from the knowledge that disease can be waterborne, people in the developed world no longer die from cholera. However, having safe water to drink is not something that most people in the world can take for granted. The World Health Organization estimates that each year in developing countries 2 million children die from waterborne diseases. Some of the more common waterborne pathogens in developing countries include: giardia, hepatitis A virus, hepatitis E virus, norovirus, shigella (dysentery), Vibrio cholera, rotaviruses, and Salmonella Typhi (typhoid fever).
Although water in developed countries is much cleaner than that found in developing countries, even the water supply systems in the developed world are imperfect and cause outbreaks of waterborne disease. The Centers for Disease Control and Prevention reported 780 disease outbreaks in the US from 1971 to 2006 that were associated with consuming contaminated water. The outbreaks affected 577,094 people. However, most waterborne disease in the US goes unreported. Many estimate that the actual number of illnesses associated with waterborne pathogens is much higher.
In the developed world, organisms commonly associated with waterborne disease outbreaks include: giardia, cryptosporidium, the pathogenic E. coli strain 0157:H7, norovirus, and campylobacter. Infection by some of the organisms associated with waterborne disease can have long-lasting effects, including arthritis, cancer, diabetes, and myocarditis (inflammation of the heart muscle), a fact that adds to the necessity of preventing waterborne diseases.
In addition, people in the field are finding that new waterborne diseases are emerging, both because our knowledge of what causes disease is increasing and because of changes in human practices and migration.
Testing the waters
Ensuring that water is safe for human consumption is a huge challenge. Any water test for detecting human pathogens must take the following into consideration:
- Generally the level of pathogens in the water is low, so large volumes of water must be concentrated to detect them.
- Tests must be inexpensive enough that they can be done frequently.
- The test should be able to be performed by someone with minimal training.
- To avoid putting people doing the test at risk, the test should not require culturing any pathogens.
- To be affordable, the test should not require a specialized lab or expensive equipment.
- The test should be fast.
Given these considerations, it is not practical to check water for all the known possible pathogens. Even if it were possible, such testing would not detect unknown pathogens. Most human pathogens are fecal in origin. So currently, the most common approach is to test water for organisms that would be present if the water had been contaminated with fecal material. The challenge has been to find appropriate organism(s) for indicating contamination. The organisms should be fecal in nature and should be similar to human pathogens in their ability to survive in the drinking water environment.
The most widely used indicators are coliforms, fecal or thermotolerant coliforms, Escherichia coli, enterococci (fecal or intestinal streptococci), and bacteriophages. Coliforms are Gram-negative, non-spore forming, rod-shaped bacteria that ferment lactose. Because these bacteria include organisms that live in the intestines of warm-blooded animals, they have long been used as indicators of fecal contamination of water. However, some coliforms also grow in other environments. Thus, the use of total coliforms in determining if water is safe for human consumption has been called into question and is now controversial.
Fecal or thermotolerant coliforms are bacteria that fit the description of the coliforms with the additional characteristic of being able to ferment lactose with acid production at a higher temperature (44° C). Finding bacteria in this group in water has been found to be a good indication of contamination by fecal material, but the correlation is not 100% accurate.
Escherichia coli is a normal inhabitant of the intestines of warm-blooded animals and is now used by some countries as the primary indicator of fecal contamination.
Enterococci are useful indicator organisms for several reasons, including the fact that they are always present in warm-blooded animals’ fecal material, and thus are good indicators of fecal contamination. In addition, their ability to persist in the environment is very similar to that of some waterborne pathogenic bacteria, so they are a good model for whether these organisms are likely to persist in drinking water.
Bacteriophages are bacteria viruses. One of the main reasons for using bacteriophages as indicator organisms is that, with respect to their ability to persist in water, they are similar to viruses that are known waterborne pathogens. In general, viruses are different from bacteria with respect to their response to environmental stresses.
There are many different tests for detecting the organisms discussed above. None of them is ideal. The bacteria discussed are most commonly identified by their biochemical characteristics (e.g., the ability to ferment lactase with acid production at specific temperatures). Some of these biochemical characteristics are detectable by growing the organisms on certain types of agar. There are also specific assays that detect enzyme activity associated with a specific bacterium or group of bacteria.
Some of the newer molecular techniques for detecting organisms are useful, but at this time are limited in their usefulness. For example, PCR will detect the nucleic acid sequences of an organism or group of organisms, but does not indicate the infectivity of the organism. The test amplifies the organism's DNA even if the organism is not alive or if the DNA is free floating in the water. Most molecular techniques also involve more highly skilled personnel, and specific equipment or infrastructure.
Proactive water quality monitoring
Waterborne disease outbreaks can result from failure at multiple levels, including (1) contamination of the original source of the water, (2) failure of treatment procedures, and (3) contamination of the water while it is in the distribution system. Ideally a system should be monitored at multiple levels. The push in recent years has also been for a more proactive approach to water quality, which would involve monitoring a water system at multiple levels, including the source.
This push results from the concern that the most common current approaches detect a problem after the water has been delivered to the people drinking it, and thus only after people have been affected. More widespread monitoring of a water system would be more likely to catch or prevent a problem before people are affected. For example, careful monitoring of the source of water in the system and careful protection of the watershed would most likely detect or prevent some contaminants from ever entering the system.
Engage your students
Here are some suggestions for engaging students in research of methods for testing and treating water for contamination by microorganisms that cause waterborne diseases.
- You may want to have students research the different methods for treating water and the advantages and disadvantages of each method. For example, treatment of water with chlorine and/or chloramines is one of the most common methods for eliminating microorganisms. However, the byproducts that chlorine forms with organic material found in water is a constant source of concern. Why is this a concern and how is this concern handled?
- You may want to have students test the efficiency of different water purification systems on water from a local pond or stream. Some of the methods they could test include treatment with chlorine, boiling, filtering through sand and/or charcoal, and using iodine tablets. One method that has found use in some third world countries is to place water in small plastic water bottles and lay them in the sun. Through research students may find other methods they wish to test. In addition, students could discuss the practicality of using these different methods. Clearly, different methods will have different advantages in different settings.
- The simplest way for students to test the effectiveness of purification methods is to have them test for total coliforms. As stated earlier, testing water for total coliforms as a measure of fecal contamination is now controversial; however, it is still considered a fairly effective way to test the effectiveness of water treatment. The Carolina™ Bacterial Pollution of Water Kit (item #652704) includes materials for detecting total coliforms. Alternatively, Carolina also carries MacConkey (item #821682) and Levine EMB (item #821662) plates, MacConkey (item #776350) and Levine EMB (item #775932) agar, and MacConkey (item #784480) and Levine EMB (item #784201) dried media. Both of these media can be used to detect coliforms. Make sure that when students perform their tests they include a control of untreated water and that they take the appropriate safety precautions. Once culture plates are set up, they should be taped shut and not opened to avoid exposure to any pathogens.
- You also may want to have students do research on the availability of clean water in different countries. What are the challenges that prevent access to clean water in these countries? What ideas do they have for overcoming these challenges? Would any of the methods they have tested be practical in some of the countries they have researched?
Exploring the issues associated with safe drinking water can appeal to students with a broad range of interests and offers many opportunities for independent research and hands-on exploration. It also provides you with a great opportunity to teach science using a real-world, interdisciplinary approach.
Ashbolt, N.J., Microbial Contamination of Drinking Water and Disease Outcomes in Developing Regions. 2004. Toxicology, Vol. 198, 229–238.
Craun, G.F., J.M. Brunkard, J.S. Yoder, V.A. Roberts, J. Carpenter, T. Wade, R.L. Calderon, J.M. Roberts, M.J. Beach, S.L. Roy. 2010. Causes of Outbreak Associated with Drinking Water in the United States from 1971–2006. Clinical Microbiology Reviews, Vol. 23(3), 507–528.
Madema, G.J., P. Payment, A. Dufour, W. Robertson, M. Waite, P. Hunter, R. Kirby, and Y. Anderson. Safe Drinking Water: An Ongoing Challenge in Assessing Microbial Safety of Drinking Water: Improving Approaches and Methods. Dufour, A., M. Snozzi, W. Koster, J. Bartram, E. Ronchi, L. Fewtrell, Editors. IWA Publishing, 2003.
Figueras, M.J. and Borrego, J.J. 2010. New Perspectives in Monitoring Drinking Water Microbial Quality. International Journal of Environmental Research and Public Health, Vol. 7, 4179–4202.
Postel, S.L. 2000. Water and World Population Growth. Journal of the American Water Works Association, Vol. 92(4), 131–138.
Rompré, A., P. Servais, J. Baudert, M. de-Roubin, P. Laurent. 2002. Detection and Enumeration of Coliforms in Drinking Water: Current Methods and Emerging Approaches. Journal of Microbiological Methods, Vol. 49, 31–54.