Discussion beyond the article: Use this reference sheet to lead a class discussion about drinking water contaminants and drinking water treatment methods. Assign pairs of students a particular contaminant and/or water treatment method covered by the reference sheet. Students can use additional resources to answer the given set of questions, below, about their assigned contaminant and/or treatment method. Let groups present their findings to the rest of the class. If time allows, ask the class to create its own reference sheet about contaminants and water treatment methods that can be shared with other students, family members or members of the local community.
Approximate time required: One 45-minute class period
For each contaminant, students should answer these questions:
- What is the contaminant and its composition?
- Where does the contaminant come from? How does it end up in water supplies?
- What are the effects of the contaminant on the human body?
For each treatment method, students should answer these questions:
- What is the water treatment method and how does it work?
- What contaminants is the treatment method best at removing?
- What contaminants is the treatment method not effective at removing?
Reference sheet for teachers:
Types of contaminants:
Microbes: Some harmful bacteria include Salmonella (which can cause typhoid and other infections), Vibrio (which can cause cholera infections) and Legionella (which causes Legionnaire’s disease). Some harmful viruses include poliomyelitis (in countries where it has not been eradicated) and numerous stomach viruses (rotaviruses, reoviruses and noroviruses). Some harmful protozoa include amoebae (single-cell microbes), Balantidium, Giardia and Cryptosporidium. Fecal matter is a common source of these microbial contaminants, which can cause diarrhea, fever and dehydration.
By-products of disinfectants: Chemicals such as chlorine and bromine are added to water to kill microbes. However, these chemicals also react with other molecules in water and can form dangerous by-products called trihalomethanes. One common trihalomethane is chloroform (CHCl3). Such by-products can be toxic to the kidneys, liver and central nervous system and may cause cancer.
PFAS molecules: Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are synthetic chemicals that contain fluorine and carbon. They are used in non-stick coatings and water-resistant coatings, contaminate water supplies and are difficult to remove from water. Some research has linked PFAS to fertility problems and a higher risk of birth defects and learning disabilities.
Arsenic: Arsenic is a heavy metal that is found naturally in soils and sediments in some regions. It can contaminate drinking water supplies if its natural concentration is high, or if it is released into the water supply through mining or agriculture. Exposure to high levels of arsenic has been linked to skin, bladder and lung cancers, as well as birth defects.
Nitrates: Nitrates are nitrogen-rich molecules that are widely used as fertilizers for farms and lawns. Water runoff carries nitrates into water supplies. High levels of nitrates can prevent red blood cells from efficiently carrying oxygen through the body.
- Lead: Lead is a heavy metal that was used nearly a century ago to make pipes that still make up much of the U.S. water system. Lead pipes are expensive to dig up and replace, so many U.S. residents still receive water through lead pipes. If water treatment facilities do not properly adjust water chemistry, the water can corrode the pipes and lead can leach into the water. Repeated lead exposure has been linked to reproductive, kidney and cardiovascular problems.
Common water treatment steps:
Coagulation: Coagulation is the addition of certain salts and/or polymers to water that help contaminant particles clump together. Coagulants are usually positively charged and attract negatively suspended solids and dissolved compounds. Common coagulants include calcium oxide and aluminum sulfate. This treatment is ineffective at removing larger contaminants that are suspended within the water column.
Flocculation: Flocculation is the process by which coagulated particles are clumped together into even larger particles, called flocs, by shaking or spinning water. Flocs can be removed by sedimentation or flotation.
Sedimentation: Sedimentation, also called gravitational settling, is the accumulation of sufficiently dense contaminant particles, formed during coagulation and flocculation, at the bottom of water treatment tanks. This treatment cannot remove dissolved contaminants or contaminants that are as dense as or less dense than water.
Filtration: Filtration is the removal of small contaminants in water by passing the water through filters with small holes. In most filtration systems, water passes through different materials with smaller and smaller holes, removing smaller and smaller contaminants. Filtration is best for removing solids that are not heavy enough to settle to the bottom of a water treatment tank or light enough to be skimmed from the water’s surface. No dissolved contaminants would be removed during filtration.
Activated carbon: Filters containing activated carbon can be used to adsorb contaminants from water or air. In water, activated carbon filtration allows for the removal of suspended organic compounds.
High-pressure membranes: High-pressure membranes filter water by forcing it to pass through a membrane with very small holes. In reverse osmosis, the membrane has holes small enough that essentially only pure water molecules can pass through. In nanofiltration, the holes are slightly larger, allowing a few minerals to pass through. These methods are financially and energetically expensive and cannot remove dissolved contaminants from the water column. Large contaminants can hinder high-pressure membrane filtration as well.
Disinfection: Disinfection uses ultraviolet light and/or chemicals such as chlorine, bromine or iodine to kill microbes. This type of treatment can be inhibited by water turbidity, or murkiness, and is not effective on contaminants that are not microbes.
- Ion exchange treatments: Ion exchange treatments use charged resin beads to grab contaminants that have opposite charges. So, positively charged resin beads grab negatively charged contaminants, and negatively charged resin beads grab positively charged contaminants. Ion exchange treatments filter out contaminants such as PFAS. This type of treatment can be inhibited by turbidity and large contaminants.