Before you do this lesson, students should already:
After this lesson, students will be able to:
Before doing this lesson, you may want to do the filtration activity. You could also have students design their own filtration device as a follow-up to this lesson. Purification should definitely be partnered with filtration in some way.
Coagulation-flocculation-sedimentation practices are essential pretreatments for many water purification systems- especially filtration treatments. These processes clump suspended solids together into larger bodies so that physical filtration processes can more easily remove them. Particulate removal by these methods makes later filtering processes far more effective. The process is often followed by gravity separation (sedimentation or flotation) and is always followed by filtration.
A chemical coagulant is added to source water to facilitate bonding among particulates. Coagulants work by creating a chemical reaction and eliminating the negative charges that cause particles to repel each other. Coagulants neutralize the electrical charges of particles in the water which causes the particles to clump together. Chemically, coagulant water treatment chemicals are either metallic salts (such as alum) or polymers. Polymers are man-made organic compounds made up of a long chain of smaller molecules. Polymers can be either cationic (positively charged), anionic (negatively charged), or non ionic (neutrally charged.)
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually dissolved in a solution. The coagulant-source water mixture is then slowly stirred in a process known as flocculation. This water churning induces particles to collide and clump together into larger and more easily removable clots, or “flocs.”
Wastewater: Even after primary and secondary treatment, disease-causing organisms may remain in the treated wastewater. To disinfect and kill harmful organisms, the wastewater spends a minimum of 15-20 minutes in chlorine-contact tanks mixing with sodium hypochlorite, the same chemical found in common household bleach. The treated wastewater, or effluent, is then released into local waterways.
A system that incorporates coagulation-flocculation followed by chlorination has been developed as a point of use technology, especially for developing countries.
It uses a small packet of powdered ferrous sulfate (a common flocculent) and calcium hypochlorite (a common disinfectant). A user opens the packet, adds the contents to an open bucket containing about ten liters of water, stirs for five minutes, lets the solids settle to the bottom, strains the water through cotton cloth into another container, and waits 20 minutes for the chlorine to disinfect the water.
The combination of particle removal and disinfection appears to produce high removal rates of bacteria, viruses, and protozoa, even in highly turbid waters. There is considerable evidence that the system has reduced diarrheal disease significantly in various locations. There is also evidence that the flocculation process helps remove arsenic; however, these systems are not an adequate substitute for high-quality centralized treatment when it can be made available.
Order as many of the following coagulants as possible.
If you can only order one, you can instead have students compare the properties of that one coagulant to other chemicals which only cause basic precipitation.
You can also order PUR packets if you want to have students see the physical difference instead of just watching the video. You can get information on ordering here: http://www.csdw.com/csdw/home.shtml.
If you are going to ask students questions after they test their coagulants:
Have each student come up with one question of their own about drinking water to ask the rest of the class. Be prepared to help some students form a question. If the answers to the questions are not available, encourage the students to search the EPA (http://www.epa.gov) website and to bring those answers back to the classroom to share with the other students.
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